# Dynamics Question

#### harhar

If friction is negligible, and masses of 100g and 50g are pushed across with an applied force of 1N of the greater mass, how do I find the magnitude of the force exerted by each of the two masses on eachother?

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#### misskitty

What do you mean by "applied force of 1N of the greater mass"? That's kinda confusing. I might be able to help you.

#### harhar

oops sorry...

I meant applied 1 N of force on the greater mass, that being the 100g mass.

#### misskitty

OOOHHH!!

Ok, so let me see if i've got this...your 100g mass is moving and the 50 g mass is stationary...right?

#### harhar

Both masses are in contact with eachother when they are being moved. Like the 50g mass is on top of the 100g one.

#### misskitty

Alright, that makes this a bit easier. What do you have for work so far if anything? Because you can count both of them as one mass since there in contact. That elminates one variable.

#### harhar

I've only found that acceleration is 6.7m/s^2

#### misskitty

Ok...I have notes on a similar problem. Let me see if I can point you in the right direction.

#### misskitty

Couldn't you use the Fnet=ma formula?

Input the total mass as m and use the 6.7 m/s^2 as your a.

#### harhar

That's what I thought but it didn't work. I think that 6.7 is the horizontal acceleration, thats why.

#### misskitty

Hmm, this is interesting. It would've been eaiser had they asked you for the coefficient of kinetic friction. lol.

Well, there is no vertical acceleration so that's out. Are you looking for WORK or FORCE?

#### misskitty

Do you have any other information?

#### harhar

I'm looking for the magnitude of the force exerted by each of the masses on the other. I've mentioned all the info thats given.

#### misskitty

Grr, unfortunately I've never seen a problem like this. All I can think of is newton's thrid law: the forces of two objects acting on one another or of one object acting on another object, the contact forces will be equal and opposite.

That's as far as my thinking takes me right now. Sorry love.

#### learningphysics

Homework Helper
Draw the freebody diagram of the top mass. Draw all the forces being exerted on the top mass. There is weight.... what else is there?

Then use F=ma in both the horizontal and vertical directions to solve for the unknowns. You know a=0 in the vertical direction, and a=6.7 in the horizontal direction.

#### chroot

Staff Emeritus
Gold Member
harhar,

Can you please state the problem exactly as it was given to you, all in one post?

- Warren

#### misskitty

How did you come up with the acceleration of 6.7 m/s^2?

#### harhar

Ok the EXACT question is the same concept but w/ different numbers cuz I just tried to use easy numbers to deal with..

2 masses in contact w/ eachother are 0.113kg and 0.139kg. No friction. The applied horiz. force of magnitude 5.38x10^-2 N is exerted on the heavier mass.

Determine:
a)magnitude of accel. of the 2 mass system.
b)magnitude of the force exerted by each of the 2 masses on the other

I've already gotten a) which is 0.213m/s^2

#### chroot

Staff Emeritus
Gold Member
I assume there's a picture, also, showing one mass above the other. You need to tell us about those parts of the question when posting your question here, or we won't be able to make sense of it.

The answer to b) is simply the weight of the mass on top. The top mass presses its weight down on the bottom mass, and the bottom mass in turn pushes back up, supporting it.

- Warren

#### learningphysics

Homework Helper
harhar said:
Ok the EXACT question is the same concept but w/ different numbers cuz I just tried to use easy numbers to deal with..

2 masses in contact w/ eachother are 0.113kg and 0.139kg. No friction. The applied horiz. force of magnitude 5.38x10^-2 N is exerted on the heavier mass.

Determine:
a)magnitude of accel. of the 2 mass system.
b)magnitude of the force exerted by each of the 2 masses on the other

I've already gotten a) which is 0.213m/s^2
Are the masses on top of each other, or just side by side with one pushing the other along? Is that mentioned in the question? That will affect the answer to b). If it is one on top of each other, have a look at my previous post.

#### learningphysics

Homework Helper
Also, you mentioned "no friction". There is no friction anywhere? Even between the two masses? Or just no friction between the bottom mass and the ground?

#### chroot

Staff Emeritus
Gold Member
lp,

I assumed he meant nowhere at all.

- Warren

#### learningphysics

Homework Helper
chroot said:
I assume there's a picture, also, showing one mass above the other. You need to tell us about those parts of the question when posting your question here, or we won't be able to make sense of it.

The answer to b) is simply the weight of the mass on top. The top mass presses its weight down on the bottom mass, and the bottom mass in turn pushes back up, supporting it.

- Warren
I'm pretty sure there can be friction between the two masses.

#### chroot

Staff Emeritus
Gold Member
"2 masses in contact w/ eachother are 0.113kg and 0.139kg. No friction." Sounds like no friction between the bodies to me.

- Warren

#### harhar

No friction between mass and ground. There's no picture...but anyways if the mass isn't on the top then I assume they're side by side b/c I haven't been able to get the answer. How do I find the force if they are side by side?

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