1.

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

Answer»

SOLUTION :MASS of the motorcar ,m=1200 kg
Initial velocity of the motorcar v=18km/h=5 m/s
Time taken ,t=4s
According to the first equation of MOTION.
v=u+at
5=25+a(4)
`a=-5 m//s^2`
Negative sign indicate that it.s a retarding motion, i.e velocity is decreasing. Change in momentum =mv-mu=m(v-u)
=1200 (5-25)=-2400 kgm/s
`F=ma=1200 xx5 =-6000 N `.
Acceleration of the motor car `=-5 m//s^2`
Change in momentum of the motor car `=-24000 kg m^(-1)`
Hence, the force REQUIRED to decrease the velocity is 6000 N .
(Negative sign indicate retardation, decrease in momentum and retarding force)


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