# A motorcar of mass 1200kg is moving along a straight line with a uniform velocity of 90 km//h. Its velocity is slowed down to 18 km//h in 4s b

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A motorcar of mass 1200kg is moving along a straight line with a uniform velocity of 90 km//h. Its velocity is slowed down to 18 km//h in 4s by an unbalanced external force. Calculate the acceleration and change in momentum. Also, calculate the magnitude of the force required.

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(a) Calculation of acceleration
Here, Initial velocity,u=90km/h
=(90xx1000m)/(60xx60s)
=25 m/s
Final velocity,v=18 km/h ltbr? =(18xx1000m)/(60xx60s)
=5m/s
Acceleration,a=? (To be calculated)
And, Time,t=4s
Now, v=a+ut
5=25+axx4
4a=5-25
4a=-20
a=-20/4
a=-5m//s^(2)
Thus, the acceleration of the motorcar is, -5 m//s^(2). It is actually negative acceleration or retardation.
(b) Calculation of change in momentum
Change in momentum =mv-mu
=1200xx5-1200xx25
=6000-30000
=-24000kg.m//s
Thus, the change in momentum is 24000 kg. m/s
(c) Calculation of magnitude of force
Force, F=mxxa
F=1200xx(-5)
F=-6000N
Thus, the magnitude of force required is 6000 newtons. The negative sign shows that this force is opposing the motion.