Static friction vs. Kinetic friction

[reising1]

Let's say a car is driving. The friction is pointed towards the motion of the car and it is static (not kinetic) friction.

Given a situation, I'm trying to come up with a way to determine if it's static or kinetic friction. What makes this car situation static friction whereas, for example, a ball on a ramp is kinetic friction?

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[sakabato93]

Let's say a car is driving. The friction is pointed towards the motion of the car and it is static (not kinetic) friction.

Given a situation, I'm trying to come up with a way to determine if it's static or kinetic friction. What makes this car situation static friction whereas, for example, a ball on a ramp is kinetic friction?

Static friction prevents motion from occurring while kinetic friction works against objects in motion.

 

[SN12357]

Let's say a car is driving. The friction is pointed towards the motion of the car and it is static (not kinetic) friction.

Given a situation, I'm trying to come up with a way to determine if it's static or kinetic friction. What makes this car situation static friction whereas, for example, a ball on a ramp is kinetic friction?

Something that is sliding is experiencing kinetic friction. Something that is rolling is experiencing static friction. Kinetic friction in sliding opposes the direction of motion. Static friction in rolling is in the direction of motion, but is playing a roll in torque, not translational force.

Rolling and sliding are completely distinct types of motion. Rolling cannot happen on a frictionless surface.

.....I think this is right. Someone please correct me if I'm remembering wrong.

 

[mehc012]

Something that is sliding is experiencing kinetic friction. Something that is rolling is experiencing static friction. Kinetic friction in sliding opposes the direction of motion. Static friction in rolling is still opposing the direction of motionin the direction of motion, but is playing a roll in torque, not translational force.

Rolling and sliding are completely distinct types of motion. Rolling cannot happen on a frictionless surface.

.....I think this is right. Someone please correct me if I'm remembering wrong.

Yes, this is (mostly) correct. If a ball rolls down a ramp, each tangential point will briefly experience static friction as it contacts the floor, but if a block slides down that same ramp, it is experiencing kinetic friction. Both are in opposition to the motion, as friction can ONLY be in opposition to motion.

Both will slow the objects, but in different ways. The block will accelerate more slowly due to the direct opposition of friction to its movement down the ramp, while the ball will accelerate more slowly because the gravitational energy is going into both translational AND rotational kinetic energies.

 

[reising1]

Yes, this is (mostly) correct. If a ball rolls down a ramp, each tangential point will briefly experience static friction as it contacts the floor, but if a block slides down that same ramp, it is experiencing kinetic friction. Both are in opposition to the motion, as friction can ONLY be in opposition to motion.

No...friction does not always oppose motion....

A car driving forward experiences friction forward as well. This prevents it from sliding backwards.

 

[mehc012]

No...friction does not always oppose motion....

A car driving forward experiences friction forward as well. This prevents it from sliding backwards.

Friction is always an opposition force. It can't be larger magnitude than the other forces on the object, or tables/ramps would push stationary objects around with their friction. It can only be equal to or less than the forces causing movement in magnitude. Thus, if a car is moving forward, there is no net backwards force and so the 'forwards' friction force must be zero or less.

 

[Synapsis]

Yes, this is (mostly) correct. If a ball rolls down a ramp, each tangential point will briefly experience static friction as it contacts the floor, but if a block slides down that same ramp, it is experiencing kinetic friction. Both are in opposition to the motion, as friction can ONLY be in opposition to motion.

Both will slow the objects, but in different ways. The block will accelerate more slowly due to the direct opposition of friction to its movement down the ramp, while the ball will accelerate more slowly because the gravitational energy is going into both translational AND rotational kinetic energies.

This isn't correct.

Thus, if a car is moving forward, there is no net backwards force and so the 'forwards' friction force must be zero or less.

And neither is this.

You don't really get a magnitude of friction less then zero. As a vector, you can denote its direction with a sign, but a friction force will never be less than zero. It will only be zero or greater.

Friction opposes two surfaces from sliding, but that does not necessarily denote that it opposes motion. In the wheels of a car, in the absence of friction, when the tires start moving (as they would to drive the car forward), the surface of the tires would slide backward relative to the ground (clockwise). In a surface with friction, static friction will oppose this sliding, and point forward in the direction of motion.

 

[mehc012]

This isn't correct.



And neither is this.

You don't really get a magnitude of friction less then zero. As a vector, you can denote its direction with a sign, but a friction force will never be less than zero. It will only be zero or greater.

Friction opposes two surfaces from sliding, but that does not necessarily denote that it opposes motion. In the wheels of a car, in the absence of friction, when the tires start moving (as they would to drive the car forward), the surface of the tires would slide backward relative to the ground (clockwise). In a surface with friction, static friction will oppose this sliding, and point forward in the direction of motion.

In the example you give with the car driving forward, they are opposing the motion of the tire surface...which is backwards. The motion of the car body is irrelevant as that is not the surface contacting the ground. However, it is true that 'motion' is perhaps not the best term to use when discussing static friction, as by definition, the surfaces never move relative to each other. How about 'the friction force always opposes the sum of the parallel component of all other forces acting on the object, and must be equal to or less than that force.'

In a block, the friction force will be in the opposite direction that it is being pulled. With a ball or tire, it will be static, but still in the opposite direction of the rotation.

 

[Synapsis]

In the example you give with the car driving forward, they are opposing the motion of the tire surface...which is backwards. The motion of the car body is irrelevant as that is not the surface contacting the ground. However, it is true that 'motion' is perhaps not the best term to use when discussing static friction, as by definition, the surfaces never move relative to each other. How about 'the friction force always opposes the sum of the parallel component of all other forces acting on the object, and must be equal to or less than that force.'

In a block, the friction force will be in the opposite direction that it is being pulled. With a ball or tire, it will be static, but still in the opposite direction of the rotation.

Well we don't know what's relevant or not because it depends on the question. A question can ask if the static frictional force is in the same or opposite direction of the motion of the car. At this point the car is relevant and you're forced to think this way.

I agree that friction often opposes motion, but we need to be careful because this isn't always the case. In order to avoid mistakes, it's good to think of it as a force opposing the sliding of surfaces which just so happens to frequently be the opposite direction of motion. Whatever works for you. But it surely won't be less then zero (otherwise we ironically get the case that you correctly rejected earlier).