# Physics question...

## Recommended Posts

A 2 kg mass (m1) and a 3 kg mass (m2) are attached to a lightweight cord that passes over a frictionless pulley.

Assume the positive direction of motion to be when the smaller object moves upward and the larger mass moves downward.

Okay...I went around trying to solve this as a normal tension problem...going like this:

T - (m1)g = (m1)a and T - (m2)g = (m1)a (g = 9.8, a = acceleration)

therefore: T = (m1)a + (m1)g and T = (m2)a + (m2)g

(m2)a + (m20g = (m1)a + (m2)g

solving that gives: a = 9.8 (meters per second squared)

the answer key says: 1.96 (meters per second squared)

My answer makes more sense to me...but I have a feeling I'm wrong (I get the feeling physics likes to chuck intuition out the window).

##### Share on other sites

solving that gives: a = 9.8 (meters per second squared)

the answer key says: 1.96 (meters per second squared)

My answer makes more sense to me...but I have a feeling I'm wrong (I get the feeling physics likes to chuck intuition out the window).

It makes sense to you that the object will fall at g, like an untethered mass, even though the 3 kg mass needs to lift a 2 kg mass? Your intuition needs some honing.

You have

T - (m1)g = (m1)a

T - (m2)g = (m2)a (you had m1, but it should be m2)

T and g have opposite signs, which is OK, but a will have a different sign for the two equations. The positive direction was defined to be down for the larger mass, so the second equation is incorrect.

## Create an account

Register a new account