Help with equations for Kinetic Energy.

[imthedan]

Pretty basic and I'm sure for you guys pretty lame, but I am taking a science class that is required for me to get my AA. I am having a real hard time and I'd appreciate some help. Ok, the question:

 

If an object with a mass of 4.0 kg has a velocity of 7.0 m/s, what is the kinetic energy?

 

I have my book and it says the equation for Kinetic energy is: KE = 1/2(mv²)

 

I have to solve in Joules. Now, what I have so far (and probably WAY wrong) is:

 

KE = 1/2(mv²)

KE = 1/2(4kg x 49m/s²)

KE = 1/2(196N)

KE = 98N

 

Now, I don't even know if I did that right -- but on top of that, I have to solve it in Joules and I don't even know where to start for that. I did a google search and there are converters that are supposed to be 1 Newton is = 1 Joule/Meter.

 

ah, I am just so confused.. I have this paper due tomorrow and have no idea how to do it. I've tried reading the book, but it just skims over this part giving the equation and what kinetic energy is.

 

Help is appreciated. Thanks.

[Cap'n Refsmmat]

You've got it right. You just forgot to square the units all the way:

 

kg x (m/s)²

kg x m²/s²

 

which is the definition of a Joule.

 

You could even simplify it down to

 

kg x m x m/s²

 

and a kg m/s² is a Newton, so

 

N x m

 

which is another definition for a Joule.

[imthedan]

Thanks for the help. I appreciate it.

 

When do I use the kg x (m/s)² equation? I know that's the definition for Joule, but how would I use that with the original question? Do I take the weight (4.0kg) multiply that by meter (7x7) and seconds (1x1)?

 

so it'll still come out to 4kg x 49m/s, right? Another thing that confuses me is do I still have to square the final value? Is it 49m/s or 49m/s²?

 

Thanks.

Edited March 5, 2009 by imthedan

[swansont]

The term is v^2, so you square everything that represents the speed: the magnitude and the units. So (7 m/s)^2 is 49 m^2/s^2

 

Also, mass and weight are not the same thing.

[imthedan]

Yeah, I had a lot of things cleared up today in class. We had a review today for our test on next Thursday. I knew that mass and weight weren't the same, but I'm new to the science terms and say weight still (actually got corrected in class today.. haha)..

 

I also had the when to use m/s and m/s² part cleared up as well.

 

Thanks for all the help guys.

[samtheflash82]

i thought that for the KE formula to work, you had to first convert kilograms into newtons by multipleying the number of kg by 9.8, which would give you the mass in newtons and once you multiply it by the velocity squared, you would get a number in newtonmeters, aka joules.

[Cap'n Refsmmat]

That wouldn't be the mass in Newtons, that would be the weight. You wouldn't get a number in Joules that way because of the way the units work out. See my post above.

[samtheflash82]

That wouldn't be the mass in Newtons, that would be the weight. You wouldn't get a number in Joules that way because of the way the units work out. See my post above.

 

ah, my mistake. you are right.