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P201 - Test 3 Sample questions.

1. Choose all the correct statements about gravity.

a) the force of gravity is the same on all freely falling objects near the earth's surface.

b) astronauts are weightless in satellites because the gravitational force is very small at that distance.

c) the moon creates its own gravitational force.

d) the moon is a freely falling object in the earth's gravitational field.

e) "freely falling" means gravity is the only force acting on an object; thus a football is "freely falling" once it leaves the punter's foot.

f) all objects on which gravity is acting must be moving with the acceleration of gravity.


2. Suppose:

a) the sun underwent a gravitational collapse... what would happen to the orbits of the planets?

b) the earth collapsed to 1/2 its present radius... what would happen to the weight of objects on the surface of the earth?

c) gravity were turned off while you throw a ball horizontally out of a window... what would be the path of the ball?

d) the cables on an elevator snapped while you were standing on the floor of the elevator. What would happen to your mass?


3. In the film "The Apple and the Moon",

a) what was the basic premise or principle that Newton wanted to show?

b) How did he show that his basic premise was true?

c) Why do we say the astronauts are "weightless" when they are in a spaceship which is circling the earth?


4. Shown is a planet in 4 different positions in its orbit around the sun.

a) Show in the drawing how much the planet has "fallen" toward the sun in going from A to B.

b) The velocity vector is shown at A. Is it longer or shorter

at B?

at C?

c) Draw the acceleration vector on the planet at D.


5. Canning factories in Denver and Omaha both fill cans with 1 pound of tomatoes. Is there a difference? If so, which can would you prefer to buy? Why?


6. The force F = 16 N pushes the blocks on a frictionless surface with an acceleration of 2 m/s/s. The mass of block 1 is 3 kg.

a) What is the force T which block 2 exerts on block 1? Use Newton's 2nd law on block 1.

b) How large is the normal force on block 1? Use Sum Fperp.

c) What is the mass of block 2? Use Newton's 2nd law on block 2.


7. Identify the correct statements. Correct the incorrect statements. According to Newton:

a) A body with no net force on it must be at rest.

b) There must always be a force in the direction of motion.

c) A large body hitting a small body exerts a larger force on it than the reaction force if it causes the small body to move.

d) The gravitational force on a body has a reaction force on the surface of the earth.

e) Walking is an application of Newton's third law.

f) The apple and the moon are both freely falling bodies pulled by earth's gravity.


8. A freely falling apple has a weight of 1 Newton. Earth's mass is 6 x 10^24 kg.

a) How strong is the force of earth on the apple?

b) How strong a force does the apple exert on the earth?

c) How big is the apple's acceleration?

d) How big would the earth's acceleration toward the apple be if it were the only force acting on the earth?

e) How far would the earth move in the first second of fall?


9. A child of weight of 400 N is swinging with a velocity of 5 m/s at the bottom of the motion. If the length of the swing is 4 m, what is the tension in each of the two chains?


10. Answer the following using Newton's appropriate law.

a) Draw a force diagram showing the forces on a rocket during liftoff. Which force is largest? What is the direction of net force?

b) Two masses collide. What is the same before and after the collision?

c) What are the conditions that allow a satellite to stay at one point above the earth?

d) Does earth's gravity pull more strongly on a block or wood or a block of iron having the same size?

e) Does earth's gravity pull more strongly on a block of wood or a block of iron having the same mass?

f) Explain why you can float in water.


Answers to Sample Test 3.

1. a) False... b) False... c) True... d) True... e) True... f) False

2. a) They would be unchanged... b) They would become four times as great. ...c) it would follow a straight line at constant speed.... d) Mass would be unchanged.

3. a) That his force of gravity F = G M m/r2 and his law of inertia F = m a would explain the empirical findings of Galileo and Kepler.... b) For Galileo F = G M m/r2 = m a and mass m cancelled; for Kepler F = G M m/r2 = m v2 / r led to law that T2 = 4 p2 / G r3. c) Because space ship is freely falling.

4. a) Extend velocity vector at A ... then draw a line down to B which extended would go through sun. b) it is longer at B since force points toward sun and thus has component along path toward B; at C, the velocity reaches its maximum value. c) It points directly toward sun since Force on planet is gravitational force.

5. Since weight (lb) is mg... since g is less in Denver, m must be greater in Denver... so buy the tomatoes packed in Denver.

6. a) F - T = m1 a... 16 Newtons - T = 3 kg x 2 m/s/s... Thus T = 16 Newtons - 6 Newtons = 10 Newtons. b) N1 - M1 g = 0... N1 - 3 kg x 10 m/s/s = 0... N1 = 30 Newtons. c) T = M2 a... .... ..10 Newtons = M2 x 2 m/s/s... M2 = 5 kg.

7. a) A body with no net force on it can be at rest or moving with constant velocity. b) There does not need to be a force in the direction of velocity but in the direction of the acceleration. c) A large body hitting a small body exerts an equal and opposite force to the force the small body exerts on it. d) The gravitational force on a body has a reaction force on the center of the earth. e) True. f) True.

8. a) 1 Newton. b) 1 Newton. c) 9.8 m/s/s. d) 1 Newton = mass of earth x acceleration of earth. acceleration of earth = 1 Newton / 6 x 10 24 kg. e) one half of acceleration value.

9. Fperp = m aperp.... 2 T - m g = m v 2 / r... 2 T = 400 N + 40 kg (5 m/s) 2 / 4 m = 450 Newtons.... T = 225 Newtons.

10. a) Thrust arrow up due to reaction force of gases being pushed out bottom of rocket.... Gravity arrow down due to weight of rocket.... Thrust must be larger so net force is up. b) Total momentum. c) It must have same period of rotation as the earth... 1 day.... It must be above the equator and moving in same direction...toward East... in its orbit. d) more force on the iron since it would have more mass in the same volume as the wood. e) Force would be same since both have the same mass. f)... water produces buoyant force up equal to weight of body.