Homework Help: Physics

Posted by Physics on Sunday, July 12, 2009 at 4:24pm.

I got two questions

Three blocks on a fricitonless horizontal surface are in contact with each other

a force F is applied to block 1 mass m1

Draw a free body diagram for each block
ok done

the accelration of the system (in terms of m1, m2, and m3)
ok done I got
a = (m1 + m2 + m3)^-1 (force F)

the force of contact that each block exerts on its neighbor
ok no idea how to do this

----->Force F | m1 |--->contact force | m2 |---->contact force | m3 |

ok so how do i do this
ok and

a block (mass m1) on a smooth horizontal surface, connected by a thin cord that passes over a pulley to a second block (m2), which hangs vertically.

Draw a free body diagram for each block showing the force of gravity on each, the force (tension) exerted by the cord, and any normal force

ok done

determine formulas for the acceleration of the system and for the tnesion in the cord. Ignore friction and the masses of the pulley and cord

ok well isn't the force of tension just the opposite in direction but equal in magntiude of the force of gravity on second hanging block?

Also isn't the accleeration zero???

Isn't the net force zero???

ok i do not know how to find the acceleration of the system

Physics - Physics, Sunday, July 12, 2009 at 4:55pm
Ok I guess I'm drawing the free body diagram wrong how do I draw it because this is not making much sense to me
Physics - drwls, Sunday, July 12, 2009 at 4:57pm
Why did you start this question by re-asking the three blocks question that we have already explained to you? I assume that you followed directions and are through with that one.

As for the problem with the two masses, cord and pulley: the acceleration is NOT zero in a frictionless situation. The dangling mass m2 exerts a weight force on the cord, which allows both masses to move with the same acceleration, a. What equations of motion have you derived from your FBDs?

The tension force T in the cord does not equal the weight force W = m2*g when the masses are accelerating.

You should have two equations in two unknowns: T and a.

T = m1*a
m2*g - T = m2*a

m2*g = (m1 + m2) a
Solve for a and T
Physics - Physics, Sunday, July 12, 2009 at 4:57pm
is this right?
------Force F onto m1--->|m1| ---contact force onto m2--->|m2|-----contact force onto m3---->|m3|
Physics - drwls, Sunday, July 12, 2009 at 6:24pm
No, it is not right. Force and mass have different dimensions.

Mass has to be multiplied by an acceleration rate to get force.

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