Chapter 1 – Question Keys

Chapter 1 – Question Keys

1

1. Below is a speed-time graph showing a cart speeding up over a certain period

of time, and then slowing down.

a) On the graph above, carefully sketch the speed-time graph for another cart that would speed up at a higher rate than the one shown above, reach the same maximum speed, and then slow down at a lower rate than the one shown above.

b) Briefly explain why you sketched your graph the way you did.

A higher rate of speeding up is shown by making the graph have a steeper upward slope, so it reaches the same height (maximum speed) in less time. A lower rate of slowing down is shown by making the graph have a less steep downward slope, so it takes a longer time to go from its maximum speed to being stopped.

PET C1 Question Keys 2

2. A ball sits on a carpeted floor. A child kicks the ball and it starts to roll. While

rolling across the carpet, the ball slows down and eventually stops on the other side of the room.

Sketch a speed-time graph for the entire motion, including the interval during which the child kicks the ball and the interval during which the ball slows down when rolling along the carpet and comes to a stop. On your graph mark with an X the moment that the boy’s foot looses contact with the ball. Below the graph explain your reasoning for choosing that particular moment.

X

The moment the foot loses contact with the ball is also the moment the speed stops increasing rapidly. This is because the speed only increases while the interaction between the foot and the ball is actually in progress. As soon as the interaction stops, the effect it has on the balls speed also stops. (The ball starts to slow down at this point because another interaction, friction, is affecting it.) We saw this in a video of a hand pushing a cart. As soon as the hand lost contact the speed of the cart stopped increasing.

3. Below are some statements about contact interactions. Read each statement

carefully and indicate whether you think it is true (T) or false (F).

If you think the statement is false, give an example of an interaction you have seen that does not fit the statement. If you think the statement is true, give an example that does fit.

a) In a contact interaction involving friction, at least one of the objects

involved has both an energy increase and an energy decrease.

T F Examples: Block sliding on table – block decrease in kinetic energy, increase in thermal energy.


Brake fade homework – wheels decrease in kinetic energy, increase in thermal energy.

b) In any contact interaction involving an elastic object the elastic potential

energy of that object always increases.

T F Counter example: Launching cart with rubber band – decrease in elastic potential energy

c) When the kinetic energy of a moving rigid object decreases during a

contact interaction, this means the object must be the energy source in that interaction.

T F Examples: Friction-type interaction – moving object slows and is energy source and energy is transferred to other object involved. Running cart into rubber band – energy is transferred to rubber band.

d) In all contact interactions there is always an increase in kinetic energy of

at least one of the objects involved.

T F Counter examples: Friction interaction – moving object loses kinetic energy. Running cart into rubber band – kinetic energy decreases.


4. The diagram below shows a speed-time graph for a low-friction cart that was moving along a very smooth track. For this situation assume the effects of friction are so small that you can ignore them. The graph consists of five straight-line segments labeled A through E. During these different parts of the motion, a hand may or may not have been in contact with the cart.

a) During which segments (A, B, C, D and/or E) does the graph provide

evidence that the cart was involved in a contact interaction? Briefly justify your choice(s).

The graph shows the cart was involved in a contact interaction in segments A, C, and E. We know this because during these segments the speed was changing and that is evidence for a contact interaction.

b) During which segments (if any) does the graph provide evidence that the

cart was an energy source in a contact interaction? Briefly justify your choice(s).

The graph shows the cart was an energy source in a contact interaction in segments C and E. We know this because during these segments the speed was decreasing which means it was losing kinetic energy. This loss of energy means mechanical energy was being transferred from the cart, making it the energy source in an interaction.

c) During which segments (if any) does the graph provide evidence that the

cart was an energy receiver in a contact interaction? Briefly justify your choice(s).


The graph shows the cart was an energy receiver in a contact interaction in segment A. We know this because during this segment the speed was increasing which means it was gaining kinetic energy. For this to happen mechanical energy must be transferred to the cart, making it the energy receiver in an interaction.

5. A soccer player kicks a soccer ball into a net. As the ball comes in contact with the net, the net stretches and the ball stops moving. Consider the time interval when the soccer ball is in contact with the net. Complete the following S/R energy diagram to describe the interaction for this time interval.

Energy Source Energy Receiver

_______________________ Interaction Contact

Ball Nety

Mechanical Energy

Decrease in kinetic energy

Increase in elastic

potential energy


6. A block sits on a carpeted floor. A child kicks the block and it starts to slide. While sliding across the carpet, the block slows down and eventually stops on the other side of the room. While sliding across the carpet, both the block and the carpet warm up.

a) Consider the time interval during which the child’s foot was in contact with

the block. Draw an I/O energy diagram for the block during this time interval. Make sure you label the appropriate interaction(s), energy change(s) and energy transfer(s).

MechanicalEnergy

Increase in kinetic energy

Contact Interaction

Block

Increase in thermal energy

MechanicalEnergy

Heat Energy

Contact Interaction

Heat Conduction/IR Interaction

b) Now consider the time interval after the block leaves the child’s foot and is slowing down as it slides across the carpet. Draw an I/O energy diagram for the block as it slides across the carpet. Make sure you label the appropriate interaction(s), energy change(s) and energy transfer(s).

MechanicalEnergy


Block


Heat Energy

Contact Interaction

Heat Conduction/IR Interaction


7. A ball sits on a carpeted floor. A child kicks the ball and it starts to roll. While rolling across the carpet, the ball slows down and eventually stops on the other side of the room.

A student writes the following explanation for why the ball slows down after it leaves the boy’s foot.

There is a contact interaction between the ball and the carpet. During this interaction there is a transfer of mechanical energy from the ball to the carpet. There is a decrease in kinetic energy of the ball and an increase in thermal energy in the carpet.

Evaluate this explanation by considering each of the following four criteria. If the written explanation is good, state why. If it is poor, state what is wrong with it and correct it.

Present Criterion YES/NO Complete: All ideas needed are included YES/NO Accurate: All included ideas included correspond to established

ideas YES/NO Logical Reasoning: Narrative connects ideas to the phenomenon YES/NO Clear: Narrative is well written

This explanation is _____POOR_____ (good, poor) because:

It says that only the carpet warms up but we know that during a friction-type contact interaction both objects involved warm up and thus increase in thermal energy. It also does not connect the decrease in kinetic energy to the slowing down of the ball. A better explanation would be: There is a friction-type contact interaction between the ball and the carpet. During this interaction there is a transfer of mechanical energy from the ball to the ball. There is a decrease in kinetic energy of the ball and an increase in both thermal energy in the carpet and the ball. Because the ball is losing kinetic energy it slows down as it rolls across the carpet.


8. Below are some statements about contact interactions. Read each statement carefully and indicate whether you think it is true (T) or false (F).

If you think the statement is false, give an example of an interaction you have seen that does not fit the statement. If you think the statement is true, give an example that does fit.

a) In all contact interactions involving friction, one of the objects decreases in

thermal energy.

T F Counter examples: Rubbing eraser on hand – hand and eraser both warmed up. Brake-fade homework – wheels and brake pads both increased in thermal energy.

b) In any contact interaction involving an elastic object, that elastic object is

always an energy receiver.

T F Counter example: Launching cart with rubber band – rubber band is energy source

c) When the kinetic energy of a moving rigid object decreases during a

contact interaction, this means the object must be the energy source in that interaction.

T F Examples: Friction interaction – moving object slows and is energy source and energy is transferred to both objects involved. Running cart into rubber band – moving cart slows and energy is transferred to rubber band.

d) In all contact interactions there is always an increase in kinetic energy of

at least one of the objects involved.

T F Counter examples: Friction interaction – moving object loses kinetic energy. Running cart into rubber band – kinetic energy decreases.


9. Manuela is preparing to go play a game of soccer. She grabs a cooler, packs it with ice, puts several warm soft drinks in it, and closes the cooler. Two hours later, Manuela and her teammates enjoy cool, refreshing sodas. Which energy diagram below best describes the interaction between the soda and the ice in the cooler? Circle the best answer.

(a)

iceHeatEnergysoda

Increase inThermalenergy

Decrease inthermalenergy

source receiver

(b)

sodaHeatEnergyice

Increase inthermalenergy


source receiver

(c)

sodaHeatEnergyice

Decrease inthermal

energy

Increase inthermal

energy

source receiver

(d)

iceHeatEnergysoda


Increase inthermalenergy

source receiver

10. One night Julie is reading a novel by the bright light

of a lamp with a 100-watt light bulb. A cold glass of water sits under the lamp. After a while Julie reaches for her glass of water. To her surprise, both the glass, and the water inside it, are quite warm.

a) What kind of interaction was mainly responsible for

causing the glass to get warm, and what are the interacting objects?

The interaction mainly responsible for warming the glass was the Infrared Interaction between the lamp and the glass.


b) Sketch a S/R energy diagram describing the interaction. Include the

interacting objects, and label the interaction type(s), energy transfers and the energy changes within the interacting objects.

Glass

Infrared Interaction


Heat Energy Lamp

Decrease in thermal energy

11. In one of your activities in the PET

class you gave a cart a shove towards a rubber band. As it ran into the rubber band, the cart slowed down, and the band stretched.

Write an explanation, using energy ideas, for why the cart slowed down as it ran into the rubber band.

Explanation: Why did the cart slow down as it ran into the rubber band?

Draw the energy diagram


MechanicalEnergy


Energy Source

Contact Interaction

Cart

Write the explanation:

When the cart hits the rubber band there is a contact interaction between them and mechanical energy is transferred from the cart to the rubber band (which increases in elastic potential energy as it stretches). Since there is no energy input to the cart, conservation of energy means that the output of mechanical energy must be accounted for by a decrease in its kinetic energy. This decrease in kinetic energy means that the cart slows down as it runs into the rubber band.


12. Rocky grabs a cold can of soda from the refrigerator and sits down on his soft

chair to watch some TV. He falls asleep almost immediately, while still holding the can. After about ten minutes he wakes up and, to his disappointment, the can of soda is no longer cold. Explain why the can warms up by filling in the following steps.

a) What is the type of interaction that causes the soda can to become warmer

and what are the interacting objects? The main interaction that causes the soda can to become warmer is the heat conduction interaction between Rocky’s hand and the can.

b) Draw a S/R energy diagram to describe the interaction. Include the

interacting objects, and label the interaction type, energy transfer and the energy changes within the interacting objects.

Can

Heat Conduction Interaction


Heat Energy Hand

Decrease in thermal energy

c) Write a few sentences to explain why the can becomes warmer.

While Rocky is holding the can there is a Heat Conduction Interaction between his warm hand and the cold soda can, during which there is a transfer of heat energy from the hand to the can. This input of heat energy to the soda can means that its thermal energy increases. This increase in thermal energy means of the soda can means that its temperature rises and so it becomes warmer.

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Chapter 1 – Question Keys