Fig. 1 shows a safety test of a bicycle helmet on a dummy head. The head is dropped onto a pad and comes to rest in a short time. A high-speed camera provid...

Fig. 1 shows a safety test of a bicycle helmet on a dummy head. The head is dropped onto a pad and comes to rest in a short time. A high-speed camera provides the velocity before impact and the stopping time. The helmet increases the time taken for the head to stop. The mass of the dummy head is kept the same in every test.

(a) What happens to the velocity of the head during the impact? [1]
(b) Which quantity is reduced when the helmet makes the stopping time longer: force on the head or mass of the head? [1]
(c) What happens to the acceleration magnitude if the same change in velocity occurs in a longer time? [1]
(d) Sketch an arrow to show the direction of the contact force from the pad on the head. [1]

The motion of a marble is filmed as it rolls down a shallow ramp and onto a horizontal track. Fig. 1 shows positions of the marble in photographs taken every 0.20 s. The spaces between the first positions increase. The spaces on the horizontal section are nearly equal because friction is small. A ruler beside the track is used to measure distance.

(a) What does the increasing separation of the first three marble positions show? [1]
(b) Which section shows approximately constant velocity? [1]
(c) Calculate the average speed on the horizontal track if the marble moves 0.50 m in 1.0 s. [2]