O-LevelPhysicsKinematics5054/22 · May/June 2017 · Q17 Marks

A car accelerates from rest in a straight line. During the first 14s, the acceleration is uniform and the car reaches a speed of 25 m/s. [Figure 1.1]

(a)(i)Calculate the acceleration of the car.[2]
(a)(ii)After the first 14s, the speed of the car continues to increase but the acceleration decreases. From 70s to 80s after the start, the car moves at a constant speed of 55 m/s. On Fig. 1.1, draw a possible speed-time graph for the car.[2]
(b)At a later time, the driver applies the brakes to stop. He is wearing a seat belt and slows down in his seat. A bag on the seat next to him slides forwards, across the seat towards the front of the car. Using ideas about the forces acting, explain why the driver slows down but the bag slides forwards.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] (a=) v(-u) / t or 25/14
[1] 1.8 m/s²
[1] initial straight line from (0,0) to (14,25)
[1] gradient of line decreases after 14 s and flat from (70,55) to (80,55)
[1] force backwards on driver/car
[1] force produced by seat belt / steering wheel or friction with seat / friction between tyres and road
[1] no/small (backward) force / friction on bag
[1] (mass of bag) resists change (from state of motion) or carries on in straight line or has constant velocity or (bag) has inertia

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