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 Form 3 Physics Newton's laws of motion questions and answers

A minibus of mass 1,500 kg travelling at a constant velocity of 72 km/h collides head-on with a stationary car of
mass 900 kg. The impact takes 2 seconds before the two move together at a constant velocity for 20 seconds.
Calculate
a).The common velocity
b).The distance moved after the impact
c).The impulsive force
d).The change in kinetic energy

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Answer Text:
a).Let the common velocity be ‘v’ Momentum before collision = momentum after collision
#(1500times20) + (900times0)# = (1500 +900) v
30000 = 2400v
v = #30000/2400# = 12.5 m/s (common velocity)
b).After impact, the two bodies move together as one with a velocity of 12.5 m/s
#Distance = velocity times time#
= #12.5 times 20#
= 250m
c).Impulse = change in momentum
= 1500 (20-12.5) for minibus or
=900 (12.5 – 0) for the car
= 11250 Ns
Impulse force F =#(impu lse)/(time)# =
#11250/2# = 5,625 N
d).K.E before collision
= #½ times 1,500 times 20^2 = 3 times 10^5 #J
K.E after collision = #½ times 2400 times 12.52 = 1.875times 10^5 #J
Therefore, change in
K.E = #(3.00 – 1.875) times 10^5 = 1.25times 10^5# J


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