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# Question 1b: 2015 AP Physics 1 free response

Acceleration of two masses connected by string-pulley system.

## Want to join the conversation?

• why is the direction of the forces correspondent with the direction of acceleration..? I thought up was positive and down was negative? I'm literally so confused. • well, you are right in both ways but your confusion is soon fixed.

1) acceleration and force always go in the same direction as one another. This is because F=ma so they will always be linked.
Now they can go (together) in any direction.

2) yes, we usually think of up as the positive direction and down as negative. (also to the right is positive and to the left is negative) This is just a way of describibg the direciton of forces, acceleration, velocity etc .... in fact any vector.

so, for example... if you fall freely through the air, the FORCE is gravity and this is your weight. You have an acceleration towards the Earth. Both force and acceleration are negative because they point down.

ok??
(1 vote)
• Why is mass added but the forces subtracted? when I tried to work through it I subtracted the masses instead of adding them.
(1 vote) • Let's say you think of Blocks 1, 2, and the string all as one system. We know that the magnitude of acceleration of the entire system will be equal to the magnitude of acceleration of block 2.
As per Newton's second law, we know that a = (net force)/(total mass). The net force will be equal to the weight of block 2 minus the weight of block 1, because the object is moving in the direction of the weight of block 2. So, the net force = m1g-m2g. If we factor the g out, we get g(m1 - m2). This should be the term in the numerator of the expression for acceleration, because we can plug it in to the Newton's second law.
The total mass of the system should be the sum of the two masses, so this is why mass is added in the denominator as (m1 + m2) when we plug this into the Newton's second law.
I hope that helped!!
(1 vote)
• Sal said that the force of tension(both in the y direction) are the same in magnitude but is that true when one object has tension in the horizontal direction and the other has it vertical?
(1 vote) • why is the direction of the forces correspondent with the direction of acceleration..? I thought up was positive and down was negative? I'm literally so confused. • 1) The direction of the force vector corresponds directly to the direction of acceleration because of Newton's Second Law's equation: F = m * a. The mass of an object is a one-dimensional quantity and only changes the magnitude of the result, not its direction. Therefore, the net force vector will have the same direction as acceleration, but not always the same magnitude

2) We can define which are the positive directions and which are the negative directions in order to make a problem easier to visualize/compute. For example, in free-fall problems, it can be helpful to think of downwards as positive, since then you have a positive acceleration.

Also, I'm just curious: why'd you just copy and paste the most popular question from the video?
(1 vote)
• Can i derive the magnitude of accelaration by finding a(system) which is F(ext)/(total mass) i know this will get me the right answer but i am from india and will be giving AP physics C in 2017 so wanted to know can i do this way in the paper...?  