Physics Chapter 8

The frictional quantity that is characteristic of two surfaces in contact is the ________________________ ____ ______________.

Coefficient of friction

The magnitude of the vertical component of gravity for an object resting on a flat surface is __________ ____ the normal force in magnitude.

Equal to

A free-body diagram contains more than one system.

False

A heavier box experiences the same frictional force as a lighter box

False

A moving object experiences more friction than a stationary object.

False

Area of contact does not affect friction.

False

Assuming that a string is ideal _always_ adds much error into your calculations.

False

For an object resting on an incline, the normal force is equal in magnitude to the x-component of the weight of the object.

False

Gravity does _not_ affect an astronaut performing experiments at the International Space Station,

False

The coefficient of friction is a vector quantity.

False

The equation for kinetic friction is a vector equation.

False

The magnitude of the tension in an ideal string changes when going over an ideal pulley.

False

The smoother a surface is, the more friction it can exert.

False

The values of static friction are constant for any given material.

False

Weight does _not_ affect friction.

False

On a level surface, the normal force and the force of gravity is acting on an object are vectors.

False (Their magnitudes are equal)

In an accelerated reference frame, apparent weight is equal to actual weight.

False (i.e. going up/down in an elevator)

Identify one of the forces that act on a locomotive that starts from rest to climb a hill.

Fw, Fn, fs, &c.

When is apparent weight _not_ equal to actual weight?

In an accelerated reference frame

Describe the shape of a Jersey barrier.

It's a low wedge-shaped wall used to divide opposing lanes of highway. It has 2 sections: a steep upper section & a less steep lower section.

The type of friction that is equal to μN is _________ ___________.

Kinetic Friction

When a car skids to a stop, the predominant form of friction slowing the car is __________ _________.

Kinetic Friction

The force that keeps you from falling to the floor when you sit in a chair is the ________ _________.

Normal Force

the only Force besides gravity affecting hanging block is ________.

Tension

When an object is accelerating, how does the apparent weight compare with the actual weight?

The apparent weight is greater than or less than the actual weight, depending on the direction of acceleration.

A block is on an incline. What is the relationship between the normal force and the gravitational force?

The normal force is less than the gravitational force.

The apparent weight is ...

The weight a scale records

An ideal string does _not_ affect a block's acceleration due to gravity because _______________________.

They have no mass.

The friction that makes jogging possible is called _______.

Traction

Coordinate systems are _always_ chosen within a frame of reference.

True

For an object resting on an incline, the normal force is equal in magnitude of the y-component of the object.

True

If a free-body diagram consists of a system of two hanging blocks, one hanging from the other, and the top block is hanging from a fixed surface, then there are only two forces acting on the system.

True

In a free-body diagram, forces that a system is exerting are _not_ shown.

True

It usually requires more force to get a car to start moving than it does to keep it moving.

True

Static friction has a range of values for any two materials.

True

The coefficient of friction is a a scalar quantity.

True

The normal force and the frictional force are related.

True

Traction makes walking possible.

True

*Which of the following is _not_ a characteristic of an ideal string?* a. Ideal strings have mass. b. Ideal strings have no stretch. c. Ideal strings do not exert pushes. d. Forces are exerted only in line with the string.

a. Ideal strings have mass.

Which type of friction is involved in the motionless tires of a car parked on a hill? a. traction b. rolling friction c. static friction d. kinetic friction

a. traction & c. static friction

*Which statement about friction is true?* a. Friction acts perpendicular to the surfaces that are sliding over one another. b. Friction is a force that opposes the relative motion of the two surfaces in contact. c. Friction is dependent on the area of contact. d. Friction acts in the same direction as that of the relative motion.

b. Friction is a force that opposes the relative motion of the two surfaces in contact.

*Which of the following is _not_ a characteristic of an ideal pulley?* a. The axle is frictionless. b. The pulley must be anchored to an immovable surface. c. The motion of the string around the wheel is frictionless. d. The pulley changes the direction of forces without diminishing the magnitude of those forces.

b. The pulley must be anchored to an immovable surface.

Which statement about kinetic friction is _not_ true? a. It is independent of sliding friction. b. It is directly proportional to the normal force. c. It is perpendicular to the contact surface. d. It is usually less than static friction for a given object.

c. It is perpendicular to the contact surface.

*The following types of motion are made possible with friction except which one?* a. Rolling b. Walking c. Sliding d. Braking

c. Sliding

*Which of the following is _not_ an important step when analyzing inclined plane dynamics?* a. Assign coordinate systems to each system element. b. Resolve all the forces acting on the system into their components. c. Determine the maximum static friction possible. d. Calculate the mechanical advantage of the inclined plane.

d. Calculate the mechanical advantage of the inclined plane.

*Which one of the following is _not_ an example of kinetic friction?* a. Shoes rubbing the ground as someone walks b. Someone striking a match c. A soccer ball rolling into the goal d. Your friend trying to push a rock

d. Your friend trying to push a rock

How do Jersey barriers minimize injuries and damage due to car accidents? a. by blocking oncoming traffic b. by exerting the right force at the right angle c. by making highway divisions more visible d. all of the above

d. all of the above

When an object moves in an upward direction, apparent weight __________.

increases

The coefficient of friction ...

is different for different substances

The normal force ...

is perpendicular to the surface.

The direction of friction is __________ the direction of motion.

opposite

The mathematical definition of normal is __________ to a surface.

perpendicular

The x-axis must be __________ to the y-axis in any reference frame.

perpendicular

Ideal strings can exert only _________ __________ on objects.

pulling forces

An ideal string acting on a block ...

pulls in line with the string

The free-body diagram of the whole system is called the __________ __________.

world diagram