Define kinetic friction .

Kinetic friction is the friction that occurs when two surfaces in contact move relative to each other.

What causes friction between two solid surfaces?

Imperfections in the contact surfaces of the objects moving, or attempting to move, over one another.

In which direction is the kinetic friction force exerted?

In a direction opposite to the motion of each surface relative to the other surface.

State two effects of kinetic friction on a surface moving over another surface.

It decreases the speed of the moving surface It increases the temperature of the contact surfaces due to heating

Friction is the component of the contact force .......... to the contact surface.

Friction is the component of the contact force parallel to the contact surface.

True or False? Doubling the surface area of contact between two objects doubles the force of friction between them.

False. The magnitude of friction depends on the nature of the surfaces and the normal reaction force , not on the size of the contact area.

Why does the coefficient of kinetic friction have no units?

It is a ratio of two forces between the contact surfaces: the frictional force the normal reaction force

What does the coefficient of kinetic friction depend on?

The material properties of the surfaces in contact.

For an object sliding on a horizontal surface with no other vertical forces, the normal force equals the object's .......... .

For an object sliding on a horizontal surface with no other vertical forces, the normal force equals the object's weight .

True or False? Ice has a higher coefficient of kinetic friction than concrete.

False. Smooth surfaces such as ice have a lower coefficient of kinetic friction than rough surfaces such as concrete.

Define static friction .

Static friction is the friction that may occur between the contact surfaces of two objects that are not moving relative to each other.

How does static friction respond when a forward force is applied to try to move a heavy object?

It increases to match the applied force It prevents the object from moving It can only increase up to a maximum value; once the applied force exceeds this, the object starts to move

What does the sudden slip of an object that begins to move indicate?

The point at which kinetic friction takes over from static friction.

How does the force of kinetic friction compare with the maximum force of static friction for the same object?

Kinetic friction is smaller , so less force is needed to keep an object moving than to start it moving.

Why is a larger forward force needed to start a heavier object moving?

Making an object heavier increases the maximum static friction, so a larger forward force is needed to overcome it.

Static friction acts in the .......... direction to the applied force.

Static friction acts in the opposite direction to the applied force.

True or False? Static friction still acts on an object once it is sliding across a surface.

False. Static friction acts only until the object begins to slide. Once it is sliding, kinetic friction acts instead.

The coefficient of static friction is typically .......... than the coefficient of kinetic friction for a given pair of surfaces.

The coefficient of static friction is typically greater than the coefficient of kinetic friction for a given pair of surfaces.

How large is the kinetic friction on an object pulled horizontally at constant velocity ?

It is equal in magnitude to the applied force, because the net force is zero at constant velocity.

Sliding occurs when two surfaces in contact move relative to each other and kinetic friction applies.

Slipping is a special case of sliding. It occurs when an object that normally rolls does not roll , so its point of contact moves relative to the surface.

Which type of friction applies to a wheel that is rolling along the road?

Static friction , because each point on the wheel makes contact with one point on the road as it rolls.

What happens when the brakes are applied hard to a bike wheel?

The surface of the wheel slides over the ground, so kinetic friction now applies instead of static friction.

What value and direction does static friction adopt?

The value and direction required to prevent an object from slipping or sliding on a surface.

Static friction is at its maximum magnitude when an object is right on the point of .......... .

Static friction is at its maximum magnitude when an object is right on the point of slipping .

True or False? Static friction can exceed the product of the coefficient of static friction and the normal force.

False. This product is the maximum static friction on a given surface. Beyond it the object slips .

AP®PhysicsCollege BoardPhysics 1: Algebra-BasedFlashcardsUnit 2: Force & Translational DynamicsKinetic & Static Friction

Kinetic & Static Friction (College Board AP® Physics 1: Algebra-Based): Flashcards

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Define kinetic friction.

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Define kinetic friction.
Kinetic friction is the friction that occurs when two surfaces in contact move relative to each other.
What causes friction between two solid surfaces?
Imperfections in the contact surfaces of the objects moving, or attempting to move, over one another.
In which direction is the kinetic friction force exerted?
In a direction opposite to the motion of each surface relative to the other surface.
State two effects of kinetic friction on a surface moving over another surface.
- It decreases the speed of the moving surface
- It increases the temperature of the contact surfaces due to heating
Friction is the component of the contact force .......... to the contact surface.
Friction is the component of the contact force parallel to the contact surface.
True or False?
Doubling the surface area of contact between two objects doubles the force of friction between them.
False.
The magnitude of friction depends on the nature of the surfaces and the normal reaction force, not on the size of the contact area.
State the equation for the kinetic friction force.
$|{\vec{F}}_{f, k}| = |μ_{k} {\vec{F}}_{n}|$
- $|{\vec{F}}_{f, k}|$ = magnitude of kinetic friction (N)
- $μ_{k}$ = coefficient of kinetic friction
- ${\vec{F}}_{n}$ = normal reaction force between the two contact surfaces (N)
Why does the coefficient of kinetic friction have no units?
It is a ratio of two forces between the contact surfaces:
- the frictional force
- the normal reaction force
What does the coefficient of kinetic friction depend on?
The material properties of the surfaces in contact.
For an object sliding on a horizontal surface with no other vertical forces, the normal force equals the object's ...........
For an object sliding on a horizontal surface with no other vertical forces, the normal force equals the object's weight.
True or False?
Ice has a higher coefficient of kinetic friction than concrete.
False.
Smooth surfaces such as ice have a lower coefficient of kinetic friction than rough surfaces such as concrete.
Define static friction.
Static friction is the friction that may occur between the contact surfaces of two objects that are not moving relative to each other.
How does static friction respond when a forward force is applied to try to move a heavy object?
- It increases to match the applied force
- It prevents the object from moving
- It can only increase up to a maximum value; once the applied force exceeds this, the object starts to move
What does the sudden slip of an object that begins to move indicate?
The point at which kinetic friction takes over from static friction.
How does the force of kinetic friction compare with the maximum force of static friction for the same object?
Kinetic friction is smaller, so less force is needed to keep an object moving than to start it moving.
Why is a larger forward force needed to start a heavier object moving?
Making an object heavier increases the maximum static friction, so a larger forward force is needed to overcome it.
Static friction acts in the .......... direction to the applied force.
Static friction acts in the opposite direction to the applied force.
True or False?
Static friction still acts on an object once it is sliding across a surface.
False.
Static friction acts only until the object begins to slide. Once it is sliding, kinetic friction acts instead.
State the equation for the static friction force.
$|{\vec{F}}_{f, s}| \leq |μ_{s} {\vec{F}}_{n}|$
- $|{\vec{F}}_{f, s}|$ = magnitude of static friction (N)
- $μ_{s}$ = coefficient of static friction
- ${\vec{F}}_{n}$ = normal reaction force between the two contact surfaces (N)
- $\leq$ means less than or equal to
The coefficient of static friction is typically .......... than the coefficient of kinetic friction for a given pair of surfaces.
The coefficient of static friction is typically greater than the coefficient of kinetic friction for a given pair of surfaces.
How large is the kinetic friction on an object pulled horizontally at constant velocity?
It is equal in magnitude to the applied force, because the net force is zero at constant velocity.
An object is on the point of moving when a 50 N force is applied. The normal force on it is 200 N. What is $μ_{s}$ ?
At the threshold of motion, static friction equals the applied force.
$|μ_{s}| = \frac{50}{200} = 0.25$
True or False?
Static friction always equals the coefficient of static friction multiplied by the normal force.
False.
The product $|μ_{s} {\vec{F}}_{n}|$ is only the maximum possible magnitude of static friction.
Define sliding.
Sliding occurs when two surfaces in contact move relative to each other and kinetic friction applies.
Define slipping.
Slipping is a special case of sliding. It occurs when an object that normally rolls does not roll, so its point of contact moves relative to the surface.
Which type of friction applies to a wheel that is rolling along the road?
Static friction, because each point on the wheel makes contact with one point on the road as it rolls.
What happens when the brakes are applied hard to a bike wheel?
The surface of the wheel slides over the ground, so kinetic friction now applies instead of static friction.
What value and direction does static friction adopt?
The value and direction required to prevent an object from slipping or sliding on a surface.
State the equation for the maximum static friction that prevents an object from slipping.
$|{\vec{F}}_{f, s, max}| = |μ_{s} {\vec{F}}_{n}|$
- $|{\vec{F}}_{f, s, max}|$ = maximum magnitude of static friction (N)
- $μ_{s}$ = coefficient of static friction
- ${\vec{F}}_{n}$ = normal force (N)
Static friction is at its maximum magnitude when an object is right on the point of ...........
Static friction is at its maximum magnitude when an object is right on the point of slipping.
True or False?
Static friction can exceed the product of the coefficient of static friction and the normal force.
False.
This product is the maximum static friction on a given surface. Beyond it the object slips.

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