Laws of Motion
ICSE Class 9 · Physics · Chapter 3
What this chapter covers
- Force: contact and non-contact forces
- Newton's first law of motion
- Linear momentum
- Newton's second law of motion
- Newton's third law of motion
- Universal law of gravitation
- Gravity and free fall
- Mass and weight
Key points
- A force changes or tends to change the state of rest or uniform motion of a body, or its shape.
- Contact forces (friction, normal reaction, tension) need touching; non-contact forces (gravitational, electric, magnetic) act at a distance and follow the inverse square relation.
- First law: a body keeps its state of rest or uniform straight-line motion unless an external unbalanced force acts. Inertia is the property behind it, and mass measures inertia.
- Static inertia (inertia of rest) and dynamic inertia (inertia of motion) explain everyday events such as passengers jerking in a bus and dust leaving a beaten carpet.
- Linear momentum p = mv is a vector with SI unit kg m s-1.
- Second law: F = Δ p / Δ t = ma. 1 N = 1 kg m s-2 = 105 dyne.
- Increasing the stopping time reduces the force: catching with hands drawn back, sand pits, air bags, packing material.
- Third law: action and reaction are equal, opposite, simultaneous, and act on different bodies, so they never cancel.
- F = Gm1 m2 / d2, with G = 6.67 × 10-11 N m2 kg-2.
- In free fall all bodies have the same acceleration g ≈ 9.8 m s-2, whatever their mass.
- Weight W = mg is a force in newtons and varies with g; mass in kilograms is constant.
- 1 kgf = 9.8 N and 1 gf = 980 dyne are gravitational units of force.
Questions answered
Distinguish between contact and non-contact forces, with two examples of each.
A contact force acts only when bodies touch (friction, tension); a non-contact force acts across a distance (gravitational, magnetic).
If the distance between two charged bodies is tripled, what happens to the electric force between them?
By the inverse square relation, it becomes one-ninth of its earlier value.
State Newton's first law of motion. Why is it called the law of inertia?
A body continues in its state of rest or of uniform motion in a straight line unless an external unbalanced force acts on it. It is called the law of inertia because it describes the property of a body to resist any change in its state.
Why do passengers fall backwards when a bus starts suddenly? Name the type of inertia involved.
Their feet move forward with the bus, but the upper body tends to remain at rest. This is static inertia (inertia of rest).
Define linear momentum and state its SI unit.
Linear momentum is the product of the mass and velocity of a body, p = mv; its SI unit is kg m s-1.
State Newton's second law and show that it leads to F = ma.
The rate of change of momentum is directly proportional to the applied force and takes place in its direction. For constant mass, Δ p / Δ t = m(v - u)/t = ma, so F = ma with a suitable choice of unit.
Define one newton. How many dyne make one newton?
One newton is the force that produces an acceleration of 1 m s-2 in a mass of 1 kg. 1 N = 105 dyne.
Why does a cricketer draw his hands back while catching a fast ball?
Drawing the hands back increases the time in which the ball's momentum becomes zero. Since force equals the rate of change of momentum, the force on his hands is reduced.
Past board exam questions
ICSE 2019 sample paper · 2 marks
A spring balance is used to find the weight of a body X on the surface of the moon. The mass of the body X is 2 kg and its weight is recorded as 3.4 N. The weight of another body Y recorded by the same balance is found to be 7.65 N. Calculate the mass of the body Y.
Show answer
Acceleration due to gravity on the moon: gm = (W)/(m) = (3.4)/(2) = 1.7 N kg⁻¹ (m s⁻²).
Mass of Y = (7.65)/(1.7) = 4.5 kg.
ICSE 2019 sample paper · 2 marks
State two differences between mass and weight.
Show answer
1. Mass is the quantity of matter contained in a body; weight is the force with which the earth attracts the body.
2. Mass is a scalar quantity and remains the same everywhere; weight is a vector quantity and changes from place to place (it depends on g).
(Also: mass is measured in kg with a beam balance; weight is measured in N or kgf with a spring balance.)
ICSE 2019 sample paper · 2 marks
Why do we need to run a certain distance before getting into a moving bus?
Show answer
Because of inertia of motion. If we step onto a moving bus from rest, our feet come into motion with the bus while the upper body tends to remain at rest, so we fall backwards. By running alongside the bus we acquire a velocity close to that of the bus, so on boarding there is little change in our state of motion and we do not lose balance.
ICSE 2019 sample paper · 2 marks
A bob hangs at rest from a rigid support by a vertical string. Copy this arrangement and clearly mark the directions of the forces that act on it and name the forces.
Show answer
Two forces act on the bob:
1. Tension (T) in the string, acting vertically upwards along the string (towards the support).
2. Weight (mg), the force of gravity, acting vertically downwards from the bob.
Since the bob is at rest, the two forces are equal and opposite.
ICSE 2019 sample paper · 3 marks
(i) State the universal law of gravitation. (ii) Express it in a mathematical form. (Explain the symbols used.) (iii) State the value of universal gravitation constant in S.I. unit.
Show answer
(i) Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them; the force acts along the line joining them.
(ii) F = G(m1 m2)/(r2), where F is the gravitational force, m1 and m2 are the masses of the two bodies, r is the distance between their centres and G is the universal gravitational constant.
(iii) G = 6.67 × 10⁻¹¹ N m² kg⁻².