Any physics geniuses?

[ec437]

I was assigned this physics problem and I'm having trouble figuring it out. And don't worry, I have changed the numbers so even if you do solve it you won't be giving me the answer.tongue sticking out smiley

A car of (mass=1.5 tons) rides on the deck of a barge (mass=9 tons) floating in calm water. The car is started, driven at 7.5mph relative to the deck for a distance of 45 feet, and then stops.
a. While the car is in motion, how fast is the barge moving in the water?

b. When the car stops, does the barge also stop?

c. After the car stops, how far has the barge moved?

I think I know how to solve the first one, but I'm having trouble with the second and third ones.

[mike@af]

This sounds like one of those trick questions, where you have to pay attention to the little deatils. I've had a question similar to this before, and this is how I answered it.

A car of (mass=1.5 tons) rides on the deck of a barge (mass=9 tons) floating in calm water. The car is started, driven at 7.5mph relative to the deck for a distance of 45 feet, and then stops.
a. While the car is in motion, how fast is the barge moving in the water?
The barge is not moving at all, it is floating in the water, only the car is moving. However, this is the assumption that the current and wind to not play a factor on the movement of the barge.
b. When the car stops, does the barge also stop?
The barge does not also stop, because it is already stopped. It is floating.
c. After the car stops, how far has the barge moved?
The barge has not moved at all, only the car has moved in the length that it has traveled which was mentioned to be the deck length of 45 feet.

See, the way I see that those are the answers is because when I had a question similar to this the velocity of the barge was not stated, therefore it was assumed that it was stationary and only the objects on the barge were moving. Of course my question dealt with an airplane instead.

[beef_bourito]

the problem i see here with the question is that the car simply can't start moving at a certain speed, it has to accelerate to that speed. in this time of acceleration a force is applied that moves the car foreward but a reactionary force must also be applied to the barge. this would move the barge in the opposite direction. it also means that when the car stops, the barge has to stop moving backwards because the force applied to the car to stop it also stops the barge. i'm probably over complicating things though.

[fredjacksonsan]

Originally Posted by beef_bourito

the problem i see here with the question is that the car simply can't start moving at a certain speed, it has to accelerate to that speed. in this time of acceleration a force is applied that moves the car foreward but a reactionary force must also be applied to the barge. this would move the barge in the opposite direction. it also means that when the car stops, the barge has to stop moving backwards because the force applied to the car to stop it also stops the barge. i'm probably over complicating things though.

The force of starting and stopping the mass of the car is equal. I'd think it's a simple proportion, based on the energy to move the car, applied to the mass of the barge in the opposite direction. Same for the velocity of the barge, a proportion.

All assuming (and you could state this in your answer) that the barge is motionless in the water to start, although if it was moving it would have a relative change of velocity.

[mike@af]

Originally Posted by beef_bourito

the problem i see here with the question is that the car simply can't start moving at a certain speed, it has to accelerate to that speed. in this time of acceleration a force is applied that moves the car foreward but a reactionary force must also be applied to the barge. this would move the barge in the opposite direction. it also means that when the car stops, the barge has to stop moving backwards because the force applied to the car to stop it also stops the barge. i'm probably over complicating things though.

I thought about that too, which would make sense of the mass of the car and the barge being a given. But what about coefficients of drag. Normally you'd figure out the differences of force required through the mass to calculate the reactionary. Are we to assume this is a perfect situation in which there is no drag on the barge from the water? Or do we assume the drag is the mass?

If the case is that we can assume we're looking for a reactionary force you can figure that out through the masses and the total acceleration of both.

[fredjacksonsan]

Yes, and since there is nothing given in the problem about the status of the barge's hull, smooth or barnacled, or if it is anchored or not, you can't really calculate it; but you could calculate the force applied and state that you can't get the answer on the barge's movement due to the hull friction not being known.

[beef_bourito]

you can assume that the friction is going to be nothing because it's a simple problem.