A crate is sitting in the center of a flatbed truck.As the truck accelerates to the east,the crate moves w/it?

Question

the crate moves with it, not sliding on the bed of the truck. in what direction is the friction force exerted by the bed of the truck on the crate?
is the answer there is no friction because the crate is not sliding?

Answer 1

The frictional force of the truck bed on the crate is east. Because the truck is accelerating, the crate is going to try and remain at it's previous constant velocity. Inertia is the tendency for an object to remain at rest (which could be 0m/s or a constant velocity higher than that). So, relative to the truck, the crate will be trying to push west, only held still by the frictional forces pushing east..

Answer 2

The movement of the crate depends upon the friction between the truck bed and the crate. If there is no friction at all , as the truck moves towards east, the crate slides towards west. Actually it is not a movement by the crate. Due to inertia, the crate is reluctant to move with the truck towards east. So its relative motion will be towards west. This is the case if the friction is really zero. But, due to friction the crate can not move backwards. It is almost attached to the truck. If the friction is maximum, it is as good as fixed on the truck. That friction is towards east and the crate will move with the truck.

Answer 3

The crate is at rest with respect to the truck means there is no relative speed between them.

Both the truck and crate are moving with the same acceleration.

But there is no external matter except the truck to make the crate move with some acceleration.

It follows that the truck must exert some force on the crate pulling it along with it.

The only way it can do it by means of friction.

Thus the frictional force exerted by the truck on the cart is
accelerating the crate in the east direction.
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To prevent the crate from moving back with respect to the truck we can erect some vertical poles on the flat bed of the truck.

Now it is easy to observe that the poles are exerting the necessary force to make the crate move with equal acceleration of the truck.

It follows that even after removing the poles, if the crate follows the truck; it means friction is doing the job done by the poles.
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Answer 4

You ask, "Is the answer there is no friction because the crate is not sliding?". My answer to you is, "No, that is not a correct statement." In fact, unless the person asking the original question is being very tricky, the question itself ("in what direction is the friction force exerted by the bed of the truck on the crate?") suggests there IS friction here.

There IS a frictional force acting on the crate, and the direction of the force is EAST, to force the crate to keep up with the truck accelerating EAST.

The difficulty with this problem is that when students are first taught about frictional forces, the examples they learn deal with objects sliding on surfaces: "Joe pushes against a heavy crate on the garage floor. The force of friction acts against the motion so that Joe has to exert a constant force (force of his muscles) to overcome the opposing force of friction (generated on the bottom of the crate by the garage floor), just to keep the crate moving at a constant speed." That is a typical example. Of course, if there were no friction (the floor were made of wet ice, for example), then once Joe got the crate moving (overcoming inertia, not friction), he could let go and the crate would keep moving a constant velocity (Newton's first law).

However, frictional forces also exist when two objects are NOT moving relative to one another. As the truck accelerates, the inertia of the crate tends to make the crate NOT accelerate with it. If there were no friction, or insufficient friction, the truck bed would move from under the crate, and the crate would slide off the truck, or at least move to the reat (assuming the track is moving forward).

So if the truck accelerates EAST, and if the crate moves with the truck (does not slide off, or even slide around), then clearly the must be an EASTWARD force on the crate. In this problem, the only force there can be on the crate is friction. (For example, since the crate is not tied to the truck there can be no tension forces transmitted by ropes.) If there were no friction there would be no force on the crate, and by Newton's first law, the crate would remain in its state of motion (for example, at rest, if the truck accelerates from a stop, or the crate remains moving slowly if truck accelerates from slow speed to fast speed, or the crate continues moving in a straight direction when the truck makes a turn).

To see how the easward frictional force acting the crate comes about, consider what goes on at the microscopic level between the truck bed and bottom surface of the crate. The bed accelerates eastward. (And lets assume the turck is heading forward, not in reverse). The inertia of the crate tends to keep the crate moving as it was before, meaning it would tend to slide toward the rear of the truck bed, it could keep up. But if there is friction this means the atoms of the truck bed adhere to the atoms of the crate, so as the crate just starts to slide, just the first start of movement, too small for us to see, the atoms of the truck bed drag along the atoms making up the very bottom surface of the crate, which pull on the atoms of the next layer, which pull on other atoms of the crate, a chain pull that soon (immediately, from the point of view of humans) has all atoms making up the crate and its contents being pulled, at just the exact force needed to make the crate have the same acceleration as the truck bed. The atomic pulling forces all together combine to make up what we feel as a frictional force. We treat all the real atomic forces as if they combined into a single frictional force, which is just an easy way to handle the essentially infinite number of atomic forces all acting in the same direction on the same object.

Of courese, frictional forces have limits. If the truck were to accelerate very fast to the east, the atomic pulling forces would reach their limits, lose their holds, which we experience as the frictional force reaching it limit. Then the frictional force only partially overcomes the inertia of the crate. The crate accelerates, but not enough to keep up with the truck, so the net effect is the crate slides backward, but not as fast as if there were no (or lesser) frictional forces. If one truck had a rough wood deck and a second truck had a deck of wet ice, and they both accelerated the same, the crate on the wood deck would slide back just a little, but the crate on the ice truck would slide completely off.

Answer 5

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Answer 8

no the answer is that there is so much friction that the frictional force is equal to the accelerational force of the truck