Forces Acting On Two Blocks : mostly conceptual

[Bostonuniden]

Homework Statement

Block A is on top of Block B, which is on a frictionless incline. The mass of Block A is less than that of Block B. The positive x-direction is defined to be up the ramp.

During the interval from t1 to t2 a hand exerts a constant force on block A in the positive x-direction. Both blocks move a distance d in positive x-direction and speed up. The blocks do not slip relative to each other, as show in the diagrams below

Image is attatched


consider an instant during the interval from t1 to t2

a) is the magnitude of net force on block A greater than, less than, or equal to the magnitude of the net force on block B

b) Draw 2 separate free-body diagrams, one for each block. Label each force with 1)type of force 2) the object on which the force is exerted 3) the object exerting the force

c) Rank the magnitudes of all the forces in your free-body diagrams that are parallel to the ramp( so only the ones that point entirely to the positive or negative x-direction

Last one I promise

d) Write Newton's second law for block A only. Include all the info about the magnitude and the direction of the forces exerted on block A, as well as the direction of the acceleration of Block A.

Homework Equations

Since these are mostly concept questions I am going to guess there aren't any. Correct me if I'm wrong.

The Attempt at a Solution

a) The magnitude of the net force on block A is EQUAL TO the magnitude of the net force on block B. I put this answer because the question states that the blocks don't slip relative to each other and from t1 to t2 their net forces are going pointing in the same direction.

b) I'm just going to list the forces instead of drawing it and uploading it. Block A: Gravity, Normal Force, Force of the push, Static Friction between blocks A and B. Block B Gravity, Normal Force, Force of push?, friction between the floor and block b

c)block A: static friction between block A and B, force of push
block B: force of push, friction between the floor and block b

d) I don't know how to answer this question but i do know that Newton's second law describes the behavior of objects when all existing forces are not balanced. I'm not sure what the question is asking me to do.



THANKS! ahead of time

 

[Doc Al]

a) The magnitude of the net force on block A is EQUAL TO the magnitude of the net force on block B. I put this answer because the question states that the blocks don't slip relative to each other and from t1 to t2 their net forces are going pointing in the same direction.

Since they move together, their acceleration must be the same. But what about their mass?

b) I'm just going to list the forces instead of drawing it and uploading it. Block A: Gravity, Normal Force, Force of the push, Static Friction between blocks A and B.

OK.

Block B Gravity, Normal Force, Force of push?, friction between the floor and block b

(1) The push is on block A, not block B; (2) How many normal forces act?; (3) Where's the friction? (The incline is frictionless.)

c)block A: static friction between block A and B, force of push
block B: force of push, friction between the floor and block b

Redo this in light of the above comments. And rank order them.

d) I don't know how to answer this question but i do know that Newton's second law describes the behavior of objects when all existing forces are not balanced. I'm not sure what the question is asking me to do.

Just take a shot.

 

[Moetasim]

a) Your answer is correct. Since the blocks do not slip relative to each other, the net force on block A must be equal to the net force on block B in order for them to move together in the same direction.

b) Your forces for block A are correct, but for block B, the force of push should be labeled as the force of gravity (since it is pulling the block down the incline). The friction between block B and the floor should be labeled as kinetic friction since the block is moving. The force of the push is exerted by the hand on block A and the force of gravity is exerted by the Earth on both blocks.

c) Your answer is correct. The forces in the parallel direction are only the static friction between blocks A and B and the force of push exerted by the hand.

d) Newton's Second Law states that the net force on an object is equal to its mass multiplied by its acceleration. In this case, we are looking at block A only, so the net force on block A is the force of the push exerted by the hand minus the force of static friction between blocks A and B. The mass of block A and its acceleration can be found using the given information. The direction of the acceleration will be in the positive x-direction, since the block is moving up the ramp. Therefore, the equation would be: Fnet = ma = (Force of push) - (Force of static friction) = ma.