## Unit 2 163 Your Turn Solutions

1. As an airplane climbs:

a. its mass increases.

b. its mass decreases.

c. its weight increases.

d. its weight decreases.

2. The weight of an object:

a. is the quantity of matter it contains.

b. refers to its inertia.

c. is basically the same quantity as its mass but expressed in different units.

d. is the force with which it is attracted to the earth.

3. The weight of a 600 gram of salami is ____ Newtons.

a. 0.06 b. 6 c. 600 d. 6000

4. A body has a weight of 400 N. Its mass is ____ kg.

a. 4000 b. 400 c. 40 d. 4

5. When a body is moved from sea level to the top of a tall mountain, what changes - the body's mass, its weight, or both? Explain.

As the distance from the center of the earth increases, the force of gravity between an object and the earth decreases so its weight decreases.

6. The weight of a man is 750 N. What is his mass?

7. The mass of a dog is 37 kg. What is its weight?

8. 2.2 pounds is about 10 Newtons. Calculate your own weight, in Newtons. Calculate your mass in kg.

Multiply your weight in pounds with 4.45 to obtain your weight in newtons.

Divide your newton weight by ten to recieve your weight in kilograms.

9. A net force is being applied to an object when it moves to the right with a:

a. constant speed of 50 miles per hour.

If speed is constant, there is no net force.

b. constant acceleration of 20 mi/hr/min.

If acceleration exists, a net force is present.

10. Is a net force being applied to an object when the object is moving downward

(a) with a constant acceleration of 9.8 m/s/s?

If acceleration exists, a net force is present.

(b) with a constant speed of 12 m/s? Explain.

If speed is constant, there is no net force.

11. Newton's second law indicates that when a body is accelerating, a net force must be acting on the body. Does this mean that when two or more forces are applied to an object simultaneously, the object must always accelerate? Explain.

Forces can cancel/balance, so acceleration may or may not occur when 2 or more forces are exerted on an object.

12. A 10.0 kg mass is acted upon by a force of 95 N. What is the acceleration which results?

13. An unbalanced 16 N force is applied to a 2 kg mass. What is the acceleration of the mass?

14. A shot-putter exerts an unbalanced force of 140 N on a shot giving it an acceleration of 19 m/s/s. What is the mass of the shot?

15. A child can exert a forward push of 32 N. What acceleration can she give to a loaded wagon with a mass of 10 kg?

16. A racing car undergoes a uniform acceleration of 8.00 m/s/s. If the unbalanced force causing the acceleration is 6.00 X 103 N. What is the mass of the racing car?

17. A racing car has a mass of 710 kg. It accelerates from rest as shown in the velocity vs. time graph above.

a. Determine the acceleration of the car in the first 3.0 seconds.

b. Determine the net force acting upon the car during the first 3.0 seconds.

c. From 3.0 seconds to 6.0 seconds, is it accurate to say that no force acts upon the car?

Explain.

It is accurate to say that since there is no acceleration, there is no net force. However, there may be (in fact there almost certainly are) forces exerted on the car.

18. A 500 gram ball lying on the ground is kicked with a force of 250 N. If the kick lasted 0.020 seconds, the ball flew off with a speed of ___ m/s.

a. 0.01 b. 0.1 c. 2.5 d. 10

19. An artillery shell has a mass of 55 kg. The shell (which starts at rest) is fired from the muzzle of a gun with a speed of 770 m/s. Using the velocity vs. time graph above, determine the average force on the shell while it is in the gun barrel?

20. On a given mass the force of 55 Newtons acts to the right and a force of 33 N acts to the left. If the resulting acceleration is 4.4 m/s/s, what mass is being accelerated?

21.A boy and wagon have a combined mass of 50.0 kg. A 75 N force pushes the boy and wagon to the right while a 25 N force of friction pulls the boy and sled to the left. The resulting acceleration is ___ m/s/s.

a. 0.50 b. 1.0 b. 1.5 c. 2.0

22. During reentry of a 11, 000 kg satellite, it is acted upon by air resistance that supplies an upward force of 140,000 N. What is the resulting acceleration of the satellite?

23. A falling coconut has a mass of 1.5 kg, and, at one point of its drop, it experiences an air resistance of 8.7 N. Calculate the magnitude and direction of the acceleration of the coconut.

24. A truck of mass 2100 kg is moving at 21 m/s and is acted upon by two constant forces: The forward force of 810 N due to the engine, and a 220 N retarding force due to friction. (a) At what rate is the truck gaining speed? (b) How far will it travel in 8.0 seconds?

25. An 82 kg person reaches terminal velocity. What is the force of air resistance?

Fair=(8.2kg)(9.8m/s)=80.36N

26. A force of 15 N gives an object an acceleration of 5.0 m/s/s. What force would be needed to give it an acceleration of 1.0 m/s/s?

27. An 85 kg firefighter slides 8.0 meters down a brass pole in a fire station. If there is an upward 620 Newton frictional force acting upon the firefighter, determine the time it would take him to reach the bottom of the pole.

28. What is the difference between static and kinetic friction?

There is really no physical difference. They are just terms used under different circumstances. Static for when an object experiencing friction is stationary (or when the force applied to an object is less than the maximum static friction force). And kinetic for when an object experiencing friction is in motion.

29. Explain what it means to say that friction always opposes motion.

The force of friction always acts in the opposite direction of motion causing an object to lose energy.

30. It is reported that the coefficient of kinetic friction is 0.44. Please write out a full explanation of what this means.

This means that the amount of force friction will provide for each newton of weight put on the surface, is .44 N.

31. A force of 42 N is needed to start a 32 kg steel box moving across a wooden floor. What is the coefficient of static friction?

Force of static friction is that which resists the initial movement between two objects in contact. Since 42 N is needed to start the box moving, it can be assumed that the force of static friction is also 42 N.

32. 35 N of force is needed to keep a 7.8 kg box sliding at a constant speed. (a) What is the coefficient of kinetic friction? (b) What is the force of friction?

Ff=35

33. Calculate the acceleration of a moving 75 kg object with a horizontal force of 65 N acting upon it. The 75 kg object lies on a horizontal plane with a coefficient of kinetic friction of 0.33.

34. Thumper jumps upon a patch of horizontal ice. If he began with a speed of 1.6 m/s and slid to a stop in a distance of 6.3 meters, what is the coefficient of kinetic friction between the fur and ice?

If the force of kinetic friction is the only horizontal force acting on the bunny (and we'll assume that it is) Ff = Fnet. Therefore:

It is necessary to find the acceleration that Thumper experiences.

35. Wood on wood has a coefficient of friction of 0.33. If it takes a force of 7.6 N applied horizontally to pull the wood along at a constant speed, what is the mass of the object?

If the object is moving at a constant velocity, Fnet=0; Fapp = Ff and Fgrav = FN

36. The mass of an elevator plus its occupants is 752 kg. The upward tension on the cable is 8950 N. At what rate does the elevator accelerate upward?

37. Galileo once performed an experiment where he dropped two rocks from the leaning tower of pisa. If the two rocks had masses of 1 kg and 5 kg respectively.

a. Determine the weight of each rock.

b. Determine the acceleration of each rock after Galileo dropped them.

If air resistance is negligible:

c. After Galileo dropped the rocks he ran down the stairs to see which hit first. Which rock did he see hit the ground first? Explain.

In order for Galileo to travel down the stairs at a higher vertical velocity than the rocks, he would need an additional downward force (besides gravity) exerted on him so that his acceleration would be higher than 10 m/s2. Assuming this happens, Galileo would see that both rocks would hit the ground at the same time (once again neglecting air resistance). However, there are some recent findings that do not support this idea entirely but the differences are extremely subtle.

38. 60 kg Rock Dropin paid an exorbitant amount of money to be brought up in a plane and thrown out. As he dropped through the air, there were varying amounts of air resistance acting upon him. For each of the following air resistances, determine Rock's net vertical force and net vertical acceleration.

a. Fair = 200 N

b. Fair = 800 N

c. Fair = 600 N

39. Some physicists say that you cannot apply a force without being forced. Explain what they mean.

They are referring to newtons third law: for every reaction there is an equal and opposite reaction. Basically, if you push something, it will push back on you. The only way to not accelerate is to have something else forcing you (i.e. the earth).

40. A boxer punches a bag with a force of 1000 N. The bag pushes the boxer with a force:

a. of less than 1000 N.

b. of more than 1000 N.

c. of 1000 N.

d. that is not determinable with the given information.

41. A 72 kilogram person is pulled down by the mass of the earth with a force of 720 N. The force with which the person pulls up on the earth is:

a. less than 720 N.

b. more than 720 N.

c. is 720 N.

d. 0 N.

42. If a 65 kg parachutist jumps off a cliff (and perhaps drinks a dew), the earth pulls her down with a force of 650 N. As a result she accelerates downward.

a. With what force does she pull the earth upward? Explain.

650 N. From newtons third law.

b. Explain why the earth doesn't accelerate a noticeable amount up to her.

Because the mass of the earth is so big that any small force divided b its mass will be very close to zero.

43. A gentleman holds a 45 N tray of food in his hand. Considering Newton's third law, what is the reaction force to the force to the force of gravity pulling down upon the 45 N tray? Explain.

The tray pulling up on the earth. Newtons third law.

44. An automobile that is towing a trailer is accelerating on a level road. The force that the automobile exerts on the trailer is:

a. equal to the force the trailer exerts on the automobile.

b. greater than the force the trailer exerts on the automobile.

d. less than the force the trailer exerts on the automobile.

d. equal to the force the trailer exerts on the road.

e. equal to the force the road exerts on the trailer.

45. The action and reaction force do not cancel out because they do not act upon the same object. Your author, Paul Hewitt, discusses this fact with a horse and a cart cartoon. The horse uses friction to force the ground backwards. In other words the action force is the horse forcing the ground backwards. What is the reaction to that force and what is the result of that force?

The reaction is the ground forcing the horse forward. The result of that force is the acceleration of the cart.

46. When a horse pulls a wagon, the force that causes the horse to accelerate forward is the force:

a. the horse exerts on the wagon.

b. the wagon exerts on the horse.

c. the horse exerts on the ground.

d. the ground exerts on the force.

47. A golf club head causes a 45 gram golf ball to accelerate forward with an acceleration of 500 m/s/s.

a. Determine the net force acting upon the golf ball during the collision.

b. If the golf club has a mass of 200 grams, determine the net force acting upon the golf club during the collision.

22.5 N. The mass makes no difference.

Glenbrook South Physics Team - Tom Henderson, John Lewis, Neil Schmidgall, Dave Smith, Suzanne Webb & Brian Wegley
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Last Updated: August 28, 1997