Ch 6: Circular Motion, Orbits, and Gravity

Question 1

For an object in uniform circular motion, the instantaneous velocity is _______ to the circle at all points.

Answer 1

tangent

p. 176

Question 2

For an object in uniform circular motion, where is the acceleration vector directed at all points?

Answer 2

toward the center of the circle

p. 176

Question 3

The time interval it takes for an object to around a circle one time, completing one revolution, is called the _____ of the motion. ______ is represented by the symbol _.

Answer 3

period
period
T

p. 177

Question 4

Rather than specify the time for one revolution, we can specify circular motion by its frequency, the number of revolutions per second, for which we use the symbol f. An object with a period of 1/2 second completes two revolutions each second. Similarly an object could make 10 revolutions in one second if its period is 1/10 of a second. The formula for frequency shows that frequency is the _______ of the period:

Answer 4

inverse

f = 1/T

Question 5

The unit we use for frequency is:

Answer 5

rev/s

We can also use Hz which is the same as 1 rev/s.

p. 177

Question 6

Frequency is often expressed as revolutions per second, but revolutions are not true units but merely the counting of events. Thus the SI unit of frequency is simply _______ ______ or __. We often see frequency given in revolutions per minute (rpm) or another time interval, but these will need to be converted to s^-1

Answer 6

inverse seconds or s^-1

Question 7

What equation related the velocity of an object in uniform circular motion to the period and the radius?

What about the frequency instead of the period?

Answer 7

v = (2π r)/T

v = 2π f r

Question 8

What is the formula for centripetal acceleration for uniform circular motion?

Answer 8

a = v²/r

or

a = (2π f)²r

Question 9

True or false: You can have uniform circular motion without completing a full circle.

Answer 9

True

p. 176

Question 10

For objects in uniform circular motion that do complete multiple full circles in the same direction, one right after the other, the motion is termed ________.

Answer 10

periodic

p. 177

Question 11

Imagine driving your car at a constant speed when it goes into a dip in the road. At the very bottom of the dip, is the normal force of the road on the car greater than, less than, or equal to the car’s weight?

Answer 11

GREATER

The car is accelerating even though it’s moving at a constant speed, because its direction is changing. When the car is at the bottom of the dip, the center of it circular path is directly above it and so its acceleration vector points straight up. Because the acceleration vector points up, by newtons second law there must be a net force on the car that also points upward. In order for this to be the case the free body diagram shows at the magnitude of the normal force must be greater than the weight. This explains the heavy feeling that you have as you drive through a dip in the road. Because you’re apparent weight, the normal force that supports you, is greater than your true weight.

Question 12

Imagine driving a car on a frictionless road, such as a very icy road. You would not be able to turn a corner. So it must be friction that allows the car to turn, and it must be _______ friction force, because the tires are not skidding. The points where the tires touch the road are not moving relative to the surface.

Answer 12

static