# Unit 6 Extra Credit

Either print this page and complete or (better yet) neatly show your answers on a separate page of paper. All answers must be expressed as complete sentences and/or all work shown to receive credit.

1. A 0.013 kg rubber stopper is attached to a 0.93 m length of string. The stopper is swung in a horizontal circle, making one revolution in 1.18 s.

A. Find the speed of the stopper.

B.Find its centripetal acceleration.

C. Find the force that the string exerts on it.

2. Suppose the mass of the rubber stopper in the example problem above is doubled, but all other given quantities remain the same.

A. How would the velocity, accleration, and force change?

B. If the radius in the example problem were twice as large, but all other given quantities remained the same, how would velocity, acceleration, and force change?

C. If the stopper were swung in the same circle so it had a period half as large as in the example, how would the answers change?

3. A runner moving at a speed of 8.8 m/s rounds a bend with a radius of 25 m.

A. Find the centripetal acceleration of the runner.

B. What supplies the force needed to give this accleration to the runner?

4. Racing on a flat track, a car going 32 m/s rounds a curve 56 m in radius.

A. What is the car's centripetal acceleration?

B. What would the minimum coefficient of static friction between tires and road that would be needed for the car to round the curve without skidding?

5. Sue is swinging a yo-yo around her head. What happens to the size of the centripetal acceleration if the mass of the yo-yo is doubled without changing the period or length of the string?

6. An athlete whirls a 7.00-kg hammer tied to the end of a 1.3-m chain in a horizontal circle. The hammer moves at the rate of 1.0 rev/s.

A. What is the centripetal accleration of the hammer?

B. What is the tension in the chain?

7. Sue whirls a yo-yo in a horizontal circle. The yo-yo has a mass of 0.20 kg and is attached to a string 0.80 m long.

A. If the yo-yo makes 1.0 complete revolution each second, what force does the string exert on it?

B. If sue increases the speed of the yo-yo to 2.0 revolutions per second, what force does the string now exert?

C. What is the ratio of answer b to a? why?

8. A coin is placed on a stereo record revolving at 33 1/3 revolutions per minute.

A. In what direction is the acceleration of the coin, if any?

B. Find the accleration of the coin when it is placed 5.0, 10, and 15 cm from the center of the record.

C. What force acclerates the coin?

D. At which of the three radii listed in b would the coin be most likely to fly off? why?

9. An early major objective to the idea that earth is spinning on its axis was that Earth would turn so fast at the equator that people would be thrown off into space. Show the error in the logic by calculating

A. the speed of 97-kg person at the equator. The radius of Earth is about 6400 km.

B. The centripetal force on the person.

C. The weight of the person

Resources for physics students are available through links on the GBS Physics Home Page. Such resources include the Multimedia Physics Studios (a collection of animations and movies which illustrate key physics concepts) and The Physics Classroom (an online tutorial written in an easy-to-understand language).