How Does Friction Affect the Acceleration of a Truck Carrying a Heavy Crate?

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The discussion revolves around understanding the forces acting on a truck carrying a heavy crate and how friction affects acceleration. The truck and crate must accelerate together, meaning they share the same acceleration due to the static friction between them. The force of static friction acts on the crate to accelerate it forward while simultaneously exerting an equal and opposite force on the truck, which can create confusion regarding the direction of forces. The key point is that static friction prevents the crate from sliding, allowing both the truck and crate to move as one unit. This interaction illustrates Newton's third law, where the forces between the crate and truck are equal and opposite.
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Homework Statement



You and a friend have just loaded a 200 kg crate filled with priceless art objects into the back of a 2000 kg truck. As you press down on the accelerator, force F(surface on truck) propels the truck forward. To keep things simple, call this Ft. What is the maximum magnitude Ft can have without the crate sliding? The static and kinetic coefficients of friction between the crate and the bed of the truck are 0.8 and 0.3.

That is the original question.

My question is when I look at the free body diagram of the truck on the x axis, Ft is moving forward (towards east). That I understand.
But friction by crate on truck is moving backward (towards west of FBD).

I'm thinking the crate is on the truck. IN terms of Newton's third law, yes it makes sense.
But when you're thinking the crate is on the truck, how could it pull (force definition) the
truck backwards, affecting Ft?

Thanks in advance for your answers.


Homework Equations





The Attempt at a Solution

 
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If the crate does not slip it has to move together with the truck. The truck is accelerating. So does the crate, and its acceleration is the same as that of the truck. What force accelerates the crate? The same force, but with opposite sign acts at the truck from the crate.

ehild
 
ehild said:
If the crate does not slip it has to move together with the truck. The truck is accelerating. So does the crate, and its acceleration is the same as that of the truck. What force accelerates the crate? The same force, but with opposite sign acts at the truck from the crate.

ehild

First of all, thanks for the reply ehild.
your explanation helps me better to understand this problem,

let me just paraphrase what you explained just so I understand your point.

1) crate moves together with the truck. So it has to have same acceleration
2) because it's accelerating, there must be force.
3) so what is that force?

and I don't think I understand
"The same force, but with opposite sign acts at the truck from the crate."

because what I don't understand is how can a crate "on the truck"
possibly pull the truck backwards (according to FBD of the textbook)
when it makes sense explained by your way (Newton's third law).
 
Yes, it is Newton's third law. There is an interaction between the truck and the crate through static friction. This force acts against the accelerating force in case of truck. At the same time, the force of static friction accelerates the crate.

ehild
 
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
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