# A Super Easy/Stupid Physics Question **URL:** \ **Category:** Factual Questions\ **Created:** [November 24, 2002, 4:31am UTC](https://boards.straightdope.com/t/a-super-easy-stupid-physics-question/138448 "2002-11-24T04:31:41Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Civil\_Twilight](https://avatars.discourse-cdn.com/v4/letter/c/db5fbb/32.png) [@Civil\_Twilight](https://boards.straightdope.com/u/Civil_Twilight)\ **Post date:** [November 24, 2002, 4:31am UTC](https://boards.straightdope.com/t/a-super-easy-stupid-physics-question/138448/1 "2002-11-24T04:31:41Z")
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! ---
**Author:** ![Race\_Bannon](https://avatars.discourse-cdn.com/v4/letter/r/5f8ce5/32.png) [@Race\_Bannon](https://boards.straightdope.com/u/Race_Bannon)\ **Post date:** [November 24, 2002, 4:37am UTC](https://boards.straightdope.com/t/a-super-easy-stupid-physics-question/138448/2 "2002-11-24T04:37:35Z")
How much mass is moving? That would be the masses A and B. ---
**Author:** ![Shalmanese](https://avatars.discourse-cdn.com/v4/letter/s/45deac/32.png) [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)\ **Post date:** [November 24, 2002, 4:40am UTC](https://boards.straightdope.com/t/a-super-easy-stupid-physics-question/138448/3 "2002-11-24T04:40:24Z")
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 ---
**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\ **Post date:** [November 24, 2002, 4:40am UTC](https://boards.straightdope.com/t/a-super-easy-stupid-physics-question/138448/4 "2002-11-24T04:40:42Z")
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. ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [November 24, 2002, 4:53am UTC](https://boards.straightdope.com/t/a-super-easy-stupid-physics-question/138448/5 "2002-11-24T04:53:02Z")
Last minute? It’s Saturday night - he’s got all day tomorrow to finish it. 😉