KNES 361 Exam 2

Friction, Momentum, Torque

What is friction?

A force of resistance between two surfaces in contact.

What direction does friction act?

Opposite to the direction of motion or motion tendency.

Where does friction act?

Over the area of contact between two surfaces.

What is the unit of friction?

Newton (N) or kg·m/s².

What causes friction between surfaces?

Surface irregularities between contacting surfaces.

Are surfaces ever 100% smooth?

No.

How does surface roughness affect friction?

Rougher surfaces produce more frictional force.

How does surface smoothness affect friction?

Smoother surfaces produce less frictional force.

How can friction act as a protective mechanism?

It provides resistance needed for activities such as walking and driving.

How can friction hinder motion?

It can resist smooth movement when pushing objects or in certain sports.

What is the formula for frictional force?

f = μ × R

What does μ represent?

Coefficient of friction.

What does R represent?

Normal reaction force exerted upward by a surface.

What is coefficient of friction?

A constant value for surfaces that describes their frictional interaction.

What factors affect coefficient of friction?

Surface material, relative motion, and type of motion such as rolling or sliding.

What does a higher coefficient of friction mean?

More friction is present between the surfaces.

What is the coefficient of friction for cartilage on cartilage?

0.005

What is static friction?

Friction that resists the initiation of motion.

When can an object begin moving?

When applied force exceeds maximum static friction.

What is maximum static friction?

The maximum frictional force resisting the initiation of motion.

What is the formula for maximum static friction?

Fm = μ × R

On a level surface, how are weight and normal reaction force related?

Weight = normal reaction force.

What is the formula for weight used in friction problems?

Weight = m × g

How can maximum static friction be calculated using mass?

Fm = μ × m × g

How does object weight affect maximum static friction?

Heavier objects have greater maximum static friction.

What happens to friction at the point of impeding motion?

It transitions from maximum static friction to kinetic friction.

What is kinetic friction?

Friction that resists motion after an object begins moving.

When does kinetic friction begin acting?

After the object comes into motion.

What is the formula for kinetic friction?

Fk = μk × R

How can kinetic friction be calculated using mass?

Fk = μk × m × g

Which is greater, kinetic friction or maximum static friction?

Maximum static friction is greater.

Which is harder: starting a box or continuing its motion?

Starting the box.

Why is starting a box harder than continuing its motion?

Kinetic friction is less than maximum static friction.

For constant velocity, how should applied force compare with kinetic friction?

Applied force equals kinetic friction.

What is linear momentum?

A property of an object due to its motion.

What is the formula for momentum?

P = m × v

What is the unit of momentum?

kg·m/s

Is momentum scalar or vector?

Vector.

Why is momentum a vector quantity?

Because velocity is a vector quantity.

What two variables determine momentum?

Mass and velocity.

Does a stationary object have momentum?

No; its velocity is zero, so its momentum is zero.

What happens to momentum as velocity increases?

Momentum increases.

What happens to momentum as velocity decreases?

Momentum decreases.

How are momentum and velocity related?

Momentum is directly proportional to velocity.

What happens to momentum when an object accelerates?

It gathers momentum as velocity increases.

What happens to momentum when an object decelerates?

It loses momentum as velocity decreases.

What must be applied to change an object's velocity and momentum?

An external force for a certain period of time.

What is impulse?

Force applied for a given period of time to change an object's momentum.

What is the formula for impulse?

I = F × t

What are the units of impulse?

Newton-second (N-s) or kg·m/s.

Is impulse scalar or vector?

Vector.

Why is impulse a vector quantity?

Because force is a vector quantity.

What does the area under a force-versus-time graph represent?

Impulse.

How can impulse magnitude be changed?

Change the amount of force and/or duration of force application.

What happens to impulse when force increases while time stays constant?

Impulse increases.

What happens to impulse when force application time increases?

Impulse increases.

How can the same impulse involve different forces and times?

A larger force can act for less time or a smaller force for more time.

What happens when impulse acts opposite an object's motion?

The object's momentum decreases.

What is an example of impulse opposite to motion?

Applying the brakes while driving.

What happens when impulse acts in the direction of motion?

The object's momentum increases.

What is an example of impulse acting with motion?

Pressing the gas pedal.

What does impulse-momentum theory state?

Impulse applied equals change in momentum.

What is the basic impulse-momentum equation?

I = ΔP

What is the expanded impulse-momentum equation?

F × t = mvfinal − mvinitial

What is another form of the impulse-momentum equation?

F × t = Pfinal − Pinitial

Which law is used to explain impulse-momentum theory?

Newton's Second Law of Motion.

What is Newton's Second Law equation used in the derivation?

ΣF = m × a

How is acceleration written using velocity and time?

a = (vfinal − vinitial) / t

How do you derive impulse-momentum from Newton's Second Law?

F = m(Δv/t), then multiply by t to get Ft = mvfinal − mvinitial.

What impulse is required to stop an object with 1000 kg·m/s momentum?

1000 N-s applied in the opposite direction.

What is an object's momentum after it completely stops?

0 kg·m/s.

Why are large forces applied over short periods dangerous?

They produce a large impact on the human body.

How are protective devices designed to reduce impact?

They increase the time over which force is applied.

What material property helps protective equipment extend collision time?

Deformable material.

What examples of protective devices are given in the slides?

Soft Walls, airbags/seat belts, running shoes, and running tracks.

Why does extending collision time reduce impact?

The same impulse can occur with less force over a longer time.

Why does a trampoline reduce the impact of a fall?

It stretches and increases the duration of impulse.

Why should a baseball player swing through the ball?

Longer contact allows maximum momentum to be imparted to the ball.

What is the law of conservation of linear momentum?

Without external forces, total momentum before collision equals total momentum after.

What is the momentum conservation equation?

P1 + P2 = P1′ + P2′

What is the mass-velocity form of momentum conservation?

m1v1 + m2v2 = m1v1′ + m2v2′

What condition is required for conservation of momentum in the slides?

Absence of external forces.

What happens to objects after an elastic collision?

They remain separated.

Is total momentum conserved in an elastic collision?

Yes.

What is the elastic collision momentum equation?

m1v1 + m2v2 = m1v1′ + m2v2′

What happens to objects after an inelastic collision?

They remain in contact.

Is momentum conserved in an inelastic collision?

Yes.

How do objects move after an inelastic collision?

Together as one mass with the same velocity.

What is the inelastic collision equation?

m1v1 + m2v2 = (m1 + m2)vfinal

What velocity do two objects have after an inelastic collision?

The same final velocity.

What type of motion does a linear force produce?

Linear motion.

What type of motion does a rotational force applied at an angle produce?

Angular motion.

What is torque?

Angular or rotational force, also called moment of force.

What is another name for torque?

Moment of force.