Calculating momentum when you know mass and KE

[connie5828]

Homework Statement

Calculate the momentum of an object of mass 8 kg if its kinetic energy is 70 J

Homework Equations

I am not sure what the equation is. Every one I have tried hasn't worked

The Attempt at a Solution

KE=MV squared?

 

[diazona]

Kinetic energy is not equal to mv².

Write down the definitions of kinetic energy and momentum. (You do know them, right?) Then combine them by solving for the variable you don't know.

 

[collinsmark]

Hello connie5828,

Welcome to Physics Forums!

Homework Statement

Calculate the momentum of an object of mass 8 kg if its kinetic energy is 70 J

Homework Equations

I am not sure what the equation is. Every one I have tried hasn't worked

Well, you know the object's kinetic energy and mass, so what does that say about the object's velocity? If you happen to figure out the object's velocity, and since you know its mass, then its momentum is ...?

You'll frequently find that physics is usually not about simply plugging in numbers into an existing equation (although that does happen sometimes). Much of the time you'll find that multiple laws/equations are necessary to be combined to figure out a specific problem. Which is why it's a good idea to reflect upon the equations to figure out what those equations really mean, and how they apply to various problems.

The Attempt at a Solution

KE=MV squared?

Close, but not quite right. You'll need the correct equation for kinetic energy (it's almost what you wrote above, but not quite), and another equation which describes momentum.

[Edit: I see diazona beat me to the answer.]

 

[connie5828]

looks like my first reply may not have posted. thanks for your help. Please correct what I am not understanding

KE=MV²/2
so equation would be 70=8²/2
70=64/2 (is this correct??)
70/32=2.188
P=MV
P=8*2.188

the problem is I am using the practice equation to try to figure this out and it says the answer is 33. My calculations are definitely off somewhere. Please help.

 

[Rasalhague]

Hi Connie, you've got the definitions right now, but you squared the wrong thing. KE is mv²/2 = 8v²/2, but you wrote 8²/2 = m²/2.

Start with the equation 70 = mv²/2. Substitute for the mass: 70 = 8v²/2. Then solve for v. That means keep doing the same thing (multiply or divide by the same numbers) to each side of the equation till you have it in the form v² = some number. Then take the square root of each side, so you have v = the square root of that number.

Then go to your other equation, momentum = mv, and replace the v with what you found v was equal to.

 

[connie5828]

totally got it. THANK YOU! Taking anatomy and Physics and anatomy I rock at Physic I learning with the help of people who have amazing science brains. Thanks!