Friday, November 13

Magnetism - Multiple Choice Practice pg. 192-193

1) b
2) c
3) c
4) c
5) a
6) a?
7) d
8) a
9) b
10) d

Wednesday, November 11

pg. 109

12) A machines can multiply input force but not input distance nor energy.
13) ?
14) 100% efficient
15) ?

Monday, November 9

Ch. 6 Energy pg. 108
Review Questions

6) It would have double the potential energy.
7) The car with greater mass has twice the potential energy as the other.
8) 4 J, 8 J
9) 16 times the kinetic energy
10) 9 times as much work and distance to stop

Monday, November 2

pg. 191 "Think and Explain"

3) "An electron always experiences a force in an electric field, but not always in a magnetic field." This is because magnets produce magnetic fields and magnets are made of atoms. Atoms contain electrons which are always in motion. Electron spin is what creates magnetic fields. If a pair of electrons spin in the same direction, the magnet grows stronger, but if a pair of electrons spin in opposite directions, the magnetic field is canceled out.
6) 50 N?
13) A motor converts electrical energy to mechanical energy while a generator does the exact opposite.

Tuesday, October 27

pg. 173
Think and Compare

1) same?
2) A, B, C
3) A, B, C
4) a) C, B+A the same
b) C, B, A

Think and Solve

2) Amperes=volts/ohms. 12/8=1.5 A
4) Resistance=voltage/current. 120/20=6 ohms.

Friday, October 23

pg. 172

7) A good conductor of electricity allows electrons to pass through it easily, and the flow of electrons converts energy into heat. Thus, a good conductor of electricity is also a good conductor of heat
8) When a current flowing through a lamp increases, the light emitted will become brighter. The more electrons, the greater light.
9) Ampere and volt do not have the same meaning. Amperes are the rate of electrical flow while voltage is electrical pressure. If they meant the same thing, why would they have different names?
10) circuit 5
11) The same amount of current flows in and out of a battery.
12) The rest stay as electric energy and travel back around the circuit.

Wednesday, October 21

Chapter 9: Electricity

My Questions:
1) How do you differentiate between positive and negative charges? Meaning, how can you tell how an object is charged?
2) Why is the charge on an electron negative? What are the historical reasons mentioned in the textbook?
3) Is it possible to create or destroy electrons?
4) What is Coulomb's law in simpler terms? What is the "square of the separation distance" between two charged particles that the law refers to?
5) Can there be a naturally produced electron current, or must they all flow on a controlled circuit?

Wednesday, September 16

Chapter 4
pg. 72

Think and Explain
10) Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. How fast does the heavier cart roll compared with the lighter cart?
The heavier cart rolls half as quickly as the lighter cart, because a force exerted on a large mass produces a small acceleration, whereas the same force exerted on a small mass produces a large acceleration.
11) If you exert a horizontal force of 200 N to slide a crate across a factory floor at constant velocity, how much friction does the floor exert on the crate? Is the force of friction equal and oppositely directed to your 200-N push? If the force of friction isn’t the reaction force to your push, what is?
The friction from the floor exerted on the crate is 200 N, because the crate is not accelerating. Yes, it is equal and oppositely directed to the push by Newton ’s third law. The reaction force is the crate pushing back on your hand, which equals the amount of force that you exert on it.
12) If a massive truck and a small sports car have a head-on collision, on which vehicle is the impact force greater? Which vehicle experiences the greater acceleration? Explain your answers.
The impact force is equal for both vehicles because of the interaction. However, the small sports car would experience the greater acceleration because a smaller mass produces a larger acceleration.
16) A stone is shown at rest on the ground.
(a) The vector shows the weight of the stone. Complete the vector diagram by showing another vector that results in zero net force on the stone.
(b) What is the conventional name of the vector you have drawn?

(a) draw an upward vector the same size as the downward vector, representing the normal force
(b) vector quantity

17) Here a stone is suspended at rest by a string.
(a) Draw force vectors for all the forces that act on the stone.
(b) Should your vectors have a zero resultant?
(c) Why, or why not?
Draw a downward vector for the force of gravity, and draw an upward vector equal in length for the force exerted by the string. The vectors should have a zero resultant, because the stone is not moving.

18) Here the same stone is being accelerated vertically upward.
(a) Draw force vectors to some suitable scale showing relative forces acting on the stone.
(b) Which is the longer vector, and why?
Draw a downward vector representing the force of gravity, draw a smaller downward arrow representing air drag, and draw a longer vector pointing upward representing the force of the string. This vector is longer because its force must be greater to accelerate the stone upward.

Multiple Choice
1) d
2) b
3) b
4) c
5) c
6) a
7) a
8) c
9) a
10) d

Thursday, September 10

pg. 35
2) Correct your friend who says, "The race-car driver rounded the curve at a constant velocity of 100km/h."
Velocity specifies direction as well as speed, so the driver would have to be driving in a constant direction. Because he turns, the sentence should be stated using "constant speed" instead of "constant velocity".
4) If a huge bear were chasing you, its enormous mass would be very threatening. But if you ran in a zigzag pattern, the bear's mass would be to your advantage. Why?
A large bear's mass would take much more force to put into motion than your mass. Because you are lighter than the bear, you would be able to use less force and have an easier time changing directions quickly.
9) When a car moves along the highway at constant velocity, the net force on it is zero. Why, then, do you continue running your engine?
You must maintain your velocity in order to keep the net force at zero.
13) The sketch shows a painting scaffold in mechanical equilibrium. The person in the middle weighs 250 N, and the tensions in each rope are 200 N. What is the weight of the scaffold?
The weight of the scaffold is 150 N, because the sum of the upward tensions equal the sum of its weights and the weight of the scaffold.
19) If you toss a coin straight upward while riding in a train, where does the coin land when the motion of the train is uniform along a straight-line track? When the train slows while the coin is in the air? When the train is turning?
The coin lands in the exactly same spot from where it was thrown, regardless of the circumstances. This is because the coin moves by its inertia until a force changes its motion, and it is unaffected by the motion of the train while it is in the air.

pg. 36
1. A rolling ball comes to a stop. How do we know that one or more forces have acted on the ball?
(a) The ball changes its motion.
2. One child pushes a cart to the right with a force of 100 N. Another child pushes the cart to the left with a 50-N force. What is the net force on the cart?
(b) 50 N to the right
3. A truck comes to a sudden stop. A crate in the back of the truck slides forward and hits the front part of the truck. Why?
(c) Both (a) and (b)
(The box slides forward due to its inertia.) (Every object continues its state of motion until acted on by a nonzero net force.)
4. A ball sits on the floor in a bus. When the driver hits the brakes, the ball slides forward. The explanation for this is that
(b) the ball was moving along with the bus yet did not experience the braking force that the bus did, so the ball kept going
5. A 20-lb dog sleeps on the floor. The support force acting on the dog is
(a) 90 N, directed upward.
6. A walker's speed is 4 m/s. How far did the walker go in 5 s?
(c) 20 m
7. A glacier is a flowing river of ice, as you will learn more about in Chapter 33. Every day, the Quarayaq glacier in Greenland flows up to 20 m. Its speed is
(a) 20 m/day.
8. Your average speed bicycling to your friend's house is 8 mi/h. Is it possible that your instantaneous speed was zero at some point along your journey?
(a) Yes
9. In a game of tug-of-war, the rope moves to the right. This indicates that
(d) All of the above
(the net force on the rope is nonzero, the team on the right pushes harder on the ground than the team on the left, the velocity of the rope is nonzero.)
10. You whirl a yo-yo in a flat circle above your head. If you let go of the string, Newton's first law tells you that the yo-yo will
(c) move in a straight line at constant speed until a force acts to change its motion

pg. 55
2) Your empty hand is not hurt when it bangs lightly against a wall. Why is it hurt if it does so while carrying a heavy load? Which of Newton's laws is most applicable here?
Newton's second law is most applicable here. The greater the mass of an object, the greater its inertia. It would be more difficult to stop your hand from banging against the wall while carrying a heavy load, so it would hit the wall with greater impact, therefore causing greater pain.
3) When a junked car is crushed into a compact cube, does its mass change? Its weight? Its volume?
Its mass is the same, because the amount of matter in the object doesn't change. Its weight doesn't change, because the force of gravity on its mass doesn't change. Its volume changes, because the amount of space in it has changed.
5) If it takes 1 N to push horizontally on your book to make it slide at constant velocity, how much force of friction acts on the book?
The force of friction is 1 N, because in order to maintain a constant velocity, the net force must be zero.
8) Two basketballs are dropped from a high building through the air. One ball is hollow and the other filled with rocks. Which accelerates more? Defend your answer.
The balls accelerate at the same rate, because all freely falling objects have the same force/mass ratio.
9) A parachutist, after opening the chute, finds herself gently floating downward, no longer gaining speed. She feels the upward pull of the narness, while gravity pulls her down. Which of these two forces is greater? Or are they equal in magnitude?
The forces are equal in magnitude; it is her weight that pulls her down now.
13) How does the terminal speed of a parachutist before opening a parachute compare with terminal speed after? Why is there a difference?
Before opening the parachute, her terminal speed is much greater than after opening the parachute. This is because air drag caused by an increased surface area (the parachute) reduces acceleration.
14) How does the gravitational force on a falling body compare with the air drag it encounters before it reaches terminal velocity? After?
The gravitational force is much greater than the air drag before it reaches terminal velocity. After it does, the two forces are balanced, so the velocity becomes constant.

pg. 55
1. How great is the air drag that acts on a 10-N sack that falls in air at constant velocity?
(b) 10 N
2. A bear that weighs 4000 N grasps a vertical tree and slides down at constant velocity. What is the friction force that acts on the bear?
(a) 4000 N directed upward
3. When your mass increases, your weight
(b) increases.
4. On a long alley, a fast-moving bowling ball gradually slows as it rolls. What is the horizontal force acting on the ball?
(b) The force of friction.
5. A cat that accidentally falls from the top of a 50-story building but hits the safety net below falls no faster than if it falls from the 20th story. The reason is that
(d) the car reached its terminal velocity.
6. If a mass of 1 kg is accelerated 1 m/s^2 by a force of 1 N, what would be the acceleration of 2 kg acted on by a force of 2 N?
(c) 1 m/s^2
7. What is the net force on a bright red Mercedes convertible traveling along a straight road at a steady speed of 100 km/h?
(a) 0 N
8. The net force acting on a sliding skateboard is somehow doubled. By how much does the acceleration increase?
(a) Acceleration doubles.
9. What is the force of gravity on a 2-kg watermelon?
(d) about 20 N
10. What is the acceleration of free fall?
(b) 10 m/s^2

Friday, September 4

The concepts in chapter 3, Newton’s Second Law of Motion, relate to the motion of my toy car in several ways. Section 3.3 talks about the mass of an object affecting its inertia, which also applies to my toy car experiment. I am comparing the motion of my car when supporting different weights. The greater the weight, the greater the mass, and consequently, the car’s inertia is also greater. This means that it would require more power to move the toy car when it is carrying greater weights. Additionally, Section 3.4 says that greater mass result in less acceleration, which also applies to the movement of my toy car.

Monday, August 31

Chapter 2: Newton's First Law of Motion
pg. 36

14) A different scaffold that weighs 300 N supports two painters, one 250 N and the other 300 N. The reading on the left scale is 400 N. What is the reading on the right scale?
The reading on the right scale is 450 N, because the magnitudes of the two upward vectors equal the magnitude of the three downward vectors. Thus, because the sum of the three downward vectors equal 850 N and the reading on the left scale is 400 N, the reading on the right scale must be 450 N.

15) Nellie Newton hangs at rest from the ends of the rope as shown. How does the reading on the scale compare to her weight?
The reading on the scale reads the same as her weight, because the sum of upward tensions is equal to the sum of its weights.

16) Harry the painter swings year after year from his bosun's chair. His weight is 500 N and the rope, unknown to him, has a breaking point of 300 N. Why doesn't the rope break when he is supported as shown at the left? One day Harry is painting near a flagpole, and, for a change, he ties the free end of the rope to the flagpole instead of to his chair as shown at the right. Why did Harry end up taking his vacation early?
The rope doesn't break because both ends of the rope are tied to Harry's bosun's chair. The total upward tension in the rope is equal to his weight, which shows that each half of the rope only needs to support 250 N. Therefore, the rope does not break.
Harry ends up taking his vacation early, because the rope broke and he fell down. With only one piece of rope tied to his chair, the rope couldn't support his full, indispersed weight. It broke, and Harry spent his vacation in the hospital. D:

Thursday, August 27

Chapter 2: Newton's First Law of Motion
pg. 35

4) If a huge bear were chasing you, its enormous mass would be very threatening. But if you ran in a zigzag pattern, the bear's mass would be to your advantage. Why?
A large bear's mass would take much more force to put into motion than your mass. Because you are lighter than the bear, you would be able to use less force and have an easier time changing directions quickly.

6) Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled forward, the ball rolls against the back of the wagon. Interpret this observation in terms of Newton's first law.
Newton's first law of motion states that an object at rest will stay at rest unless a force sets it in motion. When the wagon is pulled forward, the ball inside the wagon is at a resting state. Thus, the ball is shifted to the back of the wagon at its original resting spot.

7) Why do you lurch forward in a bus that suddenly slows? Why do you lurch backward when it picks up speed? What law applies here?
Newton's law of inertia says that when an object is in motion, it tends to remain in motion unless an external force acts upon it. You are in steady motion on a bus, so when it brakes quickly, you can't immediately stop your forward motion. When the bus picks up speed, you lurch backward because the force has just set you in a forward motion whereas previously, you were in a resting state.