標題:
Momentum (20marks) Please help me T T
發問:
Two blocks with different masses are joined by an elastic string on a smooth ground. The blocks are pulled apart to stretch the string. Which of the following statements is/are correct if the blocks are now released? 更新: (1) At any instant, each of the blocks is pulled by the same magnitude of force. (2) At any instant, each of the blocks moves with the same magnitude of acceleration. (3) The total momentum of the blocks after release is greater than that before release. 更新 2: The answer is (1) only. I don't know why (1) is correct. I also don't know why (2) and (3) are not correct. Can anyone explain briefly with this three statements? Thx for helping !!!
最佳解答:
1 is correct because both the blocks are pulled by the same string, ie the same tension, since the tension in a sring must be equal in magnitude. 2 is incorrect because their although their net force are the same, their masses are not. F=ma, F is the same for both blocks but m is not. The one with larger mass will have a smaller acceleration. 3 is incorrect due to conservation of momentum. The momentum before release is the same as after the release. The tension of the string is the internal force of the whole system. 2007-04-07 11:50:31 補充： 3: The original momentum is 0. After the release, mass M gain a momentum P.P/t (time)=T (tension). But for mass B, it gains momentum p. p/t=-TP=Ttp=-tTSo the total momentum is Tt-Tt=0 2007-04-07 11:50:39 補充： (we should use differentiation and integration. But for simpicity, we assume the change in momentum is constant in time) 2007-04-07 11:53:26 補充： me_acw:For your statement 3, I think I need to clear your misconception. W and R are NOT action-and reaction pair. They both act on the same object.The reaction force of W is the gravitation force act on Earth. 2007-04-07 11:56:20 補充： The reaction force of R is the normal force act on the ground by the block.
其他解答:
Momentum (20marks) Please help me T T
發問:
Two blocks with different masses are joined by an elastic string on a smooth ground. The blocks are pulled apart to stretch the string. Which of the following statements is/are correct if the blocks are now released? 更新: (1) At any instant, each of the blocks is pulled by the same magnitude of force. (2) At any instant, each of the blocks moves with the same magnitude of acceleration. (3) The total momentum of the blocks after release is greater than that before release. 更新 2: The answer is (1) only. I don't know why (1) is correct. I also don't know why (2) and (3) are not correct. Can anyone explain briefly with this three statements? Thx for helping !!!
最佳解答:
1 is correct because both the blocks are pulled by the same string, ie the same tension, since the tension in a sring must be equal in magnitude. 2 is incorrect because their although their net force are the same, their masses are not. F=ma, F is the same for both blocks but m is not. The one with larger mass will have a smaller acceleration. 3 is incorrect due to conservation of momentum. The momentum before release is the same as after the release. The tension of the string is the internal force of the whole system. 2007-04-07 11:50:31 補充： 3: The original momentum is 0. After the release, mass M gain a momentum P.P/t (time)=T (tension). But for mass B, it gains momentum p. p/t=-TP=Ttp=-tTSo the total momentum is Tt-Tt=0 2007-04-07 11:50:39 補充： (we should use differentiation and integration. But for simpicity, we assume the change in momentum is constant in time) 2007-04-07 11:53:26 補充： me_acw:For your statement 3, I think I need to clear your misconception. W and R are NOT action-and reaction pair. They both act on the same object.The reaction force of W is the gravitation force act on Earth. 2007-04-07 11:56:20 補充： The reaction force of R is the normal force act on the ground by the block.
其他解答:
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For statemet (1), the two block is pulled by the same force is because of the internal force in the elastic string. As two block is connected together by an elastic string, the force pulling the block is the tension in the string. We already know that tension is an internal force and should have the same magnitude. Therefore, statement (1) is true. For statement (2), as the two block block has different mass, they should have different magnitude of accerlation. By newton's second law, F=ma, we know that the net force acting on the blocks should be directional properties to the acceleration with the propertional constant is the mass. For the two blocks with different mass, they should have different acceleration as they have different mass but same net force. For statement (3), the total momentumof the blocks after release is equal to that before releasing. The reason is that momentum should be constant. Only one constrain for this theory is that there is no external net force acting on the system. The forces acting on these system is the weight of the blocks(W), the tension in the string(T) and the normal reaction(R). As W and R is an action-and-reaction pair, W=R. Furthermore, T is the internal force and do not affect the momentum of the whole system. Therefore, momentum should be conserves.
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