Why is kinetic friction always lower than maximum static friction?

[Peter G.]

I was reading about those in my textbook and they drew an example of a block resting on a ramp. The diagram showed the force of friction.

How is there friction if the block is not moving?

Thanks,
Peter G.

 

[Filip Larsen]

Look for an explanation of static friction in your textbook, or see http://en.wikipedia.org/wiki/Friction#Static_friction

 

[Peter G.]

Ok, thanks!

 

[Femme_physics]

Well, if any force is applied on the object (even if it's gravity pulling it down a slope) then there's static friction to prevent movement, or attempt to prevent it anyway (so it'll be opposite to where the force wants to cause movement). Once the force is strong enough to achieve movement on the object, static friction becomes kinetic friction, which-- curiously and interestingly enough, is always lower than the max static friction.

 

[rwisz]

Hello Peter,

Thank you for reaching out with your question. Kinetic friction is always lower than maximum static friction because it requires less force to maintain an object in motion than to get it to start moving. In the example of the block on a ramp, the force of friction is acting on the block to prevent it from sliding down the ramp. This is known as static friction, and it requires a certain amount of force to overcome it and get the block to start moving. Once the block is in motion, the force of friction changes to kinetic friction. Kinetic friction is the force that acts to slow down or stop the motion of an object. It is typically lower than maximum static friction because the surfaces are already in motion and there is less resistance between them.

Think of it this way: imagine trying to push a heavy box across a carpeted floor. At first, the box won't budge because of the static friction between the box and the carpet. But once you start pushing with enough force, the box will eventually start to move. Once it's in motion, it becomes easier to keep it moving because the friction between the box and the carpet is now kinetic friction, which is lower than maximum static friction.

I hope this explanation helps to clarify the concept of kinetic and static friction for you. Keep up the good work in your studies!

Best,