Forces on a plane exerted by a ball

[blueblast]

Could someone please provide a diagram of all the forces acting on the plane, and explain why the answer is E?

This is from the 2014 F=MA exam.

Thanks,

blueblast

 

[NFuller]

The only force the ball can exert on the plane is its weight, which points straight down.

 

[conscience]

The only force the ball can exert on the plane is its weight, which points straight down.

This is completely wrong .

Could someone please provide a diagram of all the forces acting on the plane, and explain why the answer is E?

What are your thoughts on this ?

Draw a free body diagram listing all the forces on the ball ? Think carefully about the condition that the ball rolls without slipping on the inclined plane .

 

[NFuller]

This is completely wrong .

You're right, apparently I'm too distracted right now to think straight...

 

[blueblast]

This is completely wrong .
What are your thoughts on this ?

Draw a free body diagram listing all the forces on the ball ? Think carefully about the condition that the ball rolls without slipping on the inclined plane .

This is what I have so far:

Now this is how my teacher explained it:
Every force has an equal and opposite reaction, so the friction force acting on the ball has a force that acts the other way.
The normal force that pushes the ball up also has a force that pushes the plane down, so:

But where did gravity go?

 

[conscience]

But where did gravity go?

Is force due to gravity on the ball exerted by the inclined plane ?

Note : You have been asked to find the direction of force by the ball on the inclined plane .

 

[blueblast]

Is force due to gravity on the ball exerted by the inclined plane ?

Note : You have been asked to find the direction of force by the ball on the inclined plane .

The gravity on the ball is not exerted by the plane, but doesn't it still have a weight that pushes down on the plane?

 

[conscience]

The gravity on the ball is not exerted by the plane, but doesn't it still have a weight that pushes down on the plane?

This is not how you think . Weight is not the force by which the ball pushes on the plane .Weight is the force of gravity exerted by Earth .

It is the Normal force from the ball which pushes the inclined plane in a direction perpendicular to the plane .And it is the frictional force from the ball which exerts a force along the inclined plane surface .

 

[conscience]

But presence of normal and frictional forces in the given problem is due to gravitational force exerted by Earth . If force of gravity is somehow absent , then zero normal force , zero frictional force . Ball stays in the place where it is put without exerting any force on the inclined plane

 

[blueblast]

This is not how you think . Weight is not the force by which the ball pushes on the plane .Weight is the force of gravity exerted by Earth .

It is the Normal force from the ball which pushes the inclined plane in a direction perpendicular to the plane .And it is the frictional force from the ball which exerts a force along the inclined plane surface .

It's getting clearer to me now, but I'm still confused about gravity. If I'm lying down, and someone puts a heavy book on my back, wouldn't I feel the book pushing down on me?

 

[conscience]

It's getting clearer to me now, but I'm still confused about gravity. If I'm lying down, and someone puts a heavy book on my back, wouldn't I feel the book pushing down on me?

You definitely would feel book pushing down on you .

Earth pulls you towards its center by exerting a gravitational force .Earth also pulls the book towards its center .Due to this a Normal contact force develops between you and the book , as well as between you and the floor .It is this Normal force from the book which you feel as the force from the book which pushes you down .

The normal force from the floor pushes you up .

 

[blueblast]

You definitely would feel book pushing down on you .

Earth pulls you towards its center by exerting a gravitational force .Earth also pulls the book towards its center .Due to this a Normal contact force develops between you and the book , as well as between you and the floor .It is this Normal force from the book which you feel as the force from the book which pushes you down .

The normal force from the floor pushes you up .

So regarding just me and the book, I would push up on the book(normal force). Then, the reaction force of the book pushing on me would be what I feel.

Is this right?

 

[conscience]

So regarding just me and the book, I would push up on the book(normal force). Then, the reaction force of the book pushing on me would be what I feel.

Is this right?

Yes

 

[haruspex]

The hard part in this question is choosing between A, C and E. The direction of the vector in C appears to be normal to the slope. You need to assume that is intentional.

 

[jbriggs444]

So regarding just me and the book, I would push up on the book(normal force). Then, the reaction force of the book pushing on me would be what I feel.

What you actually feel are nerve impulses caused due to the strain in your skin and muscles that are supporting the book.

Whether this strain is due to you pushing on the book or due to the book pushing on you is not answerable and not relevant. Newton's third law says that forces come in pairs. You cannot have the force of your back on the book without having the force of the book on your back. No possible experiment can can tell which one of those you "feel" because you cannot remove them one at a time to see which one was responsible for the sensation.