AP Physics B Multiple Choice Newtonian (1 prob)

[ChickenChakuro]

Hi guys, I need help with the following problem I couldn't figure out:

1) A block of mass 2 kg slides along a horizontal tabletop. A horizontal applied force of 12 N and a vertical applied force of 15 N act on the block. If the coefficient of kinetic friction between the block and the table is 0.2, the frictional force exterted on the block is most nearly
A) 1 N
B) 3 N
C) 4 N
D) 5 N
E) 7 N

The answer is E. Why?

 

[dextercioby]

What thoughts did u have about it?Something must have crossed your mind...

To get their answer,take g=10ms^{-2}.

Daniel.

 

[ChickenChakuro]

I thought that the frictional force was given by the equation Ff = mu*m*g*cos(theta). But that only comes out to 1/5*2*10=4N. So I have no idea how to factor in the vertical force.

 

[dextercioby]

What "cos" are u dreaming about ?All forces have right angles between them."cos 90°=0",IIRC.

Daniel.

 

[ChickenChakuro]

Sorry I still don't understand. Can someone please help me get this concept?

 

[dextercioby]

The problem is ambiguous,because it doesn't specify the direction of the forces...Assume the vertical force pushes the body into the table (it has the same direction & sense as the gravity force).

Since the body is moving,u have to know what is the expression of the kinetic friction force.

Daniel.

 

[ChickenChakuro]

You are correct, the vertical force is pushing the block into the table. I believe the force of kinetic friction is mu * Normal Force. Normal force here would be 15 N, right, so F kinetic friction = 3? Bah, I don't know.

 

[dextercioby]

No.In the table,the pressing force is the 15N external force and the 20N force due to gravity attraction.Therefore,the normal force is 15+20=35N

Daniel.

 

[ChickenChakuro]

Yes, but why isn't there an opposite reaction from the table upwards into the block of the same force of the block's weight, 20 N?

 

[dextercioby]

That's part of the reaction force,called NORMAL FORCE...

Daniel.

 

[ChickenChakuro]

Ok thank you for your help