Finding friction force for a 20 kg crate dragged at constant speed

[Helpme7534]

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.

 

[russ_watters]

What do Newton's laws say about constant speed motion?

 

[Helpme7534]

F=ma

 

[Helpme7534]

no acceleration?

 

[stanton]

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?]

 

[Helpme7534]

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

 

[russ_watters]

If there is no acceleration, what is the net force?

 

[Helpme7534]

85n?

 

[stanton]

no... you have information that there is no acceleration. so a=0 plug that in f(net)=ma. What is net force now?

 

[Helpme7534]

0, so 85 N is the force of friction?

 

[stanton]

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.

 

[Lsos]

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.

 

[mmludin08]

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.)