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Answers to Readers' Questions on Physical Science:

Friction Causes

The following 3 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Writing a story about a world without friction
• Adhesion force and motion
• Why is kinetic smaller than static?

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Writing a story about a world without friction

Question

February 10, 2009

I am a junior in high school, and I have an assignment that I am having trouble with. I am to write a story or take the role of a news writer about a world without friction on the surface. There is still gravity and all other laws of physics apply, and all thre of Newton's laws of motion apply except no friction on the surface of the planet. I need to explain how things move, turn, and stop. If gravity holds us down but no friction - would we be slidding around the surface? Could we have transportation - how would anything work with no ground friction - no airplanes could take off. I just don't get this - HELP!!! Lennea

Lennea - USA

17245

Answer

You are on the right track. You could imagine that everything was covered with ice or something very slippery.

Obviously, life could not exist, because even the simplest things like eating require friction.

Without friction, something that was moving or rolling on the ground, would continue moving. If a marble would roll down the side of a bowl, it would roll up the other side and back down forever.

The only way something would change directions is if it collided with another object.

If this is to be a story, you might consider an astronaut landing on such a planet. Things would be flying or rolling all over the place. And the astronaut would not be able to get back into the spaceship.

I hope these ideas help. Best wishes in your story.

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Adhesion force and motion

Question

December 17, 2005

Thanks for your reply, but I want to know more about that

Q1)why the adhesion force disappear when an object is moving?
Q2)is the separate of atoms between the two object due to the motion is one of the cause of coefficient of kinetic friction is smaller than the coefficient of static frictrion?if yes,please explain for me.

sorry for my poor english.

kwong - Hong Kong

9405

Answer

Adhesion force is similar to static friction in that once the force is broken and the objects are sliding, the molecules are sliding over each other and do not have an opportunity to settle in between. There still is some adhesion force, but the motion makes it highly ineffective.

Static friction is due to surface roughness and is not at the atomic or molecular level. Once motion is started, the object "bounces" over the irregularities and does not have a chance to settle in the crevices.

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Why is kinetic smaller than static?

Question

December 16, 2005

why the coefficient of kinetic friction is smaller than the coefficient of static frictrion?

KWONG - HONG KONG

9396

Answer

When an object is not moving, it tends to stick to the other surface from adhesive forces. Likewise, when it is moving, it glides over the rough surface.

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