  Physics Paper 2, May/June 2012
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
Weakness/Remedies
Strength

Question 11

1. State the principle of conservation of linear momentum.
2. Explain the mode of action of a propelled rocket
3. During a training session, two footballers pass a ball repeatedly between themselves. Give two reasons why the to and fro motion of the ball is not simple harmonic.
4. A ball is dropped from a height, at the same time as another ball is projected horizontally from the same height.
1. would the balls hit the ground at the same time?
5. A ball of mass 0.10kg is projected horizontally onto a vertical wall with a speed of 17ms-1. The ball makes contect with the wall for 0.15s and rebounds horizontally with a speed of 13ms-1. Calculate the:
1. change in momentum of the ball;
2. Average force exerted on the ball during its collision with the wall.
_____________________________________________________________________________________________________

Part (a) Most candidates in their definition forgot to include the key word such as closed system, total momentum or absence of external forces. This made them to lose marks
. Performance was average.
Part (b) Most candidates knew that rocket propulsion was due to Newton’s third law of
motion but they could not give details
Part (c) many candidate* did m* have anything to write due to poor comprehension of the demand of the question. Performance was poor.
Part (d) most candidates knew the balls would hit the ground at the same time but could not explain why due to poor understanding of the concept of  free falling objects
Part (e) This was very popular among the candidates and majority of them -calculated correctly. However, few candidates got it wrong due to incorrect substitution as they failed to note that momentum is a vector quantity hence reversal in direction connote a negative sign.

The expected answers are as follow:

1. Principle of conservation of linear.
2. State that in a system of colliding objects the total momentum is always conserved provided that there is no net external force acting on the system
OR
If two or more bodies collide in a closed system, the total momentum after the collision is equal to the total momentum before the collision

3. Principle of action of a propelled rocket

Gases burnt within the combustion chambers of the rocket engine are expelled downwards through the tail nozzle (exhaust) at very high speed. An equal and opposite momentum is given to the rocket which then moves upward.

c. - The acceleration of the ball is not directed towards a fixed point.
- These is no equilibrium position
-  Acceleration of the ball is not directly proportional

d. Ball projection

(i)  Yes or Both balls hit the ground at the same time
(ii) All objects are pulled towards the Earth by gravity and accelerate at a constant rate. This means that all free fall mg objects fails at the same rate.

e. Momentum Change P

1. P - m(v - u)

= 0.10 (13 +17)
= 3kgms-1

1. Average force on the ball

F= m(v-u)
t
=        Δp
Δt
=     3
0.15
= 20N