Physics Chapter 10: Circular Motion (Part 1)

A bug inside a can whirled in a circle feels a force of the can on its feet. This force acts a. in an inward direction. b. in an outward direction. c. in either an inward or outward direction. d. parallel to the force of gravity.

A.

When you whirl a tin can in a horizontal circle overhead, the force that holds the can in the path acts a. in an inward direction. b. in an outward direction. c. in either an inward or outward direction. d. parallel to the force of gravity.

A.

Whereas a rotation takes place about an axis that is internal, a revolution takes place about an axis that is a. external. b. at the center of gravity. c. at the center of mass. d. either internal or external.

A.

When you whirl a tin can in a horizontal circle overhead, the force that the can exerts on the string acts a. in an inward direction. b. in an outward direction. c. in either an inward or outward direction. d. parallel to the force of gravity.

B.

When you roll a tapered cup across a table, the path of the cup curves because the wider end rolls a. slower. b. at the same speed as the narrow part. c. faster. d. in an unexplained way.

C.

At an amusement park, you and a friend sit on a large rotating disk. You sit at the edge and have a rotational speed of 4 RPM and a linear speed of 6 m/s. Your friend sits halfway to the center. What is her rotational speed? What is her linear speed?

Her rotational speed is also 4 RPM, and her linear speed is 3 m/s.

A heavy iron ball is attached by a spring to a rotating platform, as shown in the sketch. Two observers, one in the rotating frame and one on the ground at rest, observe its motion. Which observer sees the ball being pulled outward, stretching the spring? Which observer sees the spring pulling the ball into circular motion?

The observer in the reference frame of the rotating platform states that centrifugal force pulls radially outward on the ball, which stretches the spring. The observer in the rest frame states that centripetal force supplied by the stretched spring pulls the ball into circular motion. (Only the observer in the rest frame can identify an action-reaction pair of forces; where action is spring-on-ball, reaction is ball-on-spring. The rotating observer can't identify a reaction counterpart to the centrifugal force because there isn't any.)

Train wheels ride on a pair of tracks. For straight-line motion, both tracks are the same length. But which track is longer for a curve, the one on the outside or the one on the inside of the curve?

The outer track is longer—just as a circle with a greater radius has a greater circumference.

How do the wheels of a train stay on the tracks?

The train wheels stay on the tracks because their rims are slightly tapered.

~ centrifugal force

an apparent outward force on a rotating or revolving body; "center-fleeing" or "away from the center"

~ rotational speed

angular speed; the number of rotations per unit of time

~ linear speed

distance traveled per unit of time

~ axis

is the straight line around which rotation takes place

~ centripetal force

the force directed toward a fixed center that causes an object to follow a circular path

~ tangential speed

the speed of something moving along a circular path; the direction of motion is always tangent to the circle

~ revolution

when an object turns about an external axis

~ rotation

when an object turns around an internal axis (an axis located within the body of the object); also called spin