Question

In: Physics

How much force could you exert on a large crate if you wanted to get it...

How much force could you exert on a large crate if you wanted to get it moving? One thing the answer depends on is whether you can keep your footing.

a) Imagine someone on level ground, pushing horizontally against a crate, with enough effort that their feet are about to start slipping. Nevertheless, the crate hasn't started moving. What type of force is the crate exerting on the person pushing on it, and how does the magnitude of this force compare to the magnitude of the force the pusher is exerting on the crate? Explain.

b) Draw a FBD of the pusher in this situation.. Use standard symbols for all forces, include labeled axes, and include information about your velocity and acceleration. Only include forces that act on the person pushing the crate.

c) Choose a coefficient of static friction between the pusher's feet (or shoes) and the floor (in the range of 0.50 to 0.70). Let's say the pusher's mass is 92 kg. Use N2L in conjunction with your FBD to calculate the force exerted on the pusher by the crate when their feet are about to start slipping.

d) Suppose you tried this yourself – let's assume you're just as strong as the pusher, but you weigh less. Would you be able to exert as much force on the crate as you just calculated? Why or why not?

e) The pusher, pushing as hard as possible, manages to make the crate start sliding, and keeps pushing. Draw a FBD for the sliding crate.

f) Choose a coefficient of kinetic friction between the crate and the floor (between 0.30 and 0.45). Calculate the minimum time, starting from rest, it would take to push the crate 3.5 m across the floor, if its mass is 71 kg. Hint: you'll need N2L and your second FBD to start solving this.

Solutions

Expert Solution

a) the person pushing the crate applying a force F on the crate, the crate applies a reaction force R on the person. This reaction is equal to force applied by the person i.e., F = R.

The force applied by the person F is not more than static friction between the crate and floor hence crate does not move.

The reaction R applied by crate is just equal to static friction between the foot of the person and floor hence is just start slipping, he can't apply more than this as he start slipping.

B)


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