# Newtons Laws Questions

1. Sep 29, 2007

### graten2go

[SOLVED] Newtons Laws Questions

Question: A 3 kg block moves up a 40 degrees incline with constant speed under the action of a 26-N force up and parallel to the incline. What magnitude force must act up and parallel to the incline for the block to move down the incline at constant velocity?

Here is what I have done so far:
26 N sin 40 degrees and I got 16.8. However, I think I am doing it wrong. Can someone help me out? Thanks!

2. Sep 29, 2007

### rbj

i think you need to know the force due to friction on the inclined plane, or the coefficients of sliding friction.

but if the inclined plane is assumed frictionless. there is a quick and easy (but it requires insight) answer to your question.

3. Sep 29, 2007

### rbj

actually, i need to correct myself. there is enough information to compute the forces due to friction, so without any assumption of frictionless, the problem can be solved and it is non-trivial (although it's reasonably easy). i think you do have to assume the acceleration of gravity is 9.8 m/s2 or similar.

set up a drawing of this and remember what must be true of the block if it is moving at at constant velocity.

4. Sep 29, 2007

### Staff: Mentor

Use this information to figure out the force of friction. Since the block moves up at a constant speed, the forces parallel to the incline must add to what?

Since the 26 N force is parallel to the incline, there's no need to find its component in some other direction.

Hint: What's the component of the block's weight parallel to the incline?

5. Sep 30, 2007

### ihavefish

Can you please offer more of a hint?
I used trig to find the y component of the block, but what do I do with it? Thanks!

6. Oct 1, 2007

### learningphysics

The 26N force acts parallel to the plane... what is the component of gravity parallel to the plane?

Find the frictional force for the first part... you know the net force parallel to the plane = 0

There are 3 forces involved... the 26N, the component of gravity parallel to the plane, and friction.

Once you get friction, you can do the second part... the frictional force is exactly the same, but it is the opposite direction. Again net force parallel to the plane = 0.

7. Oct 1, 2007

### learningphysics

you shouldn't multiply 26N by sin40. The 26N is parallel to the incline.