Calculate the time of the impact of this force

[UNknown 2010]

Hi my question is :

Moving the body mass was affected in 5 KG force of 20 Newton in a counter direction to the direction of movement if the speed 4 m / s. Calculate the time of the impact of this force
is it right?

pi= mvi
= (5)(9)=40 kg.m|s
pf= mvf
= (5)(-4)=-20 kg.m|s
t = (pf - pi)|f
= (-20-45)|20 = -3.25 s

 

[Doc Al]

Was the initial velocity + 9 m/s and the final velocity - 4 m/s? If so, then OK. (But the time will be positive.)

 

[UNknown 2010]

yeh (velocity + 9 m/s and the final velocity - 4 m/s)
but my answer is wrong because the time is not positive
can anybody help me ?!

 

[Doc Al]

but my answer is wrong because the time is not positive

Done correctly, the time will be positive.

Note: The force is negative.

 

[UNknown 2010]

In the book the answer was 1.5 seconds. but how they answer it I don't know

 

[Doc Al]

In the book the answer was 1.5 seconds. but how they answer it I don't know

Based on the data you provided, that answer seems incorrect. Can you post the problem exactly as given, word for word?

(What book is this from?)