Force

1. Force and its effects

A force can:

• make a stationary object move;

• increase or decrease the speed of a moving object;

• stop a moving object;

• change the direction of motion;

• change the shape of an object;

• produce several of these effects together.

Important: An object at rest may still have forces acting on it. In that case, the forces balance one another.

2. Types of forces

A. Muscular force

• Force produced by the action of muscles.

• Used while walking, lifting, pushing, running, jumping and chewing.

• Animals also use muscular force for movement and work.

• It is a contact force.

B. Frictional force

• Friction acts when an object moves or tries to move over another surface.

• It always acts opposite to the direction of motion or attempted motion.

• It is caused by tiny irregularities on the surfaces that lock together.

• Rough surfaces produce more friction than smooth surfaces.

• Air and water also exert friction on moving objects.

• Aeroplanes, ships and high-speed trains are given streamlined shapes to reduce fluid friction.

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Remember: More roughness → more friction → an object stops sooner.

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C. Magnetic force

• Force exerted by a magnet on another magnet or magnetic material.

• Like poles repel; unlike poles attract.

• It can act from a distance, so it is a non-contact force.

D. Electrostatic force

• Rubbing certain materials produces static electrical charges.

• The two types of charge are positive and negative.

• Like charges repel; unlike charges attract.

• A charged body can also attract some uncharged objects, such as small pieces of paper.

• Electrostatic force is a non-contact force.

E. Gravitational force

• Force by which the Earth attracts objects towards itself.

• Also called gravity or force of gravity.

• It acts vertically downwards near the Earth.

• It is a non-contact force and is always attractive.

• A ball thrown upwards slows down, stops momentarily and then accelerates downwards because gravity continues to act on it.

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Value of Gravitational constant =

6.674 × 10⁻¹¹ N·m²/kg²

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Value of acceleration due to gravity = 9.8 m/s

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3. Mass and weight

Examples for a 1 kg object:

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Saying “the weight of a bag is 10 kg” is common in everyday speech but scientifically incorrect. Kilogram measures mass; newton measures weight.

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4. Spring balance

• A spring balance measures weight (force) in newtons.

• Hanging an object stretches its spring; a heavier object causes a greater stretch.

• Check the instrument’s range before use.

• Smallest division = difference between two consecutive major readings ÷ number of small divisions.

• Example: if 1 N is divided into 5 parts, each small division represents 0.2 N.

• Never hang an object heavier than the maximum reading of the balance.

5. Floating and sinking

• A liquid exerts an upward force on an immersed object called upthrust or buoyant force.

• Two main forces act on an object in a liquid:

  • Weight/gravity: downward

  • Buoyant force: upward

• If weight is greater than buoyant force, the object sinks.

• If weight equals buoyant force, the object floats.

• Buoyant force depends partly on the density of the liquid.

Archimedes’ Principle

An object fully or partly immersed in a liquid experiences an upward force equal to the weight of the liquid displaced.

• Displaced liquid’s weight < object’s weight → object sinks.

• Displaced liquid’s weight = object’s weight → object floats.

Example: Pumice floats because it contains air pockets and is less dense than water.

6. One-minute revision

• Force = push or pull due to interaction.

• SI unit of force and weight = newton (N).

• Force can change motion, speed, direction or shape.

• Contact forces: muscular and frictional.

• Non-contact forces: magnetic, electrostatic and gravitational.

• Friction opposes motion and is greater on rough surfaces.

• Like magnetic poles/charges repel; unlike poles/charges attract.

• Gravity always attracts objects towards the Earth.

• Mass stays constant; weight can change.

• Liquids exert upward buoyant force.

7. Important exam questions

1. Define force and state its SI unit.

2. List four effects of force with examples.

3. Differentiate between contact and non-contact forces.

4. What is friction? State its direction and cause.

5. Explain why an object stops sooner on a rough surface.

6. Differentiate between mass and weight.

7. Explain the working of a spring balance and how to find its smallest division.

8. What is buoyant force? When does an object float or sink?

9. State Archimedes’ Principle.

10. Explain the motion of a ball thrown vertically upwards.

8. Common mistakes to avoid

• Do not write kg as the SI unit of weight; it is N.

• Do not say friction always stops motion instantly; it opposes motion.

• Do not assume an object at rest has no forces acting on it.

• Do not say gravity acts only on falling objects; it acts even at the topmost point of a throw.

• Do not confuse buoyant force (upward) with weight (downward).

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Ball activity =

While going upwards, the ball’s speed decreases because gravity acts downwards. At the topmost point, its speed becomes zero momentarily. While falling, its speed increases due to gravity.

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