# A block on an incline - What force applied to incline keeps block at equilibrium?

1. Sep 26, 2011

### Myronnie

1. The problem statement, all variables and given/known data
Ok, so I know how to solve ALL the other types of newton problems that have been thrown at us. I can't understand conceptually when two masses are stacked on each other..I've tried to google it relentlessly and am going into my prof's office hours.

The surfaces between the 10kg block and 20kg incline are frictionless.
What is the force that has to be exerted on the incline so the 10kg block does not move up or down (aka is in equilibrium). The force is directed so that the incline is pushed to the left.
The problem is attached...
I've tried making free-body diagrams but to no avail - I just don't understand what forces exist in the free body diagrams! Is the force applied to the incline in two components relative to the block? I'm just so confused...

2. Relevant equations

Newton's third law

3. The attempt at a solution
Eh...many pages of useless scribble..

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2. Sep 26, 2011

### Staff: Mentor

Here's a hint or two to get you going. Concentrate on the block. What forces act on it? See if you can figure out what acceleration it must have in order not to slide down the incline.

3. Sep 26, 2011

### Myronnie

Forces acting on the block - Normal force, Weight in two components (mgsintheta, mgcostheta), Contact force between incline and block ?

4. Sep 26, 2011

### Staff: Mentor

The contact force between incline and block is the normal force. So only two forces act on the block: its weight and the normal force from the incline.

Now apply Newton's 2nd law. Which direction is the block being accelerated?

5. Sep 26, 2011

### Myronnie

the block is being accelerated to the right opposite the horizontal component of weight (mgsintheta)..but the incline is being accelerated to the left by the force? I'm still lost =.=

6. Sep 26, 2011

### Staff: Mentor

If the force F is pushing both block and incline to the left, what direction are both accelerated?