Free body diagrams for forces on a block

[wajhea]

Hi all,

For an online physics assignment I have a question about a block resting on a slope. The question asks what forces are acting on the block.. and I was thinking normal and weight force? But it turned out to be incorrect...

list of possibilities to choose from
weight
kinetic friction
static friction
force of push
normal force

 

[Doc Al]

If the block is resting on the slope, what does that tell you?

 

[wajhea]

Does it mean that there are no forces acting on it? I don't know, I don't get this stuff.

 

[wajhea]

or static friction maybe?

 

[Doc Al]

It means that the net force on the object is zero. So what additional force is needed to balance the component of the weight acting down the slope?

 

[wajhea]

normal force?

 

[Doc Al]

No. The normal force is perpendicular to the surface of the slope, so it cannot act to balance out a force parallel to the surface.

 

[wajhea]

So could the static force balance out the forces?

 

[Doc Al]

So could the static force balance out the forces?

Absolutely! If there were no friction, the object would slide down the slope.

 

[wajhea]

Thanks...

as my answer I put down weight and static friction, but turns out that was wrong because the answer also includes normal force. Oh well...

 

[Doc Al]

Yes. Three forces act on the object.