1. Suppose you exert a 100-N horizontal force on a heavy crate of computer supplies resting motionless on an office floor. The fact that it remains at rest indicates that 100 N isn’t great enough to make it slide. How does the force of friction between the crate and floor compare with your push?

Question

2. You push harder—say, 110 N—and the crate still doesn’t slide. How much friction acts on the crate?

3. You push still harder and the crate moves. Once in motion, you push with 115 N, which is just sufficient to keep it sliding at constant velocity. How much friction acts on the crate?

4. What net force does a sliding crate experience when you exert a force of 125 N and friction between the crate and the floor is 115 N?

Accepted Answer

1. 100 N in the opposite direction. Friction opposes the motion that would occur otherwise. The fact that the crate is at rest is evidence that [latex]\Sigma F =0[/latex].

2. Friction increases to 110 N, again [latex]\Sigma F =0[/latex].

3. 115 N, because when moving at constant velocity, [latex]\Sigma F =0[/latex].

4. 10 N, because [latex]\Sigma F =125 N -115 N[/latex]. In this case the crate picks up speed—it accelerates.

Question 4.S&C.7: 1. Suppose you exert a 100-N horizontal force on a heavy cra...

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