Homework Help: Force and acceleration in object at rest

1. May 27, 2013

blueberryfive

1. The problem statement, all variables and given/known data

An object is at rest and stays at rest, but a rope attached to it is pulling with increasing force. What is the acceleration?

2. Relevant equations

F=ma

3. The attempt at a solution

The solution is that acceleration is constant and 0. Obviously a= dv/dt =0, since the object is at rest.

But if force is increasing, since F=ma, isn't acceleration increasing as well?

This is probably a stupid question, but I'm confused

2. May 27, 2013

barryj

Consider a block of wood on a table. As you gradually start pulling, or pushing, on the block, the force will obviously increase but the friction force will be equal to and opposite the force you are applying up to a maximum value. When the friction force is exceeded, then the block will accelerate according to F = ma where F is the net force on the block. The net force will be the force you are pulling with minus the friction force. Then there will be acceleration.

3. May 27, 2013

blueberryfive

Thanks, so I guess my problem is that F is the net force, not just the pulling force applied to the block.

4. May 27, 2013

barryj

That is correct.