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# Question 1c: 2015 AP Physics 1 free response

Impact of adding a third mass to our string-pulley system.

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• At, what's the meaning of the different of two blocks is moving more mass? what's the difference bwtween the weight and the mass?
(1 vote)

Think of the situation when there was no block 3. How many external forces are acting on the system which includes block 1 + block 2 + the massless rope connecting the two blocks?

Think about it and it doesn't matter whether your answer is wrong or right, just comment what you think. If it's wrong, you'll learn something new. If it's right, then there is one less thing to learn!
• why is t2 larger than t1
(1 vote)
• The tension on the line between the mass (M3) on the table and the mass on the left( M1) is caused by M1 so it is equal to the weight of M1.

The tension on the line between the mass (M3) on the table and the mass on the right( M2) is caused by M2 so it is equal to the weight of M2.

Since M2 has a greater mass than M1 the tension T2 is greater than T1.
• What would the answer be if friction existed between Block 3 and the table?
(1 vote)
• While writing Newton's 2nd law for the motion of block 3, you'd include friction force in the net force equation this time. Try it.
• at , he says block 3 but means block 2
(1 vote)
• how do you know its connected by different string
(1 vote)
• Since the masses of m1 and m2 are different, the tension between m1 and m3, and between m2 and m3 will cause the tension to be different. Think about it as when there is no m3, the tension of the string will be the same. When m3 is added into the system, there are "two different" strings created and two different tension forces.
(1 vote)
• so is there any equation for the magnitude of the tension, or do we just know that it is bigger or smaller than something? is that because things are not static?
or maybe I'm confusing this with situations where you consider friction...
(1 vote)
• Tension will be different for different strings. If 2 bodies are connected by the same string, the tension will be the same. If one body has a larger mass (say M) than the other, force of gravity will overpower tension in that case. Its equation will be- Mg - T = F.
(1 vote)
• Would the upward force exerted on Block 3 be the Normal Force or does it have another name?
(1 vote)
• I don't understand why M1 * a = T1-m1g and M2g- T2 = M2 * a. Why is the order of the magnitudes are different?
(1 vote)
• can you say "the magnitude of acceleration of block 2 is now smaller because the tension in the string has decreased (another mass is supporting both sides of the block)"?