1)(adsbygoogle = window.adsbygoogle || []).push({});

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

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?

2. Relevant equations

F = ma

3. 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)

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

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.

2. Relevant equations

None?

3. 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.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Newton's Second and Third Laws

**Physics Forums | Science Articles, Homework Help, Discussion**