Solving Dynamics Problems: When Does Friction Change Direction?

[Stalker23]

So i have multiple teachers giving me different answers to this question.

A block of mass (m) is initially sliding up a ramp at an incline of (Theta) at an initial speed (V0). The coefficient of kinetic friction between the ramp and block is (Uk), and and the coefficient of static friction is (Us).

The questions i have are

1) Does force vector for friction change direction when the block changes direction?

2) When the block is stopped for an instant does the coefficient of static friction matter?

Lets say you wanted to see how fast the block were moving after (t) seconds, you could do it using Newtons Laws or the Laws of conservation of energy.

---the first question effects the Both Methods for solving the problem.

---the second question matters because you may need to check to make sure the block will slid down the ramp after reaching the top.



Thankyou

...my dynamics teacher told me that friction DOES NOT CHANGE DIRECTION (Dr. Hendricks of Virginia Tech ESM)

 

[Vaal]

For 1 I would say in practice yes, the frictional force that you are interested in does change if the block changes direction.


For 2 the is answer is basically yes. You are right that you need to check to see if static friction is small enough that the block will slide back down but in reality the idea of friction being simply "static" or "kinetic" is a little bit of an over simplification. Frictional force really depends on how fast the object is moving or better said how long it takes the object to pass through a relatively small area. We usually just assume friction to be either kinetic or static because it only differs significantly from the kinetic value at very, very low speeds.

This has two implications:
1) Even if the object will slide back down and not stop indefinitely at the top it will have undergone a brief period at its highest point on the ramp when the magnitude of the frictional force is higher than the normal kinetic value. This will slightly affect its position relative to time during the rest of its motion.
2) It is also in theory possible for the block to be held in place by static friction when it's placed on the ramp and held still for a brief instant but not stop permanently at the highest point of its motion in the situation described above. This basically comes from the fact that the object is only very, very close to its highest position (the position where it stops for an instant) for a very, very brief time and that time won't be as long as the time needed for the highest possible frictional force, the frictional force that is associated with static friction, to take hold.

 

[arunma]

I agree with Vaal. Normally I wouldn't bother posting a redundant response, but since a guy with a PhD disagrees with us...

1.) The direction of the frictional force does change. The frictional force opposes the motion (this is not a general rule, but it's true for blocks sliding on inclined planes). When the block goes up the incline, the kinetic friction will point down the incline. When the block slides down, the frictional force will point up.

Just a guess here, but maybe the question was poorly worded and he just meant to ask if the block stops at its maximum height, or comes down.

2.) Yes. If the block stops, then the component of gravity parallel to the incline needs to be greater than the maximum static friction that the surface can offer. Otherwise it won't stop moving again.