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Old Jun 5th 2017, 12:38 PM   #1
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Question Friction Force Question

A 148-kg crate rests in the bed of a truck that slows from 61 km/h to a stop in 10 s. The coefficient of static friction between the crate and the truck bed is 0.589.
(a) Will the crate slide during the braking period?

(b) What is the minimum stopping time for the truck in order to prevent the crate from sliding?

I don't even know where to start. I tried drawing the free body diagram and summing the forces in the x and y directions to find some unknown but I just ended up with two different numbers for the normal force, and now that just seems irrelevant, because I don't know what to do with them anyway.

Last edited by sbnett; Jun 5th 2017 at 12:59 PM.
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Old Jun 5th 2017, 03:13 PM   #2
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Well to start you off, you are given some kinematic information connecting speed, and time.

so you need to convert this to useful data ie forces by finding a kinematic equation connecting this information to acceleration.

Then use Newton's law (which one?) to get the force.

When you have done this, note for your free body diagram that the system is not necessarily in equilibrium so foces do not necessarily 'balance'.

So you need to continue with Newton's Laws or the observation that one force is greater than another.
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Old Jun 5th 2017, 07:44 PM   #3
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Originally Posted by sbnett View Post
A 148-kg crate rests in the bed of a truck that slows from 61 km/h to a stop in 10 s.

I don't even know where to start.
First find the (effective) push on the crate ....

This comes from the deceleration of the truck , you have the initial speed , and the end speed , and the time , from these three numbers you can get a deceleration in meters per second squared.
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