Answers to Questions on Work and Energy

[allstar1]

My answers have ******** next to them.

1. A woman holds a bowling ball in a fixed position. The work she does on the ball
Choices:
a) depends on the weight of the ball.
b) cannot be calculated without more information.
c) is equal to zero. ************************


2. When you do positive work on a particle, its kinetic energy
Choices:
a) increases. ************************
b) decreases.
c) remains the same.
d) need more information about the way the work was done.


3. The potential energy of a spring is
Choices:
a) propotional to the amount the spring is stretched.
b) propotional to the square of the amount the spring is stretched.*******
c) depends on the orientation of the spring.
d) not covered in the reading assignment.


4. A car slows down as a result of air friction. Which is true?
Choices:
a) The car's kinetic energy decreases. ************************
b) Heat is generated.
c) The energy of the car/road/air system is constant.
d) all of the above
e) none of the above


any help is appreciated. Thank you.

 

[allstar1]

anyone?

 

[lightgrav]

I'm not sure you asked a question ...
but I would have the car+road+air Energy constant.
Friction Force transforms KE to thermal E;
you probably haven't covered Thermal topics
in detail yet; that almost means "Heat generated"

in #2, I'd stick with your answer,
but notice that if you do +'ve work on an object
while something else does negative W on it,
the object's KE can increase, decrease, or stay same.

 

[allstar1]

i would think the Kinetic energy decreases since KE=mv2/2, so if velocity decreases shouldn't the KE decrease?

 

[Doc Al]

That's also true.

 

[allstar1]

this question is very tricky

 

[arildno]

You have already found that a) is true. But you cannot yet place your mark on alternative a), because alternative d) also implies that a) is true (note that because a) is true, e) is necessarily false).

Thus, your task should now be to determine:
Is both b) and c) true as well, so that d) should be your answer?
(If either b) or c) is false, then d) is a false statement and cannot be used as an answer)

 

[allstar1]

In question 4 I think that A) is true, but i don't think that C) is true because i don't believe that "The energy of the car/road/air system is constant," since the question specifies that the car slows down as a result of air friction. Due to air friction i think that choice C) is FALSE.

 

[Doc Al]

When the car slows down, its kinetic energy is lost. Where does the energy go?

 

[allstar1]

When the car slows down, its kinetic energy is lost. Where does the energy go?

isn't the energy always constant and conserved. So as thr kinetic energy decreases, it becomes (turns) into potential energy and visa versa?

 

[Doc Al]

Mechanical energy (the sum of kinetic plus potential energy) is not conserved here. After all, the car is not (presumably) driving uphill, so its gravitational potential energy remains constant. And it says it slows down due to air resistance. (Mechanical is not conserved when dissipative forces--like air resistance or friction--act on the object.)

But the energy doesn't just disappear. It is transformed into other forms, such as thermal energy.

But if you consider not just the car, but also the air and the road... the total energy of this car/air/road system is conserved. That's the point.

 

[allstar1]

Mechanical energy (the sum of kinetic plus potential energy) is not conserved here. After all, the car is not (presumably) driving uphill, so its gravitational potential energy remains constant. And it says it slows down due to air resistance. (Mechanical is not conserved when dissipative forces--like air resistance or friction--act on the object.)

But the energy doesn't just disappear. It is transformed into other forms, such as thermal energy.

But if you consider not just the car, but also the air and the road... the total energy of this car/air/road system is conserved. That's the point.

I see you point, then since the
The car's kinetic energy decreases. And The energy of the car/road/air system is constant, Can we say that Heat is also generated, since like you said that "energy doesn't just disappear. It is transformed into other forms, such as thermal energy." So heat in this case would be thermal energy??

 

[Doc Al]

Can we say that Heat is also generated...

Yes, heat is generated.