Question 8
(a) Define uniform acceleration
(b) Forces act on a car in motion. List the:
(i) horizontal forces and their directions;
(ii) vertical forces and their directions.
(c) A car starts from rest and accelerates uniformly for 20s to attain a speed of 25 ms-1. It maintains this speed for 30s before decelerating uniformly to rest. The total time for the journey is 60s.
(i) Sketch a velocity- time graph for the motion.
(ii) Use the graph to determine the:
(a) total distance travelled by the car;
(b) deceleration of the car.
(d)
Observation
The figure above illustrates force - extension graph for a stretched spiral spring. Determine the work done on the spring.
Part (a) This was one of the popular choice of the candidates but the recorded performance was fair. Many candidates supplied the correct definition but others failed in providing key words like increase velocity. Some candidates used ‘rate’ and ‘change’ respectively to score zero.
Part (b) Many candidates attempted this question and supplied the correct response. In view of the question, correct force and wrong direction will only attract [½ mark]. Also, ‘Reaction’ is not accepted without ‘Normal’. Performance was average.
Part (c) Many candidates drew the velocity-time graph correctly and determined the total distance travelled but some were not able to give correct calculation of deceleration. Performance was average.
Part (d) In this question, area under the Force-Extension graph is required. This amount to area of the triangle. Majority of the candidates failed to convert 0.53 cm to metre correctly. Performance was below average.
The expected answer is:
- Uniform acceleration is the constant time rate of increase in velocity.
(b) (i) Horizontal Force
Thrust: forward
Friction: backward
(ii) Vertical Force
Weight of car: downward
Normal Reaction: upward force
