Page 47 - Physics Coursebook 2015 (A level)

Chapter 2: Accelerated motion
9 A car driver is travelling at speed v on a straight road. He comes over the top of a hill to find a fallen tree on the road ahead. He immediately brakes hard but travels a distance of 60 m at speed v before the brakes are applied. The skid marks left on the road by the wheels of the car are of length 140 m (Figure 2.37).
The police investigate whether the driver was speeding and establish that the car decelerates at 2.0 m s−2 during the skid.
top of hill
skid marks
60 m 140 m Figure 2.37 For End-of-chapter Question 9.
a Determine the initial speed v of the car before the brakes are applied. [2]
b Determine the time taken between the driver coming over the top of the hill and applying the brakes.
Suggest whether this shows whether the driver was alert to the danger. [2]
c The speed limit on the road is 100 km/h. Determine whether the driver was breaking the speed limit. [2]
10 A hot-air balloon rises vertically. At time t = 0, a ball is released from the balloon. Figure 2.38 shows the variation of the ball’s velocity v with t. The ball hits the ground at t = 4.1 s.
20 v/ms–1
10 0 –10
–20
1234
t /s
Figure 2.38 For End-of-chapter Question 10.
a Explainhowthegraphshowsthattheaccelerationoftheballisconstant. [1] b Usethegraphto:
i determine the time at which the ball reaches its highest point [1]
ii show that the ball rises for a further 12 m between release and its highest point [2]
iii determinethedistancebetweenthehighestpointreachedbytheballandtheground. [2]
c Theequationrelatingvandtisv=15−9.81t.Explainthesignificanceintheequationof:
i the number 15 [1]
ii the negative sign. [1]
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