A force of 25N acts on a body at rest for 0.2s and a force of 70N act for next 0.1s in opposite direction. If the final velocity of the body is 5m/s the mass of the body is?

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Solution

Let the mass of the body be m.
Case:1
Force of 25N acts on the body for 0.2s
We know that F=ma
F=ma

F=m[(v-u)/t]

25=m[(v-0)/0.2]

5=mv

v=5/m
This velocity acts as initial velocity for the next case.

Case:2

Force of 70N acts for the next 0.1s in Opposite direction.
Initial velocity=5/m
Final velocity=-5 {negative as because in opposite direction}
Force=-70N

F=ma

-70=m[(-5-5/m)/0.1]

-7=m[-5-5/m]

-7=-5m-5

-2=-5m

m=2/5=0.4Kg

Hence mass of the body is 0.4Kg

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