How Forces Affect Motion Class 9: Notes and Questions

How Forces Affect Motion is chapter 6 of the new Class 9 Science textbook. This guide walks you through the chapter in order: what a force is, balanced and unbalanced forces, friction, Newton's three laws, momentum in daily life and systems of connected objects. Each idea comes with a short worked example, and you will find important questions with model answers at the end.

What is a force?

A force is a push or a pull. It can:

• make an object at rest start moving (kicking a ball),
• change the speed or direction of a moving object (hitting a cricket ball with a bat),
• change the shape of an object (squeezing a lemon).

Force is a vector quantity: you need both its magnitude (strength) and its direction to describe it. Gravity pulls objects down, while buoyancy in water pushes up. Change either the size or the direction of a force and its effect changes.

The SI unit of force is the newton (N). We can measure a force with a spring balance, where the stretch of a spring shows the force. To get a feel for it: 1 N is roughly the upward force you apply to hold a 100 g mass steady in your palm.

Balanced and unbalanced forces

When several forces act together, what matters is the net force, their vector sum.

• Balanced forces: equal in size and exactly opposite in direction. Net force is zero. An object at rest stays at rest; a moving object keeps moving at constant velocity.
• Unbalanced forces: net force is not zero. The object accelerates in the direction of the larger force. In a tug-of-war, the rope moves towards the team pulling harder.

Finding net force along one line:

• Same direction: add. 10 N right + 6 N right = 16 N to the right.
• Opposite directions: subtract the smaller from the larger. 10 N right and 6 N left give 10 N - 6 N = 4 N to the right.

Without friction, a moving object needs no extra force to keep moving at constant velocity. To change its velocity or stop it, you need an unbalanced force.

Friction

Friction is a contact force between two surfaces. It always acts opposite to the relative motion.

• A heavy box may not move when you first push it, because friction opposes your push. It moves only when your push is greater than the maximum friction.
• Smooth surfaces such as polished marble give less friction than rough ones such as a cemented floor. A stack of coins pushed the same way travels further on a smooth table than on a wooden one.
• To keep something moving at constant velocity, you must keep pushing to balance friction. Stop pushing, and friction slowly brings it to rest.
• Friction is also helpful: it gives us grip to walk without slipping. On a wet polished floor, friction is very low, so your foot slips instead of gripping.

Newton's first law: the law of inertia

An object at rest stays at rest, and an object in motion keeps moving with constant velocity (same speed, same straight-line direction), unless a net force acts on it.

Inertia is an object's natural tendency to resist any change in its state of rest or motion. Galileo Galilei first argued that if friction and other hindrances were removed, a body moving on a horizontal plane would keep moving forever. Newton later turned this into his first law.

So if something moves in a straight line at steady speed, the net force on it is zero.

Newton's second law: F = ma

The acceleration of an object is directly proportional to the net force and inversely proportional to its mass, and it is in the direction of the net force:

F = m × a

1 N is the force that gives a 1 kg mass an acceleration of 1 m s⁻². The unit is named after Isaac Newton; the word "newton" is written in lowercase, but the symbol is a capital N.

For the same mass, more force means more acceleration. For the same force, more mass means less acceleration. That is why an empty cart is easier to push than a loaded one.

Weight: near Earth's surface every object falls with acceleration g, about 9.8 m s⁻² (often rounded to 10 m s⁻²). The gravitational force on mass m is its weight, F = mg. The value of g does not depend on the object's mass.

Worked example 1: Force to accelerate a 25 kg block at 0.2 m s⁻². F = 25 kg × 0.2 m s⁻² = 5 N.

Worked example 2: A 1500 kg car goes from 0 to 10 m s⁻¹ in 5 s. a = (v - u) / t = (10 m s⁻¹ - 0) / 5 s = 2 m s⁻². F = 1500 kg × 2 m s⁻² = 3000 N.

Momentum in daily life

Momentum is mass × velocity: p = mv, the "quantity of motion". The second law can also be stated as: the rate of change of momentum is proportional to the net force. So if the same change in momentum happens over a longer time, the force is smaller.

• A cricket fielder pulls the hands back while catching a fast ball. This increases the time taken to stop the ball, so the force on the hands is smaller.
• A car's airbag gives a soft cushion that increases the time of impact, reducing the force on the passenger.
• Glass items are packed in straw or bubble wrap so that jolts take longer, reducing the force on the glass.

Newton's third law: action and reaction

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first at the same moment.

The key point: action and reaction act on two different objects, so they never cancel each other.

• Walking: your feet push the ground backwards; the ground pushes you forwards.
• Rocket: the engine pushes hot gases down at high speed; the gases push the rocket up. The Vikram lander of Chandrayaan-3 used this idea, firing its engines in the direction of motion to slow down for a soft landing.
• Gun recoil: the gun pushes the bullet forward; the bullet pushes the gun backward.

Systems of connected objects

Two boxes tied by a string can be treated as one system with total mass m1 + m2. The string's tension is an internal force, so you ignore it when finding the system's acceleration. Only the external force counts:

a = F / (m1 + m2)

Example: blocks of 2 kg and 3 kg are pulled by 10 N. Total mass = 5 kg, so a = 10 N / 5 kg = 2 m s⁻².

Remember this

• Force: push or pull, a vector, unit newton (N).
• Balanced forces: net force zero, no change in velocity.
• Friction always opposes relative motion.
• First law: no net force means no change in velocity (inertia).
• Second law: F = ma; weight = mg.
• Momentum p = mv; more time means less force for the same change.
• Third law: equal and opposite forces on two different objects.

Important questions with answers

1. Why is force called a vector quantity? Because it has both magnitude and direction, and its effect depends on the direction in which it acts.

2. The rope in a tug-of-war does not move. What does this tell you? Both teams pull with equal and opposite forces. The forces are balanced and the net force is zero.

3. Forces of 10 N to the right and 6 N to the left act on a block. Find the net force. 10 N - 6 N = 4 N to the right.

4. An object moves with constant velocity. Is there a net force on it? No. By the first law, constant velocity means the net force is zero.

5. Find the force needed to accelerate a 2 kg mass at 5 m s⁻². F = 2 kg × 5 m s⁻² = 10 N.

6. What force accelerates a 4 kg trolley at 3 m s⁻²? F = 4 kg × 3 m s⁻² = 12 N.

7. Why does a fielder pull back the hands while catching? It increases the time to stop the ball, lowering the rate of change of momentum and so the force on the hands.

8. If action and reaction are equal and opposite, why don't they cancel? They act on two different objects. Forces cancel only when they act on the same object.

9. Blocks of 2 kg and 3 kg tied together are pulled by 10 N. Find the acceleration. a = 10 N / (2 kg + 3 kg) = 2 m s⁻².

Common mistakes to avoid

• Thinking a moving object always needs a force. Without friction, constant velocity needs no net force.
• Cancelling action and reaction. They act on different bodies.
• Adding forces that point in opposite directions. Subtract them instead.
• Including the string's tension when finding a system's acceleration.
• Writing the unit as "Newton" or "n". The word is newton; the symbol is N.

Want to practise more questions on forces step by step? Study this chapter with Joy on Learnijoy.