Need Help with AP Physics Free Response Questions?

[harmonicmotion]

Homework Statement

Homework Equations

The Attempt at a Solution

 

[harmonicmotion]

http://http://www.collegeboard.com/prod_downloads/ap/students/physics/physics_c_m_frq_02.pdf

PLEASE HELP

 

[arildno]

1. You must state the problem
Yes, that means posting the problem here. At Pf.

2. You must show your own work.

 

[harmonicmotion]

http://http://www.collegeboard.com/prod_downloads/ap/students/physics/physics_c_m_frq_02.pdf"

 

[harmonicmotion]

http://www.collegeboard.com/prod_downloads/ap/students/physics/physics_c_m_frq_02.pdf

 

[harmonicmotion]

its the first question

 

[harmonicmotion]

the second link works i think

 

[harmonicmotion]

can you try it and let me know if i should type it

 

[arildno]

Okay second link works, so what have you done so far?

 

[harmonicmotion]

are you still there

 

[arildno]

Indeed, what have you done so far?

 

[harmonicmotion]

i did a) and got 500 kg

 

[daniel_i_l]

This isn't a chat room, it takes a few minutes before you get responses.

 

[harmonicmotion]

on b i am trying to integrate and got x=(8/5)ln{1+5t}

 

[harmonicmotion]

i really do not think that is correct though

 

[arildno]

on a)+b), you are correct.

 

[arildno]

i really do not think that is correct though

But it is.

 

[harmonicmotion]

is integration the wrong method?

 

[harmonicmotion]

oh ok thank you, I am having trouble with c and d though

 

[arildno]

is integration the wrong method?

Eeh, do you even take time to read the replies you get??

Slow down a bit, will you?

 

[harmonicmotion]

what is the best method to use

 

[arildno]

oh ok thank you, I am having trouble with c and d though

Ok, now, is there some other quantity you can calculate by means of which you can calculate the force acting upon the object?

 

[harmonicmotion]

I'm sorry i think my computer is on delay because the messages don't come up until after i send something else

 

[harmonicmotion]

mass times acceleration?

 

[arildno]

mass times acceleration?

Agreed!

So, how can you find acceleration? (you already know mass)

 

[harmonicmotion]

hdhdkshhfsdhkf

 

[harmonicmotion]

dv/dt? could that be used?

 

[raman911]

use
Fnet=ma

 

[arildno]

dv/dt? could that be used?

Sure!
Isn't that what acceleration is, after all?

 

[harmonicmotion]

hjkshdgytu

 

[harmonicmotion]

sorry for the random letters your responses don't come up on my comp otherwise

 

[harmonicmotion]

wait so would dv/dt just be 8/(1+5t)

 

[harmonicmotion]

hsjsysgdsd

 

[arildno]

sorry for the random letters your responses don't come up on my comp otherwise

I understand.

Now, as for d) assuming you manage c), it is perhaps simplest to remember that the impulse over a time interval equals the change in momentum over that interval.

 

[harmonicmotion]

dhdyegstdff

 

[arildno]

wait so would dv/dt just be 8/(1+5t)

No, that is v itself!

You must differentiate that expression to get the acceleration.

 

[harmonicmotion]

isnt impulse also force times the change in time

 

[harmonicmotion]

sorry i still can't figure out c

 

[arildno]

isnt impulse also force times the change in time

Only if the force is constant.
If it is not constant, you must perform the integral of F over time.
Happily, this equal to the change of momentum over that time.

 

[harmonicmotion]

hdysudjfgf

 

[arildno]

sorry i still can't figure out c

Well, what is v?

differentiate that function!

 

[harmonicmotion]

for c i got (500)(-40)(1+5t)^-2

 

[arildno]

Oops!
Remember that the mass of the system consists of BOTH the masses, i.e, 1000+500=1500.

 

[harmonicmotion]

gyhtdesrehh

 

[harmonicmotion]

so for 500 i put 1500 and then its right?

 

[arildno]

so for 500 i put 1500 and then its right?

That is correct.

 

[harmonicmotion]

hyujiklofddss

 

[harmonicmotion]

so d is (mv(final)-mv(initial))/(time)

 

[arildno]

No, that would give you the average FORCE over that time interval.
Do not divide with time; the impulse is just the momentum difference.

 

[harmonicmotion]

hyhndhstsy