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Physics Form 4 Chapter 2 Force and Motion I

2.5 Momentum Solution
(a) Momentum of the car = 800 × 20
= 16 000 kg m s –1

Momentum of the lorry = 5 000 × 20
= 100 000 kg m s –1

(b) The lorry, due to its bigger momentum.
Figure 2.55
1. Every moving object has momentum. However, Principle of Conservation of
the momentum of a timber lorry is very much Momentum
bigger than that of a cyclist, despite both Chapter
vehicles moving at the same speed. 1. According to the principle of conservation
of momentum, when two or more bodies act
2. The linear momentum of an object is defined on each other, their total momentum remains 2
as the product of its mass and velocity constant, provided that there is no external
force acting on them.
Momentum = Mass × Velocity
SPM Tips
Elastic Collision vs Inelastic Collision
3. The SI unit for momentum is kg m s or N s. Q: Elastic and inelastic collisions are two very common
–1
types of collision. How is momentum related to
Momentum is a vector quantity. The direction these two types of collision and what are the
of momentum is the same as the direction of differences between both types of collision?
velocity.

EXAMPLE 2.13
The total mass of a lorry is 20 000 kg and the total
mass of a car is 2000 kg.
If both the lorry and the car are travelling at a
velocity of 25 m s , calculate the momentum of
–1
the lorry and the car respectively. (a) Collision between electric bumper cars in an elastic collision
Solution
Momentum of the lorry = 20 000 × 25
= 5 × 10 kg m s –1
5
Momentum of the car = 2 000 × 25
= 5 × 10 kg m s –1 (b) Collision between two cars is an inelastic collision
4
Figure 2.56

EXAMPLE 2.14 A: • In an elastic collision, the total kinetic energy of
the system is conserved. The objects elastically
A car has a mass of 800 kg and a lorry has a mass spring back to their original shapes and the
system retains all its original kinetic energy.
of 5000 kg. • In an inelastic collision, the total kinetic energy
(a) Compare the momentum of both vehicles when is not conserved . One or more of the colliding
moving at a velocity of 20 m s . objects may not spring back to its original shape
–1
(b) If both vehicles move at the same velocity, or heat or sound may be generated. The total
which vehicle will cause greater destruction kinetic energy of the system after the collision
when hitting a lamp post? is less than that before the collision.

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