# Kinetic and potential energy and speed

Could someone plz help me out??!!!  I'm so confused cuz we were never taught any of this and i have to hand in this assignment 2morrow

1.An object is dropped from a height of 10m at what height will it s kinetic energy and potential energy be equal?

2.A 100kg roller coaster car starts from rest on the top of a 30m high track.It rolls down to a 10m high deep then back up to a 15m crest. What are the speeds of the cart at (a)the deep and (b)the crest?

Help me plzzz  I know i am supposed to at least attempt the problem but I have no idea where to start.We have never been taught this stuff.If i could at least get an equation that can help i would be very grateful.

Hootenanny
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Okay, lets take question one first. If we ignore air resistance we can say that the total energy of the object must remain constant, yes?

Astronuc
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Also, in question 1, the potential energy is taken with respect to 0 m. Presumably, the object starts 'at rest'.

Okay, lets take question one first. If we ignore air resistance we can say that the total energy of the object must remain constant, yes?
yea.I guess there is constant velocity

Hootenanny
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yea.I guess there is constant velocity
No, the velocity is not constant. I said the total energy must remain constant.

Oh sry i'm not very good at physics.

Hootenanny
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Oh sry i'm not very good at physics.
No problem, so now we've established the total energy of the object is constant. What types of energy does the object posses?

Kinetic rite?

Hootenanny
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Kinetic rite?
Are you sure that it only has kinetic energy?

and potential cuz it starts at rest

Hootenanny
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and potential cuz it starts at rest
Yes, it also has potential, but not because it starts at rest. It has potential energy because it is some distance above the ground, does that make sense?

So, now we can say that;

kinetic energy + potential energy = constant

Do you follow?

yea i get it

Hootenanny
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So, if we want to find the point at which kinetic energy = potential energy we can say that;

potential energy + potential energy = 2*potential energy = constant

So, if we want to find the point at which kinetic energy = potential energy we can say that;

potential energy + potential energy = 2*potential energy = constant
Ummmm........i don't really follow

Hootenanny
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Ummmm........i don't really follow
Which bit don't you follow?

This bit:
potential energy + potential energy = 2*potential energy = constant

Hootenanny
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This bit:
potential energy + potential energy = 2*potential energy = constant
Okay, suppose we have an equation;

a + b = c

Now, suppose we say a=b, then we have;

a + a = 2*a = c

Is that a little clearer?

Okay, suppose we have an equation;

a + b = c

Now, suppose we say a=b, then we have;

a + a = 2*a = c

Is that a little clearer?
Yea i get it now

Hootenanny
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Good . So now we have;

2*potential energy = constant

Now we need to find the constant. How do you suggest we do that? use direct variation??? Hootenanny
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If the total energy of the system is constant, what is the total energy of the system before the object is dropped?

i don't understand the question:uhh:

Hootenanny
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What is the initial energy of the stone before it is dropped?

i don't know Hootenanny
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Okay, before the object is dropped find;

1. The kinetic energy of the object
2. The potential energy of the object

isn't it 0J

how do i find it?I was not given to velocity or mass

Hootenanny
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how do i find it?I was not given to velocity or mass
Firstly, what is the velocity of the object before it is dropped?

the potential energy=mass*gravity*height
=10*10*m
=100m

is the initial velocity=0ms-1

Hootenanny
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the potential energy=mass*gravity*height
=10*10*m
=100m
Correct! So since the object is at rest before it is dropped, all the energy is in the form of potential energy. So the constant in our equation must be....

0????? Hootenanny
Staff Emeritus
0????? 