What is the Force of Friction on a Block Being Pulled at Constant Velocity?

[LBP]

Homework Statement

A block of wood is pulled by a horizontal string across a rough surface at a constant velocity with a force of 20 N. The coefficient of kinetic friction between the surfaces is 0.3. The force of friction is
A) impossible to determine without knowing the mass of the block.
B) impossible to determine without knowing the speed of the block.
C) 0.3N
D) 6N
E) 20N

I thought the answer would be A but it is apparently E but I'm not sure why.

Homework Equations

Static Friction= Mu(s)*(Normal Force)
Kinetic Friction=Mu(k)*(Normal Force)
Normal force=m*g

The Attempt at a Solution

From similar problems I started by trying to figure out if the block is moving by comparing the kinetic friction to the pulling force. However to get that I am missing the mass of the block (Kinetic friction=0.3*(m)*9.81). I am obviously thinking about this wrong. Any guidance?

 

[Doc Al]

The velocity is constant. What does that tell you?

 

[LBP]

It would seem that there are no forces acting on the block if there is a constant velocity.

 

[Doc Al]

It would seem that there are no forces acting on the block if there is a constant velocity.

There are certainly forces acting on the block (you're told that), but the net force is zero.

 

[ppzmis]

what is Newton's 1st law? Be worth having a think about it

 

[Tim D.]

They are correct! You don't have to worry about us (static friction) in this problem though. Static friction is when the velocity and/or acceleration is not constant or mainly when you are trying to start an object, the us will be greater then uk. If the acceleration is constant there are forces acting on it but the net force is zero. Remember for future problems that due to calculus there are a lot of problems that can be solve with very little info. With knowledge of kinematics, dynamics, calculus, and then with a little practice of "rearranging" equations you will be surprised how much you can solve ;)

 

[LBP]

So to make sure I understand this: no net force doesn't mean there is no movement, but rather no acceleration. And since there is no acceleration there is no "force hump" to get over so there is no frictional force. Then since forces have equal and opposite reaction forces the velocity is equal to the Kinetic friction? Though if that's true why isn't it the negative velocity?

 

[Doc Al]

So to make sure I understand this: no net force doesn't mean there is no movement, but rather no acceleration.

Correct.

And since there is no acceleration there is no "force hump" to get over so there is no frictional force.

No. If there were no friction, then the block would accelerate since it's being pulled.

Then since forces have equal and opposite reaction forces the velocity is equal to the Kinetic friction? Though if that's true why isn't it the negative velocity?

(1) All forces have equal and opposite reaction forces, but that's not relevant here.
(2) A velocity is not a force!

Hint for this problem: What forces act on the block? (Only two are relevant.) What must they add to?

 

[LBP]

Earlier I meant to say there would be no Static friction, not any friction at all. Is that correct?

The two forces are Tension or pulling force and kinetic friction. They must add to 0 if there are no other forces, meaning they are equal to each other?

 

[Doc Al]

Earlier I meant to say there would be no Static friction, not any friction at all. Is that correct?

Yes. It will be kinetic friction, not static friction.

The two forces are Tension or pulling force and kinetic friction. They must add to 0 if there are no other forces, meaning they are equal to each other?

Exactly!