Magnitude of the Force of Friction

AI Thread Summary
The discussion revolves around calculating the force of friction acting on a wooden crate being dragged at a constant speed. Since the crate is moving at a constant velocity, the net force is zero, indicating that the force of friction must equal the applied force of 85 N. Participants clarify that if the force of friction were equal to the pulling force, the crate would not be able to move. Additionally, there is a mention of a separate scenario involving a painter pulling himself on a platform, which requires drawing a free body diagram and calculating forces. The key takeaway is that the force of friction in the crate scenario is indeed 85 N, balancing the applied force.
Helpme7534
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A wooden crate with a mass of 20.0 kg is dragged across a floor at a constant speed by a force of 85.0N. The magnitude of the force of friction here is ____ N.

I'm not even sure how to start.
 
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What do Newton's laws say about constant speed motion?
 
F=ma
 
no acceleration?
 
Helpme7534 said:
no acceleration?
If it is dragged with constant speed, yes. Try to draw a free body diagram and find forces acting on the object. Now let's see. I can first find two forces.
Hint: 'apple and Newton', and [the block is being moved by what?]
 
k i appreciate the help but i need to no how to do it,
f=ma yes i no that, but i don't no how to get force of friction
 
If there is no acceleration, what is the net force?
 
85n?
 
no... you have information that there is no acceleration. so a=0 plug that in f(net)=ma. What is net force now?
 
  • #10
0, so 85 N is the force of friction?
 
  • #11
Yes, zero. But Sorry, force of friction is not 85n. We are pulling this with 85n. If force of friction equals the force pulling, the object won't move. Take a look at your another post in which I submitted by opinion.
 
  • #12
Whoa I think you're wrong there stanton...and Helpme7534 is right. Don't confuse the guy.

If force of friction equals the force pulling, the object will move if it was already moving.
 
  • #13
A Painter of Mass 70kg stands on a platform of mass 10kg and pulls himself with an acceleration of 10 cm/sec^2. He pulls each rope with a force F as shown below.

a). Draw a free body diagram for the painter.
b). Compute the value of F.
c). Compute the contact force which the painter exerts on the platform.

(sorry i couldn't draw the image. which is man pulling down the two ropes from two directions.)
 
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