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Constant Net Force Model- Newton’s 2nd Law of Motion
Do your work in your logbook or on a separate paper.
Must be completed in class. Due at the end of the next class period. Date due ________
1. An elevator is moving up at a constant velocity of 2.5 m/s, as illustrated in the diagram below: The man
has a mass of 85. kg.
a. Construct a force diagram for the man.
b. What force does the floor exert on the man?
2. The elevator now accelerates upward at 2.0 m/s2.
a. Construct a force diagram for the man.
b. What force does the floor now exert on the man?
3. Upon reaching the top of the building, the elevator accelerates
downward at 3.0 m/s2.
a. Construct a force diagram for the man.
b. What force does the floor now exert on the man?
4. While descending in the elevator, the cable suddenly breaks. What is the force of the floor on the man?
For each of the problems below, you must begin your solution with a force vector diagram of some sort. Some
might require more than one diagram.
5. A 4600 kg helicopter accelerates upward at 2.0 m/s2. What lift force is exerted by the air on the
propellers?
6. The maximum force that a grocery bag can withstand without ripping is 250 N. Suppose that the bag is
filled with 20. kg of groceries and lifted with an acceleration of 5.0 m/s2. Do the groceries stay in the
bag?
7. A student, standing on a scale in an elevator at rest, sees that his weight is 840 N. As the elevator
rises, his weight increases to 1050 N, then returns to normal. When the elevator slows to a stop at the
10th floor, his weight drops to 588 N, then returns to normal. Draw a motion map for the student during
his elevator ride. Determine the acceleration at the beginning and end of the trip.
8. A sign in an elevator states that the maximum occupancy is 20 persons. Suppose that the safety
engineers assume the mass of the average rider is 75 kg. The elevator itself has a mass of 500 kg.
The cable supporting the elevator can tolerate a maximum force of 30, 000 N. What is the greatest
acceleration that the elevator's motor can produce without snapping the cable?
For these problems, you will have to use kinematics graphs or formulas as well as Newton's 2nd Law.
9. A race car has a mass of 710 kg. It starts from rest and travels 40.0m in 3.0s. The car is uniformly
accelerated during the entire time. What net force is acting on the car?
10. Suppose that a 1000 kg car is traveling at 25 m/s. Its brakes can apply a force of 5000N. What is the
minimum distance required for the car to stop?
11. A 65 kg person dives into the water from the 10 m platform.
a) What is her speed as she enters the water?
b) She comes to a stop 2.0 m below the surface of the water. What net force did the water exert on
the swimmer?
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12. During a head-on collision, a passenger in the front seat of a car accelerates from 13.3 m/s
(≈ 30 miles/hour) to rest in 0.10 s.
a) What is the acceleration of the passenger?
b) The driver of the car hold out his arm to keep his 25 kg child (who is not wearing a seat belt) from
smashing into the dashboard. What force must he exert on the child?
c) What is the weight of the child?
1 lb
d) Convert these forces from N to pounds. (x 4.45N ). What are the chances the driver will be able to
stop the child?
13. A 20 kg mass is pulled by a tension force and resisted by a friction force of 5N as shown below.
Ff
20 kg
FT
a. Draw a force vector map for the block.
b. If the acceleration is 3m/s2 what is the tension?
c. How long will it take for the block to slide 30.0m?
14. An applied 25 N force pushes on a 5.0 kg block resting on a frictionless horizontal surface.
25 N
a. Draw a force vector map for the block..
b. What is the acceleration of the block?
c. What is the normal force on the block?
5 kg
15. A 70.0 kg box is pulled by a 350.N tension force. The
kinetic friction is 50N. Draw the force vector map for
the box.
a) Calculate the acceleration of the box.
b) calculate the normal force acting on the block.
70kg
17. A 20 kg block (A) rests on a frictionless table; a cord attached to the
block extends horizontally to a pulley at the edge of the table. A 10
kg mass (B) hangs at the end of the cord.
a) Clearly draw and label the force vector map for forces acting on
the system as a whole.
b) Calculate the acceleration of the block and mass as an entire
system.
c) Draw a force vector map for block A alone.
d) Calculate the tension in the cord while only using block A as your
system.
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18. To the right is a picture of an Atwood's Machine: two masses attached to a
frictionless, massless pulley (pretty neat how physicists dream up equipment
like this, huh?).
The mass of block A is 5.0 kg, and the mass of B is 2.0 kg.
a) What is the acceleration of the system when the blocks are released?
b) How long will it take for block A to fall 2.0 m?
19-23 are all talking about the same situation
19. While being thrown, a net force of 132 N acts on a baseball (mass =
0.140kg) for a period of 4.5 x 10-2 sec. What is the magnitude of the
change in momentum of the ball?
20. If the initial speed of the baseball in question 3 is vi = 0.0 m/s, what will its speed be when it leaves the
pitcher's hand?
21. When the batter hits the ball, a net force of 1320 N, opposite to the direction of the ball's initial motion,
acts on the ball for 9.0 x 10-3 s during the hit. What is the change in momentum of the ball? What is the
final velocity of the ball?
22. What force does the ball exert on the bat in the question above? Explain.
23. A rocket, weighing 4.36 x 104 N, has an engine that provides an upward force of 8.90 x 105 N. It
reaches a maximum speed of 860 m/s.
a. Draw a force vector map for the rocket.
b. How long must the engine burn in order to reach this speed?
24. A tennis player returns a 30. m/s serve straight back at 25. m/s, after making contact with the ball for
0.50 s. If the ball has a mass of 0.20 kg, what is the force she exerted on the ball?
25. Discuss the following in terms of impulse and momentum:
a. Why are padded dashboards safer in automobiles?
b. Why are nylon ropes, which stretch considerably under stress, favored by mountain climbers?
c. Why is it preferred that railroad cars be loosely coupled with slack between cars?
26. If an 18 wheeler and a Smart Car traveling at equal speeds have a head-on collision, which vehicle will
experience the greatest force of impact?
Which will experience the greatest change in momentum?
Which will experience the greatest acceleration?
Assume that the car shown below is going at a constant speed
27. Construct a qualitative motion map of the car.
28. Is the car experiencing acceleration? If your response is
yes, in what direction is the acceleration? Explain how you
know.
29. Construct a qualitative force vector map for the car when
it's at the top of the hill. (Justify the relative forces in your force diagram.)
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30. Suppose the speed of the car is 11.1 m/s and the radius of curvature (r) is 25 m; determine the
magnitude of the centripetal acceleration of the car.
31. If the mass of the car is 1200 kg, what
32. Now, put numbers on your force vector map for the car.
33. At what speed would the centripetal force equal the force of gravity?
34. Suppose the car were going faster than the speed that you calculated for question 7; describe what
would happen to the car.
Assume that this car is going at a constant speed.
35. Construct a qualitative motion map of the car in Figure 2.
36. In what direction is the car in Figure 2 experiencing acceleration? Explain how you know.
37. Construct a qualitative force vector map for the car when it's at the bottom of the hill. (Justify the relative
sizes of the forces in your force vector map.)
38. Suppose the speed of the car in Figure 2 is 15.6 m/s (≈ 35 mph) and the radius of curvature (r) is 23 m;
determine the magnitude of the centripetal acceleration of the car.
39.
etal acceleration?
40. Now, put numbers into your force vector map for the car in figure 2.
41. If the driver of the car weighs 540 N, what is the magnitude of the upward force that the seat exerts on
the driver?
42. A 45 kg boy sits on a horse on a carousel 5.0 m from the center of the circle. He makes a revolution
every 8.0 s.
a. Calculate his speed (Hint: speed 
distance 2 r
)

time
t
b. What is the centripetal force acting on the boy?
43. A 35 kg child is on the outside edge of a merry-go-round with a radius of 2 meters. The child
experiences a net force of 500N. What is the speed of the merry-go-round?
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