Kinetic friction or static friction

In summary, the conversation discusses the relationship between a moving box, applied force, kinetic friction, and the resulting acceleration. It also considers the scenario where the applied force decreases and how it affects the box's movement. The speaker questions the relationship between the frictional force and the applied force and expresses surprise at the box slowing down when the applied force decreases.
  • #1
VHAHAHA
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Can anyone explain this case ?
If a moving box is pulling with a force F and the kinetic friction is 0.4F, of course, it will move in the direction of applied force with an acceleration and the net force is 0.6F
But what if the force suddenly decreases to 0.3F ?
The box is still moving so the friction is still kinetic friction?? But if it is the case, then the box will slow down! This seems quite strange to me@@
Can anyone explain please?
 
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  • #2
1. " the kinetic friction is 0.4F, of course,"
Of course??
Doesn't seem like an ofcoursity to me!
Why should the frictional force be proportional to the pulling force?
It might be, obviously, but generally, friction is proportional to the NORMAL force between the surface and te box.

2. "But if it is the case, then the box will slow down!"
So what?
Kinetic friction occurs whenever there is a strict velocity difference between the surface and the box.
 
  • #3
Do you really find that surprising? If the driving force drops below the friction force, naturally the object will slowdown. If you suddenly ease on the accelerator, do you find it surprising that your car coasts down to a stop?
 

What is the difference between kinetic friction and static friction?

Kinetic friction is the force that opposes the motion of an object when it is already in motion, while static friction is the force that opposes the motion of an object that is at rest.

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

The coefficient of friction is a constant value that represents the amount of friction between two surfaces. It is used to calculate the force of both kinetic and static friction.

What factors affect the amount of kinetic and static friction?

The amount of kinetic and static friction is affected by the type of surfaces in contact, the roughness of the surfaces, and the force pressing the surfaces together.

How is kinetic friction calculated?

The force of kinetic friction can be calculated by multiplying the coefficient of friction by the normal force, which is the force pressing the surfaces together. The formula is Fk = μkN, where Fk is the force of kinetic friction, μk is the coefficient of friction, and N is the normal force.

How can kinetic and static friction be reduced?

Kinetic and static friction can be reduced by using lubricants between the surfaces, making the surfaces smoother, or reducing the weight or force pressing the surfaces together.

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