Kinetic Friction

[ReadandPlay]

Does kinetic friction always opposes motion?

What about when you are ice skating does the friction add to you sliding more (motion)?

Also does static friction point in a direction (for example towards mg)?

Thanks...

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[techfan]

Yes friction of any kind (kinetic or static) always opposes motion. Think of it is way, when you have a piece of sandpaper and rub it against the wall to remove paint does it matter which way you go? No, it doesn't matter which way you move it the paint in that direction will be removed.

For ice skating. no the friction will always work against you. Think of a hockey puck on ice, does it get accelerate (continue to get faster) after a soft push? No, it eventually slows down and comes to a stop. Ice skates just have a smaller point of contact with the ice than hockey pucks, but the principle is the same.

Static friction simply acts to keep the object still. If you are trying to push a heavy box on a flat road do you gain an advantage by pushing on one side over the other (assuming you are pushing in a direction parallel to the road)? No, it's going to take the same amount of effort whichever side you're pushing on. What you're confused about is the normal force. The normal force is the force that acts to keep the box from falling through the floor and to the center of the Earth. It's the force that floor is putting on the box. The static friction force is the normal force (mg if the object is on a flat surface, or mgcos(theta) if the object is on an inclined plane) times the coefficient of state friction (mu_s)

Here's a Khan Academy video if you want a step by step

 

[NextStepTutor_1]

@techfan is spot on. But to add a little more to the skating example (because it's actually a pretty cool phenomenon):
The puck and the skate both slow down - friction is acting in the opposing direction of motion and eventually stops the puck and the skate over time. But why do they seem to exhibit different movement characteristics? Why does the skate seem to go further than the puck when the same force is applied?
As techfan noted, the point of contact with the ice is greatly reduced compared to the puck, so much so that the pressure (Force / Area) is much greater on the ice. When the pressure of ice is increased (and you can refer to the phase diagram for H2O) the H2O prefers its densest form - water. Water allows the skate to "glide" which is why you see ice skaters smoothly skate across the ice as opposed to ice pucks that are affected by minor chips and bumps.

Not specific info needed for the MCAT - but a nice example that pulls in concepts you should be able to apply.

Cheers!


http://forums.studentdoctor.net/forums/next-step-mcat-tutor-office-hours.970/

 

[ReadandPlay]

Thank you both techfan and NextStepTutor for your explanations and the insights.

 

[Czarcasm]

Yes friction of any kind (kinetic or static) always opposes motion. Think of it is way, when you have a piece of sandpaper and rub it against the wall to remove paint does it matter which way you go? No, it doesn't matter which way you move it the paint in that direction will be removed.

For ice skating. no the friction will always work against you. Think of a hockey puck on ice, does it get accelerate (continue to get faster) after a soft push? No, it eventually slows down and comes to a stop. Ice skates just have a smaller point of contact with the ice than hockey pucks, but the principle is the same.

Static friction simply acts to keep the object still. If you are trying to push a heavy box on a flat road do you gain an advantage by pushing on one side over the other (assuming you are pushing in a direction parallel to the road)? No, it's going to take the same amount of effort whichever side you're pushing on. What you're confused about is the normal force. The normal force is the force that acts to keep the box from falling through the floor and to the center of the Earth. It's the force that floor is putting on the box. The static friction force is the normal force (mg if the object is on a flat surface, or mgcos(theta) if the object is on an inclined plane) times the coefficient of state friction (mu_s)

Here's a Khan Academy video if you want a step by step

Is this always true though? The wheels of car push against the ground and are able to rotate due to rolling/static friction which in this scenario I'm told would act in the same direction as the moving car.

 

[Cawolf]

Is this always true though? The wheels of car push against the ground and are able to rotate due to rolling/static friction which in this scenario I'm told would act in the same direction as the moving car.

It is always true. The kinetic friction of the tires opposes the direction of motion of the tires - which is opposite of the direction of the translational motion of the car.