# Finding the velocity of an inclined mass

## Homework Statement

As shown in the figure, ( a mass sliding down an incline of 60°) a car of 1000 kg mass start from the rest, it's velocity after 5s is? ( the kinetic fraction coefficient 0.1)

E1=E2
K1+U1=K2+U2
E1+Wf=E2

## The Attempt at a Solution

-Fk= mg cos 60 (0.1) = -490
K1+U1 + Wf = K2 + U2
We should cancel K1 and U2 because they equal zero
Then
Mgh -Fk d = 1/2 mv^2
Cancelling both m from each side
gh -Fk (vt) = 1/2 v^2
9.8*h+490(5v) = 1/2 v^2

Then I don't know what to do

Doc Al
Mentor
Since you want the speed at a given time, why not find the acceleration and then use kinematics. Start with Newton's 2nd law.

Since you want the speed at a given time, why not find the acceleration and then use kinematics. Start with Newton's 2nd law.

I was doing the wrong law for this equation.

mg sin 60 - Fk = ma
1000*9.8*sin60 - ( 0.1 * 1000*9.8*cos60)= 1000a

8487.07-490=1000a
7997.04/1000
= 40m/s

haruspex
Homework Helper
Gold Member
I was doing the wrong law for this equation.

mg sin 60 - Fk = ma
1000*9.8*sin60 - ( 0.1 * 1000*9.8*cos60)= 1000a

8487.07-490=1000a
7997.04/1000
= 40m/s
Good.