Forces Acting on Substances and Mechanisms

In our daily lives, we are always surrounded by forces that affect how things move and behave. Gravity pulls us toward the Earth, friction slows down moving objects, and magnets attract certain metals. Even when we push, pull, or lift something, we are applying a force.

These forces are at work all around us, often without us noticing.

When objects collide, they can behave in different ways. Sometimes they bounce apart with no loss in energy; this is called an elastic collision. Other times, they stick together or change shape; this is an inelastic collision. By learning about these types of forces and collisions, we can better understand everyday situations, from how cars move and stop, to how balls bounce in sports.

KEY IDEAS

• Conservation of linear momentum.

• Types of collisions and their effects.

• Types of force and their applications.

• What force is.

FORCES A force is a push or a pull which acts on an object. This interaction can cause the object to start moving, stop moving, speed up, slow down, change direction or change its shape. The results of this interaction are known as effects of force. Force is a vector quantity, meaning it has both magnitude (how much force) and direction.

Unit of force The standard unit of force, by the convention of International System of Units (SI), is the newton (N). One Newton is defined as the force required to accelerate a one-kilogram mass at a rate of one metre per second squared (1 N = 1 kg·m/s²).

Types of Forces

Forces can be broadly put into two main categories. These are

1. Contact forces,

2. Non-contact/field forces (or forces at a distance).

Contact Forces

Contact forces are types of force that result when the source of the force is in a physical touch with the body to which the force applies. Examples of contact forces:

1. Muscular force (Applied force): This is the force applied by the action of muscles in living beings (humans or animals). It is the force we use for everyday activities like lifting, pushing, pulling, walking, and internal bodily functions like breathing and digestion. Its daily applications include:

a. Pushing a door,

b. Pulling a cart,

c. Lifting a bag,

d. Chewing food,

e. An animal ploughing a field.

2. Normal force: It is defined as the support force by a surface on an object in contact with it, acting perpendicular to the surface. When an object rests on a surface, the surface exerts a push, upward, on the object such that the push is perpendicular to the surface. This supporting upward push is called normal force. Instances of normal force include:

a. A book resting on a table (supported by the normal force from the table).

b. A person standing on the ground (supported by the normal force from the ground).

3. Tension force: This is the force transmitted through a string, rope, cable, or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the string and pulls equally on the object on its opposite ends. Its daily applications include:

a. pulling a bucket of water from a well using a rope.

b. pulling in a tug-of-war

c. plucking stretched guitar strings.

4. Spring force: It is the restoring force exerted by a spring to return to the natural length when stretched or compressed, acting in the opposite direction of the displacement.

This is the force applied by a compressed or stretched spring to any object that is attached to it. The force acts to restore the spring to its original (equilibrium) shape.

Some of its daily applications include:

a. Pens with click mechanisms use springs to extend and retract the pen tip.

b. Door closers have springs that automatically pull doors shut after opening.

c. Spring mattresses and cushions contain coils that compress and return to shape for comfort and support.

d. Trampolines use springs to provide the bouncing action when someone jumps on them.

e. Mechanical clocks and watches rely on wound springs to store and release energy gradually.

f. Suspension systems in vehicles include springs to absorb shocks and maintain stability.

g. Wound-up toys use internal springs to store energy and create motion when released.

h. Clothespins (clothes pegs) and paper clips include springs to maintain a firm grip on objects.

i. Spring-based weighing scales measure weight based on how much the spring stretches.

j. Safety pins use springs to enable secure opening and closing mechanisms.

5. Upthrust: Upthrust is also known as buoyant force, is a vertical upward force that a fluid (a liquid or gas) applies to an object placed in it. Its daily applications can be seen in the following activities:

a. When you swim, you feel lighter in water. That is because upthrust pushes your body upward, helping you to float.

b. Boats and ships float on water because the upthrust balances their weight. Their shape helps them displace enough water to stay afloat.

c. Plastic or wooden fishing floats stay on the surface of water due to upthrust.

When a fish pulls on the line, the float may dip, showing a change in balance.

d. Submarines control upthrust by filling or emptying tanks with water or air. This helps them sink or float as needed.

e. A hot air balloon rises up the atmosphere because heated air inside it is lighter, creating greater upthrust from the air outside it than the balloon’s weight.

Figure 6.1: Contact forces Non-contact/Field forces (Forces at a distance) These are forces which act on objects which are not in physical touch with the source of the force. Examples of non-contact forces

a. Gravitational Force: This is the force of attraction between any two objects, in space, with mass. The more massive the objects and the closer they are, the stronger the gravitational force between them is. On Earth, gravitational force is experienced as force of gravity which pulls everything towards the centre or core of Earth. Daily applications of gravitational force are as follows:

i. Keeps us and objects on the ground.

ii. Responsible for the orbits of planets around the sun and the Moon around Earth.

iii. One of the best examples of gravitational force is a falling object. When you drop a book, it falls to the ground due to the gravitational force pulling it downward.

iv. The gravitational pull of the Moon and the sun on Earth’s oceans causes the rise and fall of the level of the sea. This is known as tidal movement.

Figure 6.2: Gravitational force

b. Magnetic Force: This force is applied by magnets to magnetic materials like iron, nickel, cobalt or other magnets. It can be attractive (pulling together) or repulsive (pushing apart). Magnets have two poles, North and South. Two South poles or two North poles (Like poles) near each other repel, while a North pole and a South pole (Unlike poles) near each other attract. This creates the law of magnetism which states, “like poles repel, unlike poles attract.

Daily uses of magnetic force include the following

i. Magnets help keep refrigerator and freezer doors tightly closed. A magnetic strip inside the rubber seal ensures the door stays shut, preserving cool air and saving energy.

ii. Magnets are used in the speakers, microphones, and vibration motors of smartphones and laptops. Some laptop lids also use magnets to trigger sleep mode when closed.

iii. Electromagnets are used in traditional mechanical doorbells. When pressed, the circuit activates an electromagnet that pulls a metal striker to hit a bell.

iv. Magnets are used in MRI (Magnetic Resonance Imaging) machines, which produce detailed internal body images without using X-rays.

v. Motors in electric fans and washing machines use electromagnets to convert electricity into motion.

c. Electrostatic Force: This force exists between electrically charged objects or particles. Like charges repel each other, while opposite charges attract. This force arises from the accumulation of electric charges on the surface of an object, often due to friction.

Daily applications of electrostatic force can be seen in the following situations

i. Clothes cling together due to static electricity after drying.

ii. Spray painting uses electrostatic forces for even coating.

iii. Lightning occurs due to charge buildup in clouds.

iv. Hair stands up when rubbed with certain materials due to static buildup.

v. Plastic wrap sticks to surfaces using electrostatic adhesion.

vi. Charged balloons attract small paper pieces.

vii. Electrostatic charges help in inkjet printing.

Frictional Force

Friction is a contact force that opposes the relative motion or the tendency of such motion between two surfaces in contact. It acts parallel to the surfaces.

Types of Friction

₁. Static friction: The friction that acts on objects when they are stationary and prevents them from starting to move. For example, the force that keeps a heavy box from moving when you first start to push it.

2. Kinetic friction (Dynamic friction): The friction that acts on objects when they are moving. Its magnitude is less than that of static friction. It takes the following forms:

a. Sliding Friction: Occurs when one solid surface slides over another. E.g., pushing a book across a table.

b. Rolling Friction: Occurs when an object rolls over a surface. E.g., a ball rolling, wheels of a car. Rolling friction is generally much less than sliding friction, which is why wheels are so useful.

Figure 6.3: Sliding friction Some Good Effects/Advantages of Friction ₁. Walking/Running: Friction between our shoes and the ground allows us to push off and move forward without slipping.

2. Gripping: Friction allows us to hold objects.

3. Writing: Friction between a pen/pencil and paper allows the ink/graphite to transfer.

4. Braking: Brakes in vehicles use friction to slow down or stop.

5. Lighting a matchstick: Friction generates heat to ignite the match.

6. Holding nails and screws: Friction keeps them in place in wood or walls.

7. Sharpening tools: Friction reduces the area of the cutting edge or piercing tip of tools.

8. Grinding/milling: Friction causes grains of cereals, food materials and other substances to be ground or milled.

Bad effects /disadvantages of friction ₁. Wear and tear: Friction causes rubbing surfaces to wear out overtime E.g., soles of shoes, machine parts and tires.

2. Energy loss: Friction converts useful kinetic energy into heat, reducing the efficiency of machines. For example, parts of machines and engines get hot due to friction.

3. Slows movement: We need to apply more force to overcome friction when moving on a rough surface. E.g., an untarred road or the one tarred with raw chippings.

4. Bruises: Friction causes bruises to be sustained during falls.

5. Fire: Overheating caused by friction in machines and engines may result in fires.

Key question

1. Write down your answer in your notebook.

2. In what ways do you think friction can be reduced?

Factors Affecting Friction

Two main factors influence the amount of friction between solid surfaces. These are:

1. Nature of the surfaces in contact: Rougher surfaces generally have more friction than smoother surfaces. This is due to the interlocking of irregularities on the surfaces.

2. Normal force (how hard surfaces are pressed together): The greater the force pressing the two surfaces together (the normal force), the greater the friction. This is why it’s harder to push a heavy box than a light one across the same surface.

Fluid friction/ Drag Fluid friction, also known as drag, is the force that opposes the motion of an object through a fluid (a liquid or gas).

Factors influencing fluid friction ₁. Viscosity of the fluid: Viscosity is a measure of a fluid’s resistance to flow. Thicker, more viscous fluids (like honey) cause more friction than thinner fluids (like water or air).

2. Shape of the object: Objects with a streamlined shape (pointed at both ends and broader in the middle, like fish or an airplane) experience less fluid friction.

3. Speed of the object: Fluid friction increases with the speed of the object moving through the fluid. The faster an object moves through a fluid, the greater the drag.

4. Surface area of the object: A larger surface area exposed to the fluid generally leads to greater fluid friction.

5. Nature of the fluid: Denser fluids generally exert more drag.

6. Temperature of the fluid: For gases, fluid friction generally increases with temperature.

For liquids, viscosity (and thus fluid friction) often decreases with increasing temperature.

Ways of Reducing Friction

In many situations, reducing friction is desirable to improve efficiency by reducing its bad effects. Some of the ways to reduce friction are as follows:

1. Lubrication: Applying lubricants like oil, grease, or graphite between moving parts creates a thin film that separates the surfaces, reducing direct contact and friction. This is common in engine parts, bicycle and motorbike chains.

2. Streamlining: Designing objects with smooth, tapered shapes (streamlined shapes) allows fluids (air or water) to flow past them more easily, reducing drag. Examples include the bodies of airplanes, racing cars, ships, birds and fish.

3. Using rollers/Ball bearings: Replacing sliding friction with rolling friction significantly reduces frictional force. Ball bearings are used in rotating machine parts such as wheels and motors.

4. Polishing or powdering surfaces: Making surfaces smoother reduces the irregularities that interlock, thereby decreasing friction.

5. Using appropriate materials: Some materials have naturally lower coefficients of friction (e.g., PTFE, also known as Teflon).

6. Reducing contact area (in some specific cases of fluid friction): While for solid friction, area of contact has little direct effect (it is more about normal force and surface nature), for fluid friction, minimising the frontal area can reduce drag.

Figure 6.4: A streamlined body of shark Increasing Friction Sometimes, friction is essential and needs to be increased for safety or functionality. Some of the ways by which this can be done are as follows:

1. Treads on tires and shoes: The grooves (treads) on tires and shoe soles increase friction, providing better grip on the road or ground, especially on smooth and slippery ones.

2. Brake pads: Brake pads in vehicles are made of materials that create high friction when pressed against the brake discs or drums, allowing the vehicle to slow down or stop effectively.

3. Using rougher surfaces: For example, gymnasts apply coarse powder to their hands for a better grip. Kabaddi players might rub their hands with soil.

4. Increasing the normal force: Pressing surfaces together more firmly increases friction during grinding or milling.

5. Stopping relative motion: Static friction is generally higher than kinetic friction.

Rougher surfaces are put together to keep things in place.

Daily applications of forces ₐ. Think about it

i. Why does a ball, thrown upwards, always come down?

ii. How can you pick up small pieces of paper with a comb after running it through your hair?

iii. Why is it easier to skate on ice than on a rough road?

iv. How do boats float?

b. Try this

i. Push a toy car on a smooth floor and then on a carpet. What do you observe about its motion?

ii. Drop a feather and a small stone simultaneously. Which one reaches the ground first? Why?

iii. Bring two magnets close to each other in different orientations. What happens?

iv. Rub a balloon on your sweater and try to stick it to a wall or bring it near small pieces of paper.

c. Understanding the “why”

i. Pushing/Pulling: When you push a door (muscular force), you are applying a force to move it. When you pull a drawer open (muscular force), you are applying a force to change its position.

ii. Resisting motion (Friction): The toy car stops sooner on the carpet because the frictional force between the carpet and the wheels is greater than between the smooth floor and the wheels. Friction always opposes motion.

iii. Lifting (Upthrust/Buoyancy): When an object is immersed in a fluid (like water or air), it experiences an upward force called upthrust or buoyant force. If the upthrust is equal to or greater than the object’s weight, the object floats. This is why ships made of heavy steel can float; their shape displaces a large volume of water, creating a large upthrust. Animals like fish use upthrust to control their depth in water.

iv. Magnetism in daily life: Refrigerator magnets stick due to magnetic force.

A compass needle aligns with Earth’s magnetic field to show direction.

The concept map of forces illustrated below will give you insight into types of force, their examples and applications. Study it carefully to address the key questions above.

Figure 6.5: Concept map of forces Upthrust Upthrust determines whether an object will float or sink in a fluid. To understand this concept, we need to perform the following activity hands-on. Do this in your mixed ability groups.

Activity 6.2 Investigating effects of upthrust on objects Aim: To observe and explain how upthrust determines whether an object will float or sink in a fluid.

What you need

• A clear container or bucket with water

• Small plastic bottle with cap

• Stone or metal spoon

• Wooden block

• Rubber ball

• Small spring balance (optional)

• String What to do

1. Look at each object and predict whether it will float or sink. Write your predictions down.

2. Drop each object into the container of water one at a time.

3. Observe which objects sink, which float, and which stay in between.

4. Discuss the results as a group.

5. Hold an object in the air, then slowly lower it into the water while holding it.

6. Record your observation.

Key question: Which type of force could be pushing the object up?

Viscosity Viscosity is the measure of a fluid’s resistance to flow. This force affects how easily fluids flow.

Activity 6.3 Investigating viscosity Aim: To determine the use of various liquids based on their viscosity.

What you need

• A small ramp (e.g., a smooth board or tray tilted with books)

• Timer

• Measuring cup

• Marker

• Liquids to test (e.g., water, cooking oil, shampoo, liquid soap, honey)

• Spoons or small cups for pouring What to do

1. Mark a start and finish line on the ramp.

2. Pour a spoonful of each liquid at the top of the ramp and start the timer.

3. Time how long it takes each liquid to reach the bottom.

4. Record the time for each liquid.

5. Compare results and rank the liquids from fastest to slowest.

Activity 6.4 Exploring methods of reducing friction Aim: To explore methods that reduce friction if it is a nuisance.

What you need

• Various small, identical objects. E.g., wooden blocks

• A standard surface, such as a floor or table

• Sandpaper

• A smooth surface such as a polished tile or Formica

• Fine powder

• Straws or rounded pieces of stick

• Lubricating oil

• Ramp What to do

1. Create a ramp.

2. Slide the various small objects down the ramp

3. Measure how far an object slides off the ramp onto the standard surface.

4. Modify the roughness and smoothness of the surface.

a. Place a sheet of sandpaper on the sliding surface (increases friction).

b. Replace the sandpaper with the smooth surface.

c. Sprinkle some fine powder (like talcum powder) on the surface.

d. Place a few straws or round pieces of stick under the object to act as rollers.

e. If safe and appropriate, apply a tiny amount of oil to the surface (demonstrating lubrication, with caution and proper cleanup).

5. Measure how far the object slides with each modification.

Key questions Discuss your findings under the following captions:

1. Which methods were most effective in reducing friction?

2. How do these methods relate to real-world applications, e.g., oil in engines, ball bearings in wheels, polishing floors, etc?

3. How do you think friction can be reduced most effectively?

Collisions In physics, a collision is a dynamic event where two or more objects exert forces on each other over a relatively short period. Collisions can be broadly classified into two main categories. These are:

1. Elastic collisions

2. Inelastic collisions The key difference between them lies in the conservation of kinetic energy.

1. Elastic collisions An elastic collision is when objects collide and bounce off without loss of energy. These are collisions in which the colliding particles are not permanently deformed, and the total kinetic energy of the system remains constant. This means that the sum of the kinetic energies of the colliding objects before collision is equal to the sum of their kinetic energies after collision. In an ideal elastic collision, there is no loss of kinetic energy to other forms such as heat, sound and energy forms which may cause deformation.

Characteristics of Elastic Collisions

ₐ. Momentum is conserved: The total momentum of the system remains the same before and after the collision.

b. Kinetic energy is conserved: The total kinetic energy of the system before the collision is equal to the total kinetic energy after the collision.

c. Objects separate/Reversible: The colliding bodies bounce off each other and remain as distinct objects.

Real-world examples of elastic collisions While perfectly elastic collisions are ideal and do not truly occur in our everyday life experiences, some scenarios come very close.

Examples include

1. Steel ball bearing: The collision between two steel ball bearings is a classic example that approximates an elastic collision.

2. Atomic and subatomic particles: Collisions between atoms and subatomic particles, such as those in a gas, are considered to be almost perfectly elastic.

3. Newton’s cradle: The interaction between the swinging balls in a Newton’s cradle demonstrates the principles of an elastic collision.

Let us perform the following activity to demonstrate the understanding of Newton’s cradle.

Activity 6.5 Demonstrating Newton’s cradle Aim: To explore the effect of an elastic collision.

What you need

• Five small metal balls (e.g., marbles, ball bearings, or even round beads)

• Strong string or thread

• A stiff cardboard box, wooden frame, or hanger (as a support)

• Glue or tape

• Ruler and scissors What to do

1. Make 5 holes in the top of the frame or hanger, evenly spaced.

2. Tie strings around each ball, making sure each hangs at the same height and barely touches the next.

3. Suspend each ball from the frame so they are in a straight line and just touching each other.

4. Pull one of the balls at the end of the row to the side and let it go (as shown in the diagram below).

5. Record your observation(s).

Figure 6.6: A set-up of Newton’s cradle

For a one-dimensional elastic collision between two objects with masses m1 and m2, and initial velocities u1 and u2, their final velocities v1 and v2 can be determined using the conservation of momentum and kinetic energy equations as follows:

1. Conservation of momentum

a. m₁u₁+ m₂ u₂= m₁ v₁+ m₂ v₂

2. Conservation of Kinetic Energy

a. 1 2m₁u₁ ²+ 1 2m₂ u₂ ²= 1 2m₁ v₁ ²+ 1 2m₂ v₂ ²Worked examples 6.1. A 5 kg object is moving at a velocity of 3 m/s. What is its momentum?

Solution

Mass = 5kg, v= 3 m/s p = m × v = 5 kg × 3 m/s = 15 kgm/s 6.2. Two toy cars, one with a mass of 1.2 kg moving at 2 m/s, and the other with a mass of 1.4 kg at rest, collide elastically. Calculate their total momentum.

a. before collision

b. after collision

Solution is in Annex 6.0.

Inelastic Collisions

In contrast to elastic collisions, an inelastic collision is one where objects collide and the total kinetic energy of the system is not conserved. While momentum is still conserved in an inelastic collision, some of the initial kinetic energy is transformed into other forms of energy, such as thermal energy (heat), sound energy, and the energy needed for a permanent deformation of the colliding objects.

Characteristics of inelastic collisions ₁. The total momentum of the system is conserved.

2. The total kinetic energy of the system decreases during the collision.

3. The collision process is not reversible due to the energy lost to other forms.

Types of inelastic collisions ₁. Inelastic collision: The objects may or may not stick together after the collision.

2. Perfectly inelastic collision: This is a specific type of inelastic collision where the objects stick together after impact and move with a common final velocity. This results in the maximum possible loss of kinetic energy with no loss in momentum.

Real-world examples of inelastic collision Most collisions in the world are inelastic. Examples include

1. Car accidents: A significant amount of kinetic energy is converted into the crumpling of the car bodies, sound, and heat.

2. A bullet embedding in a target: When a bullet is fired into a block of wood and becomes embedded, it’s a perfectly inelastic collision.

3. A ball of clay dropped on the floor: The clay deforms and does not bounce, indicating loss of kinetic energy.

Mathematical representation: For a perfectly inelastic collision of two bodies with masses m1 and m2, initial velocities u1 and u2, where the bodies stick together and move with a common final velocity, v;

m1u1 + m2u2 = (m1 + m2) v

Figure 6.7: Elastic and inelastic collisions compared.

Worked example 6.3

Two cars, Car A with a mass of 1000 kg moving at 20 m/s, and Car B with a mass of 1500 kg moving at 10 m/s in the same direction, collide and stick together. Find their combined velocity after the collision. Solution is in Annex 6.0.

Activity 6.6 Modelling and Analysing Collisions

Aim: To investigate whether momentum is conserved in collisions between toy cars.

Hypothesis: The total momentum of the system (cars before collision) will equal the total momentum after collision, within experimental error.

Materials

• Two small toy cars (ideally with similar wheels to reduce friction differences)

• A smooth, level surface or track

• Measuring tape or ruler

• Stopwatch or motion-tracking app/video analysis software

• Mass balance (to measure car masses) Method

1. Measure the mass of each toy car.

2. Place the cars on a smooth track.

3. Push one car towards the other (initially at rest) to create a collision.

4. Record velocities before and after collision using a stopwatch and distance marks, or by analysing video footage.

5. Repeat for different initial velocities and car mass combinations (e.g., equal masses, unequal masses).

Data Collection

a. Mass of each car

b. Initial velocity of moving car(s)

c. Final velocity of both cars after collision Data Analysis

a. Calculate initial momentum.

b. Calculate final momentum.

c. Compare before and after values to check for conservation (consider percentage difference).

Conclusion

a. State whether momentum was conserved in the collisions.

b. State whether this collision was elastic or inelastic, and how you know.

c. Discuss experimental errors (e.g., friction, air resistance, timing inaccuracies).

Analysis Complete the following tasks in your notebook

a. Explain how this experiment using toy cars can be used to predict the outcome of a collision between real cars on the road.

b. List precautions that people can take to avoid collisions. If time is available, turn this into an advisory poster and display it for a gallery walk.

1. Explain how a car’s brakes use friction to stop the car.

2. Compare and contrast elastic and inelastic collisions using real-world examples, and explain what happens to the kinetic energy in each type of collision