Main content

## Physics library

### Course: Physics library > Unit 19

Lesson 2: AP Physics 1 free response questions 2015- Question 1a: 2015 AP Physics 1 free response
- Question 1b: 2015 AP Physics 1 free response
- Question 1c: 2015 AP Physics 1 free response
- Question 2ab: 2015 AP Physics 1 free response
- Question 2cd: 2015 AP Physics 1 free response
- Question 3a: 2015 AP Physics 1 free response
- Question 3b: 2015 AP Physics 1 free response
- Question 3c: 2015 AP Physics 1 free response
- Question 3d: 2015 AP Physics 1 free response
- Question 4: 2015 AP Physics 1 free response
- Question 5: 2015 AP Physics 1 free response

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Question 3c: 2015 AP Physics 1 free response

Analytically analyzing how doubling spring compression impacts stopping distance.

## Want to join the conversation?

- Isn't this simple harmonic motion? Why does Sal say that the spring would go on forever?(1 vote)
- This question is about work and energy, not simple harmonic motion.

The spring won't "go on forever" because it transfers all of its potential energy, Usp, to the block's Kinetic Energy. After it lets go of the block, the spring doesn't have any energy left, so it just stops at equilibrium. You are probably thinking of a spring with no block, which would be simple harmonic motion.(2 votes)

- How is distance = kinetic energy/friction energy?(1 vote)
- I think you misunderstood. The work done by frictional force completely dissipates kinetic energy. Hence, frictional force*distance = kinetic energy.(1 vote)

- Is the amount and type of work that Sal shows good enough in order to get a 5? Or does more work need to be shown?(0 votes)
- I think he would get a 6.

Sal is in a league of his own :)(3 votes)

- Starting at4:02, Sal writes the equation for the stopping distance as kinetic energy plus work done by friction. Why does he not include potential energy in that? What happens to the potential energy?(1 vote)
- After D=0, PE=0, and so PE does not matter after that(2 votes)

## Video transcript

- [Voiceover] Alright,
now let's tackle part c. Use quantitative reasoning,
including equations as needed, to develop an expression for the new final position of the block. Express your answer in terms of D. Alright, I'm gonna set up a
little table here for part c. Oops, sorry about that. Sometimes my pen is not
functioning properly. Alright so part c. Let me set up two scenarios. So I have scenario one. Scenario one, where we
compress the spring, our delta x is equal to d, and then we have scenario two, scenario two, where we
compress the spring by twice as much, is equal to two d, and let me set up my table now, so, just like that and like this, and let's just think about a few things. So the first thing I wanna think about is the potential energy,
so the potential energy when the spring is compressed. So potential energy
when spring compressed. When, I'll just write
compressed, when compressed. Well in this scenario I'll
call it the potential energy for scenario one, it's
equal to one half times the spring constant, times how
much we compress it squared. Now what about scenario two? This potential energy
is going to be equal to one half times the spring
constant, times how much we compress it, it's now
twice as much, squared, well this is equal to one half
times the spring constant, times four d squared, and I can out the four up front, this is equal to four times
one half, four times one half, times our spring
constant, times d squared, which is equal to four
times the potential energy when we just compress the spring by d. So we already see a little
bit of what we talked about in part b, you compress
your spring twice as much, you're going to have four
times the potential energy, because the potential energy
doesn't grow proportionately with how much you compress
it, it grows with the square of how much you compress it. Alright, now let's think
about kinetic energy Kinetic energy when x is equal to zero, so right when the spring,
when we lose contact with the spring, the
spring is no longer pushing on the block, well our
kinetic energy is going to be equal to what our potential
energy was when the spring was actually compressed. Another way of thinking about
it, all that potential energy has now been turned into kinetic energy. Now what about over here? Well, the kinetic energy in this scenario, like we just saw before,
that's gonna be equal to the potential energy when the
spring was compressed. All of that potential energy
gets turned into kinetic energy and this is equal to four
times u one, four times the potential energy in
scenario one, which is the same thing as four times, which
is equal to four times the kinetic energy in scenario one. So we have four times the kinetic energy. Four times kinetic energy,
kinetic, kinetic energy. So then we have stopping
distance, stopping, stopping distance. We know here this is three d and we know, and then we can say "What's
this?", this question mark. Well let's just think a
little bit about this. We know that if we have
that kinetic energy at x equals zero, so we know that k one plus
the work done by friction, so let me make it clear,
this right over here, that is work done by friction,
work done by friction, and it's gonna be negative work
cause the force of friction is acting in the direction
opposite of the change in x, so the kinetic energy plus
the work done by friction is going to be equal to zero. This work cancels out all of this energy. When we think about it, it's
turning it all into heat. And so let's think about what
the work done by friction is equal to. Well the work done by friction
is equal to, is equal to, the coefficient of friction
times the mass of the block, times the gravitational
field, times how far, over what distance that
force, this right over here is the force of friction,
times over what distance that force was applied, so times three d. And to be clear, this force is going in the opposite direction of our change in x, so because of that this
will be a negative, and so we can say, we can
say that the kinetic energy at x equals zero, and now I
can just write it as minus mu, the coefficient of
friction, times mass, times the gravitational field, times
three d is equal to zero. We can add this to both
sides and we can get k one is equal to mu times
m times g times three d, and if you wanted to
solve for distance here, you can divide both sides
by the force of friction. So divide both sides by mu
times m times g, and you get three d, and I'm just
swapping the sides here, is going to be equal to the
amount of kinetic energy we have right as x equals zero,
divided by mu times m times g and you can just view this
as the force of friction. The force, I'll just call it the force of friction right over there. So if you wanna figure
out your stopping distance you just figure out your kinetic energy, right when you, right at x equals zero, right when you start entering
into the frictiony part of your platform and then you divide that by the force of friction,
and that will give you your distance traveled. So the distance here, distance,
so I can just put some arrows right over here,
our distance is going to be equal to k two divided by the force of, the force of friction. Well k two is equal to four times k one, is equal to four times k one. and our force of friction
is going to be the same We have the same coefficient of friction, we have the same mass, we have the same gravitational field, so
divided by force of friction, and this we already know, k one
divided by force of friction is equal to three d, so this
is all going to be equal to four, this is going to
be four times three d, three d, which is equal to 12 d, 12 d. So this is all a
mathematical way of saying you compress it twice as
much, you're going to have four times the potential energy when your spring is compressed, which
means you're going to have four times the kinetic
energy at x equals zero, which means it is going to
take, you're going to have four times the stopping
distance, so instead of stopping at three d or in six d as
what the student proposed, you are now stopping at 12 d, so that is our stopping distance. Did we answer all of, yeah
we answered all of part c. Use quantitative reasoning,
including equations as needed, to develop an expression
for the new final position of the block. Express your answer in terms of d. Yep, feel good about that.