# Hand pushes block

1. Jul 29, 2007

1. The problem statement, all variables and given/known data
Not an actual HW problem, but related to alot of probs i'm doing.
Lets say a hand pushes a block across a surface (with friction).
You take pic of 2 timepoints
1) when hand pushes block
2) After hand has let go and block is slowing due to friction

Now when I draw the FBD for situation #2, I am confused about the horizontal forces. (i know that vertical forces cancel)

I know that 1 vector (kinetic friction) will point opposite the motion.

2. Relevant equations
BUT: Is there a forces that goes in the direction of motion???
If there isn't, then how is the block moving??
If there is, then what force could that be, seeing as the hand is no longer in contact with the block??

Thanks in advance for any explanation!
3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 29, 2007

### PhanthomJay

yes, correct.

What does your FBD show you?
The hand sure helped. What would happen to the object after the hand was released if there were no friction? Why?
Right, good point.

3. Jul 29, 2007

My problem is in drawing the correct FBD, so at moment im not relying on FBD for info
But this is how i think it should look: (W and N are sposed to be equal lengths)

Yes I know hand got it moving, but if FBD is sposed to show forces on it, then there should be no force in pos x direction (right), because the hand is no longer in contact with block, right?
But if no force in in pos x, then according to diagram, Fnet is to the left, and object is moving to the left (according to the diagram)???

If there was no friction, then block would move at constant velocity in pos x direction. (and accel would be zero)

Last edited: Jul 29, 2007
4. Jul 30, 2007

### PhanthomJay

Yes, your FBD is OK. You have correctly noted that there is only one horizontal force acting, the friction force, which is the net force, acting left. But per Newton 2, if the net force is left, then the acceleration is left, not necessarily the motion, which in this case is still to the right until the block stops. As you also have correctly noted, you don't need forces to keep a body moving. Forces retard or accelerate the motion; they do not keep it in motion, as noted in Newton 1.

5. Jul 30, 2007