# Did I do this question correctly? (1 Viewer)

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#### andgabbana

Consider the frictionless roller coaster shown:
http://i299.photobucket.com/albums/mm286/lanvin12/physics-2.jpg [Broken]

If a 12 000-kg car starts at rest from Point A, calculate

a) the total energy of the system

b) the speed of the car at point B

c) the force that must be applied to bring it to a stop at point E

d) the work done to bring it to a stop at point E

Here's what I did...

a)
E(T) = E(K) + E(P)
= 1/2(mv^2) + mgh
=1/2(12000 x 0) + (12000 x 9.8 x 95)
=1.1x10^7 J

b)
E(T1) = E(T2)
1/2(V1^2) + gh(1) = 1/2(v2^2) + gh(2)
9.8 x 95 = 1/2(v2^2) + (9.8 x 65)
931 = 1/2(v2^2) + 637
V(2) = 24m/s

c)
1/2(V1^2) + gh(1) = 1/2(v2^2) + gh(2)
9.8 x 95 = 1/2(v2^2) + (9.8 x 25)
V(2) = 37.04m/s

F = mass ([Vf^2 - Vi^2] / [2 x delta d])
= 12000 ([0^2 - 37.04^2] / [2 x 7])
=-3.2x10^4 J

d)
W=E(K)
=1.1x10^7 J

Do you see any mistakes?

Last edited by a moderator:

#### LowlyPion

Homework Helper
Welcome to PF.

Mostly good.

c) however must have a stopping force that absorbs all the potential energy, as in d) which you figured correctly.

#### andgabbana

Welcome to PF.

Mostly good.

c) however must have a stopping force that absorbs all the potential energy, as in d) which you figured correctly.
Thanks.

I don't think I understand...? I just used the formula F = ma for c). Do I change my answer to a positive? And I'm not sure how to fix d)...

#### LowlyPion

Homework Helper
Thanks.

I don't think I understand...? I just used the formula F = ma for c). Do I change my answer to a positive? And I'm not sure how to fix d)...
Not quite.

9.8 x 95 = 1/2(v2^2) + (9.8 x 25)
That's incorrect. That gives the KE at the top of the last hill. It's the whole 95 m of PE that needs stopping

#### andgabbana

Not quite.

That's incorrect. That gives the KE at the top of the last hill. It's the whole 95 m of PE that needs stopping
c)
I think I need to change my approach?
F = W/d
=(1.1x10^7 J) / 7m
=1.6x10^6 J

d)
W=E(P)
=1.1x10^7 J
...I thought the work required to stop the roller coaster is equal to the total energy of the system?

#### LowlyPion

Homework Helper
c)
I think I need to change my approach?
F = W/d
=(1.1x10^7 J) / 7m
=1.6x10^6 J

d)
W=E(P)
=1.1x10^7 J
...I thought the work required to stop the roller coaster is equal to the total energy of the system?
It is.

But you could have done it the first way too by taking all the potential as kinetic energy and figuring deceleration.

But not J for force.

#### andgabbana

It is.

But you could have done it the first way too by taking all the potential as kinetic energy and figuring deceleration.

But not J for force.
oops... my mistake!
c)
F = W/d
=(1.1x10^7 J) / 7m
=1.6x10^6 N

#### andgabbana

It is.

But you could have done it the first way too by taking all the potential as kinetic energy and figuring deceleration.

But not J for force.
Just to confirm, did I finally do c) and d) correctly?

#### LowlyPion

Homework Helper
Just to confirm, did I finally do c) and d) correctly?
Looks OK. Total energy from potential needs to be accounted for after all.

#### andgabbana

Looks OK. Total energy from potential needs to be accounted for after all.
Thank you!

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