Chapters 7-8 Test Review: ANSWERS

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1. What is true of the tangential speed of an object with uniform circular motion?  tangential speed is constant 2. When calculating gravitational force, what is the official name of “Big G?”  constant of universal gravitation 3. What is centripetal force?  net force directed toward the center of an object’s circular path 4. Give some examples of forces that can be a centripetal force.  friction, tension, gravity 5. If the distance between two objects is tripled, what happens to the gravitational force?  it is 1/9 of the original 6. To calculate gravitational force the distance between two masses must be known. How is the distance measured between the two masses?  center to center 7. What is the centripetal acceleration if: vt = 7.0 m/s; r = 0.34 m? v 2 (7.0m / s) 2  a  t   140m / s 2 c r 0.34m 8. What is the centripetal force if: m = 90.0 kg; vt = 13.2 m/s; r = 11.5 m? mv 2 (90.0kg)(13.2.m / s) 2  F  t   1360N c r 11.5m 9. How are the directions of centripetal acceleration, tangential acceleration, tangential speed, and centripetal force related?  centripetal and tangential accelerations are perpendicular, centripetal acceleration and centripetal force both point toward the center of the circle, tangential acceleration and tangential speed are both tangent to the circle 10. If you make a sharp right turn, which way will a loose backpack in the backseat go? What is the cause of this?  to the left side of the car, inertia 11. Does gravity act on an astronaut in space? Why does he fell weightless?  yes, there is no normal force 12. When Mr. Miles couldn’t remove the axle nut from his VW Beetle, what did he do? Why?  put a pipe on the end of his breaker bar, since torque is the product of force and distance he was able to increase the torque by increasing the distance 13. How much more force does it take to open a door if you push at the middle as opposed to the handle?  twice as much 14. You and your friends go to the park and ask you to push them on the merry-go-round. If you apply a force of 500 N tangent to the edge which is 1.5 m from the axis, what is the torque?    F  d  (500N)(1.5m)  750N  m 15. David was playing football in the backyard when his friend, Nathan, got his finger stuck between two fence boards. David ran inside the house and came out with a butter knife. When Nathan saw this he yelled, “No, don’t cut off my finger!” But David, being quite the young engineer, had PHYSICS GOHS

other plans. What type of simple machine did David have in mind for freeing Nathan’s finger? What are the five other types of simple machine?  lever; pulley, inclined plane, wheel and axle, wedge, screw 16. What provides the centripetal force for a car turning a corner? A moon orbiting a planet?  friction between tires and road, gravity between moon and planet 17. Two race cars enter a turn side by side and exit the turn side by side. Which car has the greater tangential speed?  car on outside of turn because of greater radius 18. A bicyclist is riding at a tangential speed of 13.2 m/s around a circular track with a radius of 40.0 m. If the combined mass of the bicycle and rider is: 86.5 kg, what is the centripetal acceleration? What is the centripetal force? v 2 (13.2m / s)2  a  t   4.36m / s 2 c r 40.0m mv 2 (86.5kg)(13.2.m / s)2  F  t   377N c r 40.0m 19. Two students at their desks in physics class are sitting 2.50 m apart. If one student has a mass of 50.0 kg and the other has a mass of 60.0 kg, what is the gravitational force of attraction between the two students?

m1m2 11 Nm2 (50.0kg)(60.0kg) 8  Fg  G  (6.67310 2 )  3.20 10 N r 2 kg (2.50m) 2 20. How would the gravitational force between the Earth and the moon change if…  the mass of the Earth was doubled?  force would be double  the distance between the centers of the Earth and Moon was doubled?  force would be one forth  the mass of the moon was halved?  force would be one half  the distance between the centers of the Earth and Moon was halved?  force would be four times as much 21. Why is the force of gravity so much larger on the Earth than it is on the moon?  The mass of the earth is very large compared to the moon 22. What force causes the Moon to move around the Earth?  Gravity 23. Which direction does the force on the Moon point?  towards center of the earth 24. Which direction will the following object go if the force causing to move in a circle suddenly disappeared? a A. v B. a

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a a P v P

 directly up, in a straight line, tangent to the circle 25. As car is turns a corner, what force keeps the car from skidding? Which way is it directed?  friction, towards the center of the turn 26. What will happen to the car if it hits a patch of ice?  friction will disappear and car will travel in a straight line, tangent to the circle 27. If you knew the height of the bleachers, you could determine the acceleration due to gravity by timing how long it took something to fall. What would be the most likely cause for getting a value that was much larger than expected?  Human error in timing 28. Do a hammer and a feather fall at the same rate? Why or why not?  Only in a vacuum; air resistance 29. If you apply a force to an object, why might it accelerate a rate below what was expected?  A force in the opposite direction such as friction 30. Assuming the height of the bleachers is 7.5 meters and a coin takes 1.0 seconds to hit the ground, what is the coin’s average speed?  7.5m/s 31. Sketch a distance-time graph of an object moving at constant speed.  Straight line with constant slope 32. Sketch a velocity-time graph of an object that is in equilibrium.  Horizontal line 33. What are action-reaction pairs? Give an example.  simultaneous equal but opposite forces resulting from the interaction of two objects, example: the force of a book on a table and the force of the table on the book  The wind applies a 500N force to a 200kg boat. The water resists the motion of the boat with a force of 300N. What is the magnitude of the resulting acceleration?  Fnet = 500N – 300N = 200N a=F/m = 200N/200kg = 1 m/s2