Kinetic and static friction

In summary, the conversation discusses a crate of mass 150.0 kg at rest on a horizontal storage room floor with a coefficient of static friction of 0.70 and kinetic friction of 0.35. The discussion then moves on to the effects of pulling on a horizontal rope attached to the crate with a force of 200 N and 1000 N, and what force is needed to start the crate sliding. The conversation also mentions the difference between kinetic and static friction and how it affects the force needed to move the crate.
  • #1
bmandrade
63
0
A crate of mass 150.0 kg lies at rest on a horizontal storage room floor. The
coefficient of static friction between the crate and the floor is 0.70, and the
coefficient of kinetic friction is 0.35. (You may assume g = 10.0 m/s2).

a) Suppose someone pulls on a horizontal rope attached to the crate. They slowly
increase the tension in the rope. Draw a free body diagram of the system.

b) Suppose someone pulls on a horizontal rope attached to the crate with a force of
200 N. Does the crate remain at rest or start moving? If it moves does it have
constant speed or continue to accelerate? If it accelerates, what is its acceleration?
Under these conditions what is the frictional force on the crate?

c) Answer part (b) again if the pull on the rope is 1000 N.

d) What force on the rope is needed to start the crate sliding?
 
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  • #2
What part are you having trouble with?
 
  • #3
well i got part A now I am stuck on part B
 
  • #4
Well, your going to have to see if the force of friction is less than or greater than the force that is being applied.
 
  • #5
ok i got that normal force is 1500 and then i multiplied that times the coefficient which is .7 which gave me 1050 but then I am not sure is I am suppose to subtract that from the 200N which is the force from the rope also i don't know how to do what you just ask
 
  • #6
Well basically you just look at what you have. You are pulling the crate at 200N while the force of friction is 1050. So is 200N greater than 1050N. No, so that box ain't going nowhere.
 
  • #7
oh so in order to move the box you will need a force greater than 1050 right?
 
  • #8
Yes sir. You need to make sure you understand the difference between kinetic friction and static friction. You need more force to get it moving then to keep it moving. Since the block is at rest you are correct in using the coefficient of static friction in your equation.

You showed that you didn't have enough force to overcome the force of friction, however, had the block already been moving then you would have used the coefficient of kinetic friction and therefore would have (in the part c) had enough force to move the block.
 
  • #9
All makes sense now, Thank you so much!
 
  • #10
Your welcome
 

1. What is the difference between kinetic and static friction?

Kinetic friction is the force that opposes the motion of two surfaces in contact when they are sliding against each other. Static friction, on the other hand, is the force that prevents an object from moving when a force is applied to it.

2. How is the coefficient of friction related to kinetic and static friction?

The coefficient of friction is a measure of how much two surfaces in contact resist the motion between them. It is used to calculate both kinetic and static frictional forces, with a higher coefficient indicating a greater resistance to motion.

3. What factors affect the amount of kinetic and static friction?

The amount of kinetic friction is affected by the type of surfaces in contact, the force pushing the surfaces together, and the speed of the relative motion. For static friction, the main factor is the force applied to the object, with a greater force resulting in a greater static frictional force.

4. How do kinetic and static friction impact the movement of an object?

Kinetic friction slows down an object that is already in motion, while static friction prevents an object from moving in the first place. These frictional forces can greatly impact the speed and direction of an object's movement.

5. Can kinetic and static friction ever be completely eliminated?

No, it is impossible to completely eliminate friction between two surfaces. However, different materials and lubricants can be used to reduce the amount of friction and make the movement of objects smoother.

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