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a) A boy throws a tennis ball vertically upwards from a truck moving at a constant velocity. Give the reason why the ball lands back exactly the same point where it was projected.
b) Define impulse in terms of momentum.
c) A trailer of mass 30 tonnes travelling at a velocity of 72km/h rams onto a stationary bus of mass10 tonnes.
The impact takes 0.5 seconds before the two vehicles move off together at a constant velocity for 15 seconds.
Determine:
i).The common velocity
ii) The distance moved after the impact
iii) The impulsive force on the trailer on impact
d) Give reasons why a safety seat belt used in a vehicle:
i) Should have a wide surface area
ii) Should be slightly extensible

Answer Text:
a).The ball has the same horizontal velocity as the truck.
b).Impulse is equal to change in momentum
c).i).#m_1u_1 + m_2u_2 = v(m_1 + m_2)#
#30,000 times 20 + 0# = V(30,000 + 10,000)
#V = (600,000)/(40,000)#
V = 15m/s
ii).S = Vt
= #15 times 15#
= 225m
iii).#F = (m(v - u))/t#
= #(30,000 (20 - 15))/0.5#
F = 300,000N
d)i)To increase the area on which the force acts to reduce pressure due to impulsive force.
ii).To prolong the period of time the force acts to reduce the impulsive force.