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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 9: Friction

# Force of friction keeping velocity constant

Calculating the coefficient of kinetic friction (correction made in next video). Created by Sal Khan.

## Want to join the conversation?

• There is one thing that puzzles me. If friction is the abrasion between two surfaces, and the repulsion of electrons keeps the surfaces apart, then doesn't that mean that there's no friction at all?
• Electrons are ridiculously small particles. At scales that small, even the smoothest surface* is jagged. It's these jagged points that cause the friction. The electron repulsion keeps the two surfaces apart, but they will still have grooves that fit together that resist motion dependant on the shapes of the grooves on the two surfaces and how they fit together.
At an atomic level these peak and valleys aren't touching, but it still requires energy to raise the peak out of the valley because of the change in elevation. Times this by millions per square centimeter and you have friction.

*there are some crystalline solids that are perfectly smooth when pure

On another note, even the interaction between the electric fields of the two surfaces can cause friction
• What IS the coefficient of static and kinetic friction? and how is it useful? Is it a ratio that stays constant in every situation?
• Yes they are both ratios. They are constant unless the normal force and force of gravity change, which would only happen on some distant planet!
• So the coefficient of static friction is actually .59 right? The typo is still there when it says .72
• yes you have it right. he meant to divide 50 by 49 times square root of 3 but accidentally made an calculator error and typed in 40. Good Question! :D
• I'm a little confused...

If the block has a constant velocity of 5m/second but it isn't moving, how does that work? That doesn't really make sense to me.
(1 vote)
• block isnt accelerating (changing direction, speeding up or slowing down) but it is moving.
• Please explain why do we take smaller steps while walking on a slippery(smooth) surface? How does that help us to balance ourselves? What would happen if we would take long steps then?
• wow; cool question...

Larger steps would mean a larger perpendicular distance between the contact point (foot) and the line of force of the body weight. This would produce a greater moment and increased chance of slipping.
• what is the difference between static and kinetic friction.??
it is very confusing
• Static friction is the force that resists movement when the object is not moving.
Kinetic friction is the force that resists movement when the object is moving.
Here's a very exaggerated example to help imagine what's happening.
Imagine an object and surface that has some deep visible interlocking teeth. You have to push the object hard to move it out of the grooves, and then you keep it moving so that it never gets all the way into any groove just a little bit. So it takes more effort to start the movement than to keep the movement the same.
• What if the incline is flat, yet the force is being applied at an angle? How would you calculate the force of friction then?
• To calculate the friction force, you must first calculate a certain component of the force being applied at an angle. This component of force must parallel to the surface.
What to do next of course depends on the situation you are dealing with. Regardless, the key concept is always to first consider the force component parallel to the surface which the object, on which these forces are applied, touches.
This is in fact just doing what you normally do in any situation, which means that the difference in situations does not make this answer lose generality, but in addition you first have to calculate a component force using vectors, angles and trigonometry.
• I am a little confused about the coefficient of static friction always being greater than or equal to the coefficient of kinetic friction. My confusion stems from when I change the angle of the incline, the ratios change. It appears that the coeff. of kin. friction is just the tangent (angle of incline). So when the angle approaches 90 degrees (vertical) kinetic friction will be greater than static friction unless the "budging force" changes with the incline.
• Just because the frictional force is changing does not mean the coefficient of friction is changing. That value is experimentally determined for different materials acting on different materials. When you figure out the frictional force you need to multiply the frictional coefficient by the normal force, right? Then if you are increasing the incline, the normal force changes because the force of gravity is then affecting the body differently, depending on which way you shift it. When you shift the plane near vertical, the portion of the gravitational force vector in the component of the x-direction grows very small (try it); because the normal force is perpendicular to the plane, this means the normal force is very small, and the body is no longer supported.
• But why is the normal force still the same? shouldn't it be different because the one is wood and the other ice?