Friction and Gravity Guided Reading and Study Answers

Chapter 3 Answer Key

Study Guide

My Notebook

1. Answers will vary, but could include main ideas from the summary or 1 main idea from each Reading (3).
2. Answers will vary, but should include 3 different explanations for the word "pound": (a) customary unit of weight, or force, on a scale (noun); (b) to hit or strike with great force (verb); (c) stray dogs or cats are often taken to a local building called a "pound" and held until an owner is found (noun).
3. (a) The force cause by the person is to the right (or forward). (b) Friction is pushing against the box to the left. (c) The person's force it greater than friction because the figure says box is moving right or forward.

Guided Reading

3.1 Forces

1. attracts
2. weight
3. pounds
4. direction

3.2 Friction

5. friction
6. surfaces
7. reduces

3.3 Forces and Equilibrium

8. balanced
9. speed
10. unbalanced

Let's Review

1. c
2. a

Check Your Understanding

Reading 3.1

1. newton, pound
2. direction and strength of the force
3. Answers are:
1. 600 N
2. 45 N
3. 15 N
4. Contact forces only act when objects are touching each other. Forces that act through a field can exist when the objects are not touching.
5. The force increases.
6. Your mass does not change if you go to the Moon, but your weight decreases. Your weight depends on the strength of gravity, and gravity is weaker on the Moon.
7. 160 N
8. 4 kg

Reading 3.2

1. sliding friction, rolling friction, static friction, air friction
2. Ice has a thin layer of water on top of it, so objects easily slide on the water.
3. If you are sitting on a chair that is on a floor that is not perfectly level, friction helps you stay on the chair.
4. Oil reduces friction in the car's engine.
5. B and D
6. The force of friction increases.
7. Ball bearings, nonstick coating on frying pans, and air hockey tables all reduce friction.
8. Rubber soles on rock climbing shoes, brakes on a car and the tread on a bicycle tire all increase friction.
9. False. It's not possible to completely get rid of friction.
10. D
11. False. Friction converts the energy of motion into heat.
12. Water reduces friction between two surfaces because it keeps the surfaces from touching. However, friction between water and an individual surface causes it to wear away. Water running over rocks causes the irregular tiny bits on the surface of the rocks to break off. This happens gradually over a very long period of time.

Reading 3.3

1. When all forces are balanced, the net force is zero.

2. Yes. If an object is moving at a constant velocity, then the net force is zero.
3. 5 N
4. 9 N to the right
5. A
6. 175,000 N
7. Answers may vary. Students should recognize that force from the person's weight and a normal force from the floor or chair are present. Students may infer that the chair provides only part of the normal force if the person's feet are on the ground. In that case the floor would provide a normal force as well. Sample answer on right:
   
8. B
9. 100 N

Connection: Forces in Equilibrium

1. Answers may vary. Two of the action-reaction pairs in a typical building are: The weight of the roof as a downward force and the load-bearing walls as an upward force; The weight of the wall-roof structure as a downward force and the foundation providing the upward force.
2. New York City is an ideal place for skyscrapers because it has a thick layer of stable bedrock just below the topsoil, providing a very solid base on which to build a foundation for a tall, heavy structure.
3. Brunelleschi's dome design featured a double-wall structure. The dome was constructed using a great deal of brick. Brunelleschi also used timbers linked together with metal fasteners to balance the outward forces on the dome. The linked timbers served a function similar to hoops on a barrel.

Activity: Friction and Forces

1. Answers may vary.
2. Answers may vary.
3. Answers may vary.
4. Answers may vary.
5. Answers may vary.

Chapter 3 Review

Vocabulary

Reading 3.1

1. force
2. pound
3. newton
4. weight
5. compression
6. tension
7. energy
8. vector
9. extension
10. kilogram
11. mass
12. matter
13. gram

Reading 3.2

14. friction
15. sliding friction
16. static friction

Reading 3.3

17. balanced
18. equilibrium
19. normal force
20. free-body diagram
21. net force

Concepts

Reading 3.1

1. Objects with energy are able to change themselves or other objects.
2. strong nuclear force, electromagnetic force, weak force, gravity
3. Weight is the force of gravity pulling on an object.
4. The force of the water against a boat's propeller allows the boat to accelerate. The force of a bicycle's brakes slows it down. The force of friction between a car's tires and the road allows the car to turn.
5. strength and direction
6. Answers are:
1. 
2. 
   
3. 
   
7. Friction is a contact force. The force of gravity acts through a force field.
8. The force increases in strength.
9. Compression and extension are both forces in springs. They both increase in strength when the spring is moved farther from its natural length. Compression is a decrease in a spring's length, and extension is an increase in length.
10. Answers are:
1. mass
2. force
3. force
4. mass
11. Weight is a measure of the force of gravity on an object. An object with a greater mass also has a greater weight.

Reading 3.2

12. There is a layer of liquid water between the blade and the ice.
13. More force is needed to start an object sliding. Static friction is usually greater than sliding friction.
14. Sliding friction is less when an object weighs less. A heavier object pushes harder into the irregularities on the surface, making friction greater.
15. Friction can be reduced with liquids such as oil or with ball bearings.
16. If you try to pull a nail upward, friction acts downward to keep the nail in. The direction of friction is against the direction the nail is trying to move. The friction acts between the nail and the wood or other material the nail is hammered into.
17. Friction changes the energy of motion into heat.

Reading 3.3

18. An object can be moving if the net force is zero. It must be moving at a constant velocity.
19. Answers are:
1. normal force
2. upward
3. The strength of the normal force is the same as the strength of the force of gravity on the object.
20. The racecar is slowing down because the net force is against the motion of the car.
21. Answers are:
1. drag, lift, thrust, and weight
2. drag and thrust
3. lift and weight
22. Answers are:
1. air resistance
2. 
   

Math and Writing Skills

Reading 3.1

1. 222 N
2. 
   
3. 4 cm
4. 70 N
5. 49 N
6. 45 kg
7. 392 N
8. 5102 kg
9. Her mass would be 60 kg. Her weight would be 222 N.

Reading 3.2

10. 40 N
11. Cars are designed with an aerodynamic shape to reduce air resistance. Their wheels have bearings that allow them to roll smoothly. The tires on a car are made of rubber because it creates a large friction force against the road.

Reading 3.3

12. Answers are:
1. 40 N left
2. 25 N up
13. The normal force is 9 N. It can be increased by putting other books on top of the first book or by pressing down on the book.
14. Yes, if the 100 N force and the 200 N force are pointing in the same direction, and the 300 N force is pointed in the opposite direction, then the forces will be in equilibrium.
    
15. The unknown scale reads 550 N.
    
16. Yes, the box is in equilibrium. All of the forces are balanced.
17. Answers are:
1. sliding friction
2. 75 N
3. 
   
18. Answers are:
1. static friction
2. 
   

Test Practice

1. A
2. C
3. C
4. B

Project

Criteria	Excellent	Good	Needs some improvement	Needs much improvement	Your score
Car is completed on time and meets the design criteria	4	3	2	1
Car is able to travel 5 meters	4	3	2	1
Car performs well in speed competition	4	3	2	1
Poster includes diagram of construction, shows how the rubber band moves the car, and includes competition data	4	3	2	1
Overall design, creativity, and construction	4	3	2	1
Total

Friction and Gravity Guided Reading and Study Answers

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