Newton's Second and Third Laws

[snoopygal327]

1)

Homework Statement

A force of 1.5 N is applied to a block on a flat surface and the block moves with a constant velocity. Approximately what applied force is needed to keep the block moving with a constant velocity that is twice as large as before?

Homework Equations

F = ma

The Attempt at a Solution

At first I thought the answer would be 3 N, but now I'm not sure. When you originally apply the force of 1.5 N the force of friction must be 1.5 N as well so that the block moves with a constant velocity right? So then if you increase the force to 3 N would friction increase to 3 N so that the block moves with a constant velocity?


2)

Homework Statement

A 90 kg man and a 60 kg boy each have one hand extended out in front and are pushing on each other. Neither is moving. Eventually the boy begins to slide across the floor. Compare the forces between their hands.

Homework Equations

None?

The Attempt at a Solution

When neither is moving the forces between their hands are equal according to Newton's third law. But once they start moving does this still hold true? My guess is that the forces are still equal but have increased in magnitude so that's what causes the boy's acceleration.

 

[Redbelly98]

(1)
Recall the general formula for friction force. How does it change (or does it) when velocity is doubled?

(2)
I agree with your answer.

p.s. Welcome to PF.

 

[snoopygal327]

Okay, I remember now that friction force doesn't increase with velocity. In that case, pulling with a force of 3 N would cause acceleration. Then is it even possible to move the block with twice the constant velocity?

 

[LowlyPion]

Okay, I remember now that friction force doesn't increase with velocity. In that case, pulling with a force of 3 N would cause acceleration. Then is it even possible to move the block with twice the constant velocity?

Yes. Any excess of force over the constant motion state of 1.5N will serve to accelerate the block. Once at double the speed how much force is required to maintain it at that speed?

 

[snoopygal327]

Oh so does that mean that after the block has accelerated, you only need to apply a force of 1.5 N to keep it moving at twice the constant velocity?

 

[LowlyPion]

Oh so does that mean that after the block has accelerated, you only need to apply a force of 1.5 N to keep it moving at twice the constant velocity?

If there was more it would accelerate no?

If less it would slow.

Looks like it to me.

 

[snoopygal327]

Thank you!