Magnitude of Acceleration after initial impulse?

[jumpingjack90]

Magnitude of Acceleration after initial impulse?

Homework Statement

A block with a mass of m = 3.04 kg is at rest on an incline. The angle of the incline is θ = 20.3° with respect to the horizontal. The coefficient of static friction between the object and the incline is μs = 0.645, the coefficient of kinetic friction is μk = 0.449.

a.) What would be the magnitude of the object's acceleration after an initial impulse set it in motion downhill on the incline?
b.)What would be the magnitude of the object's acceleration after an initial impulse set it in motion uphill on the incline?

Homework Equations

F=(uk)(mg)(cos(theta))
ma=mgsin(theta)-mg(uk)(cos(theta))

The Attempt at a Solution

I have no idea what to do. I tried using the above equations and found that
a=(3.04)(9.8)(sin(20.3))-(9.8)(3.04)(cos(20.3))(0.449) / (3.04) = -0.726 m/s^2 which is the wrong answer. ?

 

[Doc Al]

I tried using the above equations and found that
a=(3.04)(9.8)(sin(20.3))-(9.8)(3.04)(cos(20.3))(0.449) / (3.04) = -0.726 m/s^2 which is the wrong answer. ?

Looks OK to me. Are you sure you gave the magnitude of the acceleration?

 

[jumpingjack90]

ok. found the answer to part 1. How do you do part 2 when the block is traveling uphill?

 

[Doc Al]

How do you do part 2 when the block is traveling uphill?

Identify the forces acting on the block, then apply Newton's 2nd law. (That's how you should solve part 1 as well. That way you know how to derive the formula for acceleration.)

 

[merryjman]

What Doc is saying is, if you look at the forces involved, one of them has changed its direction and therefore its sign. If you just try to plug into equations you were given, you won't see that.