Question

Question #e7033

Answer 1

Force is defined as change in the rate of momentum

i.e `(dP)/(dt)`

Now,we know, momentum(`P`) = `mv`

So, `F = (d(mv))/(dt) = m(dv)/(dt) + v (dm)/(dt)`

So,either there should be change in rate of velocity or change in rate of mass for a force to exist.

But here,the question says, that mass and velocity both are constant.

So,externally no force is acting for the movement of the box,so it can't have any acceleration.

Now,suppose someone needs to apply a force to stop the box in `1s`,

So,here change in momentum will be `m(v-u)=9.3(0-0.85)=-7.91 Kg.m.s^-1`

So,rate of change in momentum i.e force required wil be `(m(v-u))/t = -7.91/1=-7.91 N`

The negative sign means,the force has to applied against the direction of its initial velocity,which is very obvious.

Answer 2

`-7.91N`

Explanation:

I'm assuming that you are saying that its acceleration is `0.85m"/"s^2` not just `0.85m"/"s`.

So, we use Newton's second law of motion,

`F=ma`

`:.F=9.3kg*0.85m"/"s^2`

`F~~7.91kg*m*s^-2`

`F=7.91N`

To stop it, this force will have to act on the opposite way, so we will write it as

`F=-7.91N`