Question

A sled traveling at a speed of `3.0` `ms^-1` slows to a stop `4.0` `m` from the point where its passenger rolled off. What is the magnitude of the horizontal net force that slows the `110` `N` sled?

Answer 1

First step is to find the acceleration (deceleration) of the sled, which turns out to be `-9/8` `ms^-2`.

Then we can use Newton's Second Law to find `F=ma=110/9.8*-9/8=-12.63` `N`

Explanation:

To find the acceleration of the sled, we use:

`v^2=u^2+2ad`

Rearranging:

`a=(v^2-u^2)/(2d)=(0^2-3^2)/(2*4)=-9/8` `ms^-2`

The minus sign just means the acceleration is in the opposite direction to the velocity: the sled is slowing down.

We want to use Newton's Second Law to find the force:

`F=ma`

We're not given the mass of the sled, but we are told its weight is `110` `N`. We know the weight force is given by `F=mg`, so we can rearrange to find:

`m=F/g=110/9.8` `kg`

So:

`F=ma=110/9.8*-9/8=-12.63` `N`

Again, the minus sign just means the force is acting in the opposite direction to the sled's initial motion.