Topic 4 - Dynamics

Question 1

Momentum

Answer 1

• Product of mass and velocity

- Unit : kgms-² / Ns

Question 2

Newton’s First Law

Answer 2

A body will remain its state of rest or uniform velocity unless an external force acts on it.

Question 3

Inertia

Answer 3

Property of a body to resist changes to its initial state of motion.

Question 4

Newton’s Second Law

Answer 4

Rate of change of momentum is directly proportional to the net force acting on it and takes place in the direction of force.

F = ma

Question 5

Newton’s Third Law

Answer 5

When two bodies interact, they exert equal magnitude of forces which acts in opposite direction on one another.

Question 6

Weight

Answer 6

Force of gravity acting in the body

Question 7

Principle of Conservation of Momentum

Answer 7

In a closed system, the total momentum remains constant if no external forces act on it.

Question 8

Explain how momentum is conserved.

Answer 8

• When two bodies collide, there’s a change in momentum. (N2L)
• Hence, this give rise to force.
• Force exerted on the bodies are equal in magnitude and opposite in direction. (N3L)
• Lost in momentum of body A is equal to the gain of momentum of body B.
• Total change in momentum is zero.
• Momentum is conserved.

Question 9

Elastic collision

Answer 9

• Total kinetic energy is conserved.
• Relative speed of approach = relative speed of separation

u1 - u2 = v2 - v1

Question 10

Inelastic collision

Answer 10

Total kinetic energy is not conserved.
- lost in the form of other energy (internal energy)

• Inelastic (separates)
• Perfectly inelastic (moves together after collision)

Question 11

When does the momentum is not conserved?

Answer 11

• There’s an external force acting.

• Friction force
• Gravitational force

• Not a closed system.

Question 12

Example :

A ball was released from a certain height. The ball then bounces off when it hits the ground. Is the momentum conserved?

Answer 12

Collision : between ball and ground

For PCM, the ground must gain momentum as the ball loses momentum.

• change in momentum if the ground = change in momentum of the ball.
• Ground exerts a reaction force.
• momentum is conserved.

Question 13

Example :

An object is undergoing free fall. Is the momentum conserved?

Answer 13

Answer 1 :

• Momentum is conserved.
• Lose in momentum of the ball = Gain in momentum of the ground.
• Acts in opposite direction.
• Total momentum = 0

Answer 2 :

• Momentum is not conserved.
• Presence of external force, gravitational force.
• Technically, the ball does not collide with other bodies.
• Momentum is not conserved.