# Finding the magnitude of the friction force.

Ripe

## Homework Statement

An 8kg ball moves down a 12m high inclined plane with a speed of 6m/s. By the time it reaches the bottom, the speed is measured to be 12m/s. What is the frictional force opposing the motion? The inclined plane has an angle of 30 degrees with respect to the ground.

## Homework Equations

Potential energy = mgh
Kinetic energy = 1/2mv^2
Work done by external forces = ΔE
Work done: Fs (or Fscos(theta))

## The Attempt at a Solution

The first step is to calculate the potential energy and kinetic energy, which is 960J and 576J respectively. Hence ΔE=960-576=384J.

Now, since the height is 12m and the angle is 30 degrees, the length of the inclined plane is 12/sin30 or 24m.

Finally, 24f=384 which yields an answer for f as 16N.

I might have gotten the correct answer, but in the book it says that the answer is supposed to be 22N, so I'm assuming I'm missing something, perhaps it has something to do with the initial speed which I haven't included in my calculations.

azizlwl
I think you forget initial KE.

Ripe
Please elaborate. I assumed it was 0.

Ripe
I'd very much like help on this one!

Homework Helper
Gold Member
A bit unclear on the wording, but the initial speed is 6 m/s at the top of the ramp. Watch your signage.

Ripe
Not sure If I follow what you're saying. Could you elaborate?