A Super Easy/Stupid Physics Question

Factual Questions
1

This one should be a no-brainer for someone who knows what they’re talking about, but I’m not one of those people so I’ve gotta ask.

Let’s say that we have Object A on a frictionless table on Earth. Then let’s say we tie a string to Object A and run the string over a pulley wheel at the edge of the table so it hangs over the table’s edge. Then, let’s tie Object B to the end of the string dangling off the table. When released, Object B will drop to the floor. The force will be transfered through the string and have its direction changed by the wheel. Object A will be pulled along until it is forced to stop by being pulled off the table or whatever. When using the calculation F = ma for this situation, is m the mass of Object A or the mass of Objects A AND B?

If I missed something important, I’m sorry. Please correct my ignorance.

Thanks guys!

2

How much mass is moving? That would be the masses A and B.

3

To calculate the FORCE, only use the mass of a. to calculate the ACCELERATION, use the mass of a + b.

eg, if g = 10, a = 10, b = 40,

F = ma
F = 10 * 10
F = 100 N

F = ma
100 = (10+40) * a
a = 2 m/s

4

Suppose T is the tension in the string. Although F=ma is a vector equation, here I use it as a scalor.

For the hanging mass

m_B x g - T = m_B x a (a being the acceleration )

or T = m_B x (g - a) ----- 1

For the sliding mass

T = m_A x a ----- 2

Using 1 and 2

m_B x (g - a) = m_A x a

or a = (m_B x g )/ (m_A + m_B)

And next time don’t wait for the last minute for your homework.

5

Last minute? It’s Saturday night - he’s got all day tomorrow to finish it. :wink: