# Kinetic and potential energy and speed

• crpcrpcrp
In summary, the conversation discusses a physics question about an object being dropped from a height and finding the point at which its kinetic energy and potential energy are equal. The conversation goes through the steps of finding the constant energy and solving for the height at which the object's potential energy is equal to its kinetic energy. The participants also discuss the types of energy the object possesses and how to find the constant using equations and math.

#### crpcrpcrp

I'm so confused because 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?

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.

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

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

Hootenanny said:
Okay, let's 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

crpcrpcrp said:
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.

crpcrpcrp said:
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?

crpcrpcrp said:
Kinetic rite?
Are you sure that it only has kinetic energy?

and potential because it starts at rest

crpcrpcrp said:
and potential because 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

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

Hootenanny said:
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

crpcrpcrp said:
Ummmm...i don't really follow
Which bit don't you follow?

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

crpcrpcrp said:
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?

Hootenanny said:
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

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?

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:

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

i don't know

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

crpcrpcrp said:
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

crpcrpcrp said:
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?

crpcrpcrp said:
0?
If the total energy of the object must be constant then the total energy must be the same as the total energy at the start. Does that make sense?

I understand