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A car travels at a uniform velocity of 20 m/s for 5s. The brakes are then applied and the car comes to rest with uniform retardation in a further 8s. How far does the car travel after the brakes are applied?
Initial Velocity = u = 20 m/s
Time traveled after brakes = t = 8s
Distance traveled after brakes = s
Applying v = u + at (where a is the retardation)
0 = 20 + 8a
8a = -20
a = -20/8
= -2.5 m/s2 (note the minus sign for retardation)
Applying s = ut + 1/2at2 (for the distance traveled after brakes) s = (20)(8) + 1/2(-2.5)(8)2
= 160 - (64)(5)/4
= 160 - 80
= 80 m
A motorist, traveling at 90 km/h, applies his brakes and comes to rest with uniform retardation in 20s. Calculate the retardation in m/s2.
Initial Velocity = u = 90 km/h = (90 X 1000)/3600 m/s = 25
Final Velocity = v = 0
Time taken = t =20s
Applying v = u + at
0 = 25 + 20a
- 20a = 25
a = -25/20
= -1.25 m/s2
Reference:
Abbott, A. F. "Speed, Velocity, and Acceleration." Ordinary Level Physics, 4th ed. Heineman Educational Books Ltd. 1984
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Mechanics Problems
