[askeese]

I have a 4000 lb container on wheels on a flat bed truck with side rails. The load is secured with a strap across the face of load and connected to the side rail. What force is exerted on the strap when the truck accelerates? Thanks for your help!

[Capuchin]

I don't quite understand the situation.

If the truck accelerates and there is no friction between the truck bed and the container, then no force is applied. The container will simply roll off the back of the truck.

If there is friction then without the full numbers I can't do much to help.

If you want to illustrate the situation a little better to me then I can help further.

[askeese]

When the truck accelerates, and no friction between the truck and the container, there should be a force exerted on the strap which is holding the container against the back of the truck, keeping the container from rolling off. What confuses me, is the force the container exerts when the truck accelerates (similarily when the truck brakes and the container tries to go forward).

I hope that clarified your question. If not, I can attach a sketch.

[Capuchin]

ah so there is no friction but the belt is holding the weight in the truck.

Okay.

If you imagine yourself accelerating along side the truck at the same rate and looking outside the window. This is the truck's frame of reference.

In this reference frame, the truck is not accelerating, the weight is accelerating backwards. In this reference frame it is plain to see that F = ma, where a is the weight's acceleration (and is equal, in the stationary frame, to the truck's acceleration).

Maybe this is a complicated way of looking at it.
The answer is F = ma where a is the truck's acceleration, and m is the weight's mass. The F is applied by the tension in the strap.

I hope this is clear, but I fear it isn't as clear as it could be, please feel free to ask anything that isn't clear :)

[askeese]

That does make sense, however from take off, is the force calculation different (greater)? I am not sure the best way to word this, but when you pop the clutch and take off suddenly, there seems to be a greater force exerted on the strap than when you are at a constant velocity. I guess it is similar to a safety harness, when a person jumps off a ledge and the safety harness catches him, he exerts a force of nearly 5000 lbs.

So if it is only F=ma, the worst case scenario would be if you tilted the truck up vertical, and the force would equal mass of 4000lb times gravity (32.2 ft/sec2)?

[Capuchin]

of course there is a greater force when you pop the clutch, that is the point of fastest acceleration. When you are in constant velocity there is no acceleration... therefore there is no force.

Both the truck and the weight are moving at the same speed, and will continue to do so until something decelerates.

In the case of the safety harness, the person is decelerating very quickly, this gives the large force.

When the truck first starts, the truck has acceleration of x, whereas the box has no acceleration. The strap has to provide the difference in force.

When the truck is moving at constant velocity, both box and truck are moving at the same speed - the strap doesn't have to do anything.

If you tilted the truck vertical, then the strap would have to provide force both to counter the weight due to gravity and the acceleration of the truck, so that would be worst case, yes.