Homework Help: Kinematics of wooden box on slide

1. Feb 8, 2005

theCandyman

Can I have some tips on where to start this problem? I have no work because I can see no work to do.

"A wooden box is placed on the left end of a 3.60-m long plywood ramp. The left end of the ramp is raised until the box begins to slide. What is the magnitude of the acceleration of the box after it starts to move?"

This is all the information. I have no way to find the coefficent of friction, no angle, and no mass for the box.

2. Feb 8, 2005

ZapperZ

Staff Emeritus
The answer is: acceleration = 0.

However, that answer assumes something that isn't given in the problem, and it can make it rather complicated.

When you increase the angle of the ramp, the weight along the inclined will start pulling the box down. Now, the problem here is that typically, the static friction tends to be larger than the kinetic friction. In answering your question, I have made the allowance that as you increase the angle of the ramp, you continue to give the box a slight push. The box will move and then stop when the ramp angle is small, but at a particular angle, it will then start to move with constant velocity. It means that at this angle, the frictional force is exactly equal the the component of the weight along the incline, so they balance out. So acceleration is zero.

If you do not give it a slight push, the object will remain stationary, and remains stationary beyond this angle because of what I said about static friction above. By the time it moves, it will be accelerating because the component of the weight along the incline will definitely be larger than the frictional force. If this occurs, or if this is what is really being asked in your problem, then your problem cannot be answered because it requires explicit knowledge of the ratio of the coefficient of static friction to the kinetic friction.

Zz.

3. Feb 8, 2005

theCandyman

Sorry, there is some misinformation in my first post. I had not thought to look in my text book for the coefficent of friction between wooden surfaces (the homework is done on-line through MasteringPhysics). According to my text the coefficent of static friction is 0.5 and kinetic friction is 0.2 (both for wood - wood).

I can see that I have to find where gravity overcomes the static friction (since gravity and friction are the only forces on the box, it is not pushed) but I still have no clue where to start because I cannot find the normal force.

4. Feb 8, 2005

ZapperZ

Staff Emeritus
Well, other than annoying me with the omission, I think you have revealed your bad physics-solving technique.

When you say that you cannot find the normal force, it tells me that you might be one of those students who tend to plug in numbers WAY in the beginning of problem solving technique, rather than going through the whole problem symbolically and plugging in numbers only at the very end. There are many instances in physics where variables cancel out at the end. Such problems will trap students with this kind of bad habits.

Solve the problem symbolically. Assign symbols such as m, etc, to the everything that you need to use. And then see at the end if they don't cancel out.

Zz.

5. Feb 8, 2005

theCandyman

I have found that the angle between the normal force and the force of gravity is the same as the angle of the incline. I can find it from here.