Question

Guys help me please
1) A block of mass m is placed on a smooth wedge of inclination theta with the horizontal. The whole system is accelerated horizontally so that the block does not slip on the wedge.Find out the magnitude of the force exerted by the wedge on the block.
2) If the force( here i'm talking about normal force N) exerted by the floor of an elevator on a man is somehow reduced, then the man will feel himself light. Am i right??

Answer 1

1. Okay, so imagine this block on a plane. If it is accelerating by a force but not moving, the component forces must equal, does that make sense? If the acceleration due to gravity down this wedge is in the diagonal vector, you need to also consider that the diagonal vector of this horizontal force is equal to it.
In this case, gravity pushing the block down = mgsin(theta)
There's a good image here that gives you an idea: http://faculty.wwu.edu/~vawter/physicsnet/topics/Dynamics/InclinePlanePhys.html

Answer 2

Answer to 1st question is mg/cos theta

Answer 3

2. I think this might be referring to a situation where the cable of the elevator breaks. At this point, the net force mg is pointing in the negative downward direction. Your apparent weight = wieght (mg) - ma in this situation. When accelerating downward, you feel your weight is less than normal because the required support force of the elevator is less than normal.

Answer 4

2) that means i was correct.
1) that's what i was thinking. But correct answer is mg/cos theta.
I don't know how.

Answer 5

I'm not really understanding the first problem, but if you took the component of the accelerating force that prevents the block from moving as
ma cos(theta) and set it equal to mg which is the normal force and what should cause the block to accelerate, and then divide by cos theta, you would get the prescribed answer.