Kinetic friction free body diagram

[bmandrade]

A Physics 1AL student uses a force P of magnitude 80 N and angle θ = 70 (with respect to the horizontal) to push a 5.0 kg block across the ceiling of her room. The coefficient of kinetic friction between the block and the ceiling is 0.40.

a) Draw a free body diagram of the system.

b) What is the magnitude of the block’s
acceleration?


This is a question that I can't really figure out. Please help

 

[Kurdt]

Have you drawn the free body diagram?

 

[bmandrade]

no i don't know how to do it i drew a line which is the ceiling and the box under it but i don't know what to do after that

 

[Kurdt]

What are the components of force acting on the box? Put them in the diagram.

 

[bmandrade]

ok well there is a normal force upward and Force due to gravity pointing down right? Plus there is the Static friction which is a horizontal force. However I don't know how to deal with the force being exerted on the box. I figure out that I have to break it down in components but I don't know if the y-component will be added or subtracted for the vertical forces.

 

[Kurdt]

The y-component is pushing the box up onto the ceiling isn't it. The weight and normal force from the ceiling act in the same direction toward the floor. You know the normal force will be whatever resultant force is pushing the box into the ceiling and yo also should know how the normal force and the friction force are related.

 

[bmandrade]

I thought Fg and the normal force were pointing opposite directions I guess not. But if it is how you say it is then that means that the normal force will be the same as the y-component because that is the only opposite force right? and I do know that kinetic friction is equal to the coefficient of friction times the normal

 

[Kurdt]

Well with the box being on the ceiling the normal force is in the same direction as the weight. It is not just equal to the y-component though. It is equal to the resultant force (i.e. the vector sum of the y-component and the weight).

 

[bmandrade]

oh ok so the sum of vertical components = fn - Fg - 80 x sin70 so that Fn will be equal to the weight + the y component

 

[raiderfc90]

hey B good I was about to ask the same question

 

[Kurdt]

oh ok so the sum of vertical components = fn - Fg - 80 x sin70 so that Fn will be equal to the weight + the y component

As long as that is a vector sum then yes. Remember the weight and y-component are in opposite directions.