physics momentum conceptual problem?

Question

Two blocks, A and B, are connected by a compressed string. The mass of bock A is twice that of block B. Immediately after release, compare the net force on block A to the net force on block B. The table is frictionless and the mass of the spring is zero.

1)There is no net force on block A or block B.

2)Fnet of A is equal in magnitude and direction to Fnet B and not zero.

3) Fnet B, will be in the opposite direction and half as large as Fnet A.

4)Fnet A, and Fnet A are both directed toward the left with Fnet A larger than Fnet A, but not twice as large. Block A is pulling block B.

5) Fnet A, will be in the opposite direction and half as large as Fnet B.

6) Fnet A is equal in magnitude and opposite in direction to Fnet B, and not zero.


I think it is 5 because A is twice as massive as B so it would take more force to move it the the same and they will be going in opposite directions. Do you agree?

Answer 1

Yeah, the answer is still (6).

Definitions:
p0 = net momentum of system before release = 0
p = net momentum of system after release = 0
mA = mass of block A
mB = mass of block B
vA = velocity of block A after release
vB = velocity of block B after release

p = p0 ──► mA·vA + mB·vB = 0
mA·vA = -mB·vB
vA = -(mB/mA)·vB = -(mB / (2·mB))·vB = -½·vB

Now, a force is just the time rate-of-change of momentum, so find the derivative of the momentum of each block to find the force on it:

FA = d(pA)/dt = mA·d(vA)/dt = (2mB)·d(-½·vB)/dt = -mB·d(vB)/dt

FB = d(pB)/dt = mB·d(vB)/dt

So you see that the force on block A is equal in magnitude and opposite in direction to the force on block B. Your intuition was on the right track, thinking that the difference in mass would have some effect. However, the effect is that the larger block will just move slower, even though the force on it is the same (you can throw a baseball farther than you can throw a bowling ball with the same force).

Answer 2

the right answer should be 2.

because the 2 block are connected by a compressed string, which means they are acting as one single object.

momentum is: mass times the velocity

the direction of the momentum is the same as the velocity

since both blocks have the same velocity and in the same direction, the momentum of the blocks will be in the same direction

The answer is 2

Answer 3

a) Ur answer alongside with ur reasoning for question "A" is honestly splendid . (cheers !) b) Impulse is the fabricated from tension x time and because tension is mass x acc. So enable the Time and acc be a million s and a million m/s^2 For vehicle A - mass = 1400 kg time = a million s acc. =a million m/s^2 For vehicle A - mass = 1900 time = a million s acc. =a million m/s^2 Impules A = tension x time = mass x acc. x time = 1400x1x1 = 1400 Ns Impules B = tension x time = mass x acc. x time = 1900x1x1 = 1900 Ns So i think of that B has greater advantageous exchange in impulse if the two the automobiles come to take a seat back ! U could verify the 2d ans. lol , U may well be top nevertheless ! ^ _ ^ C) because forces exist in pair , in accordance to newtons 3 rd l;aw of action - each and every action has an equivalent and opposite reaction ! There fore the two adventure equivalent tension of impact ! desire this could help ............. could I ask wherein class are U in !

Answer 4

The answer is 6 by Newton's Third Law.
You are correct that it will take more force to make the more massive block accelerate at the same rate as the smaller block. The problem with this argument is that they don't accelerate at the same rate.

Answer 5

Your answer is 6) by Newton's 3rd law of motion. The forces exerted against masses A and B will thus be equal and opposite.