Kinetic and gravitational energy

In summary, the conversation discusses how to calculate the kinetic energy and gravitational energy of an object at a height of 15m above the ground, traveling at 25m/s. The formulas for kinetic energy and work are mentioned, but the issue of not knowing the mass of the object poses a problem. It is suggested to solve the problem generically first, using the conservation of energy and the information given about the object's velocity and position at two different times. The direction of the object's velocity will determine if it is a 1 or 2 dimensional problem.
  • #1
msimard8
57
0
Q: An object at a height of 15 m above the ground is traveling at 25m/s. Find its

a) kinetic energy
b) gravitational energy

If I find out how to do (a), I am sure I will figure out B. The problem is I don't know how to figure out (a)

I know that Kinetic Energy = 1/2 mass (speed)^2

I also know that work = (change of kinetic energy)

Both these formulas involve masses. How do you solve?

I do know that masses cancel out when you are dealing with the law of conservation of energy, but I am only determining the energy at this specific point. Help.
 
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  • #2
In my humble opinion this exercise is not well elaborated. For what I know and it's logic, an object of 100 kg at a height of 15 m above the ground traveling at 25m/s has more kinetic energy than other of 3 kg at the same situation (the kinetic energy formula shows that very clear). In order to calculate his kinetic energy you must know is mass. In any case, I think you must have to admit that the mass of the object is 1 kg. Check the solutions.

PS- As i said this is a modest opinion, wait for the experts.
 
  • #3
thnx

thanks for your opinion

i still think its solvable though. I will wait for the experts.
 
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  • #4
I really don't understand. I have an exam tommorrow.

My recent thought processes include

ET= ek + eg
= 1/2mv^2 + mgh

but how is that possible.. you need a mass?
HELP
 
  • #5
In physics courses, "solving a problem" does not always (in fact rarely) means getting a number.

Even when you're given all the necessary numbers to get a numeric result, you'll generally want to first solve it as generically as possible. Only then do you plug in the numbers.

Just use the information you were given to solve it as much as much as you can. Often times, the information you weren't given turns out to be irrelevant (mathematically, terms might cancel).

With the information you gave and assuming ideal situations (in a vacuum, etc), we can say the only force is that of gravity, and the energy is conserved.
So we can look at the system at two different times:
1) When the object is at h=15m with v=25m/s, and
2) When the object hits the ground.

This is helpful because when the object is at ground-level, we know its potential (gravitational) energy is zero, and the total energy is equal to the kinetic energy.
Solve for its kinetic energy when it hits the ground, and you have an expression for total energy that contains a mass. Now this will cancel out the m's in the original kinetic and potential energy expressions.

However, you need to know the direction of the velocity in its initial state. If the object is traveling up or down at 25m/s initially, it's a 1 dimensional problem. If it's traveling in any other direction, it's a 2 dimensional problem.

Hope this helps. I've been awake for far too long, so if anything doesn't make sense.. please ask. ;)

--
Edit: made things a little clearer.
 
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What is Kinetic Energy?

Kinetic energy is the energy an object possesses due to its motion. It is calculated by multiplying the mass of the object by the square of its velocity and dividing by 2 (KE = 1/2mv^2).

What is Gravitational Energy?

Gravitational energy is the potential energy an object has due to its position in a gravitational field. It is calculated by multiplying the mass of the object by the acceleration due to gravity and its height (PE = mgh).

How are Kinetic and Gravitational Energy related?

Kinetic and gravitational energy are related in that an object's kinetic energy can be converted into gravitational energy and vice versa. For example, when a rollercoaster car reaches the top of a hill, its kinetic energy is converted into gravitational potential energy.

What are some examples of Kinetic Energy?

Examples of kinetic energy include a moving car, a person running, a spinning top, or a swinging pendulum.

What are some examples of Gravitational Energy?

Examples of gravitational energy include a ball sitting on a shelf, a book on a table, or a satellite orbiting the Earth.

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