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3 physics MCQs
發問:
1. 圖片參考：https://s.yimg.com/rk/HA00274964/o/8355862.jpg 2. 圖片參考：https://s.yimg.com/rk/HA00274964/o/795938851.jpg 3. 圖片參考：https://s.yimg.com/rk/HA00274964/o/2021563862.jpg please show your steps clearly thanks!! 更新: 3) could you please explain also why the other options are wrong? thanks! 更新 2: but Q and q should be of the same sign? is it ok assume that they are of opposite signs while deciding whether the other options are correct? 更新 3: but you said the answer should be B...? that means they should be of the same sign? 更新 4: sorry but i still don't understand it is known that Q and q should be of the same sign, isn't it inappropriate to explain why the other options are wrong by saying 'If Q and q are of opposite signs..'? sorry to be so annoying but I'm really confused. Thanks for your help
最佳解答:
1. Let N be the normal reaction and Ff be the frictional force. Hence, consider the horizontal components, we have N.sin(5) + Ff.cos(5) = 2400 x 10^2/60 i.e. N.sin(5) = 4000 - Ff.cos(5) ------------ (1) Consider the vertical components, N.cos(5) = 2400g + Ff.sin(5) ------------ (2) (1)/(2): tan(5) = [4000 - Ff.cos(5)]/[2400g + Ff.sin(5)] solve for Ff give Ff = 1893 N The answer is option A 2. Let T be the tension in the string. The required centripetal force = Mv^2/(L.sin(theta)) Hence, T.sin(theta) = Mv^2/(L.sin(theta)) T = Mv^2/[L.sin^2(theta)] The answer is option D 3. The answer is option B. The curvature of XY shows that the force between Q and q is a repulsive force. Hence, Q and q must be of the same sign. 2014-08-11 11:51:26 補充： Your suppl question. Option A is wrong. If Q and q are of opposite sign, there is an attractive force. q will not be able to travel from X to Y. 2014-08-11 11:56:47 補充： Option C is wrong. If Q and q are of opposite signs, an attractive force exists. When the charge is released at Y, its speed at X will be higher than that at Y. 2014-08-11 11:58:20 補充： Option D is wrong. If Q is a -ve charge, the potential at Y will be higher than that at X. 2014-08-11 19:46:04 補充： Q:is it ok assume that they are of opposite signs while deciding whether the other options are correct? A: You are given the shape of the path XY in the question. Should Q and q be of opposite sign, you wouldn't be able to get the path as given in the question.... 2014-08-11 19:48:53 補充： (cont'd)...When q is released from X, it would be attracted towards the left hand side. When it is released from Y, it would follow more or less the field lines and move towards the left. 2014-08-11 23:57:46 補充： Q: you said the answer should be B? that means they should be of the same sign? A: This is the case. Given the shape of path XY, the force is repulsive. Hence, Q and a must be of the same sign, i.e. either both +ve or both -ve. An attactive force would not give the path XY as shown in the question. 2014-08-12 15:52:06 補充： As the "words limit" has exceeded, I give the answer in the 意見 column. 2014-08-12 15:55:08 補充： Q:it is known that Q and q should be of the same sign, isn't it inappropriate to explain why the other options are wrong by saying 'If Q and q are of opposite signs..'? A:The question doesn't give anything on the signs of Q and q... 2014-08-12 15:55:41 補充： (cont'd)...Hence, options A,C and D are correct if and only if the option description is valid for Q and q have the same sign and also Q and q have opposite signs. 2014-08-12 15:59:06 補充： (cont'd)...take option A for example, the description is valid for Q and q have the same sign, but NOT for Q and q have opposite signs. Hence, it is wrong.
3 physics MCQs
發問:
1. 圖片參考：https://s.yimg.com/rk/HA00274964/o/8355862.jpg 2. 圖片參考：https://s.yimg.com/rk/HA00274964/o/795938851.jpg 3. 圖片參考：https://s.yimg.com/rk/HA00274964/o/2021563862.jpg please show your steps clearly thanks!! 更新: 3) could you please explain also why the other options are wrong? thanks! 更新 2: but Q and q should be of the same sign? is it ok assume that they are of opposite signs while deciding whether the other options are correct? 更新 3: but you said the answer should be B...? that means they should be of the same sign? 更新 4: sorry but i still don't understand it is known that Q and q should be of the same sign, isn't it inappropriate to explain why the other options are wrong by saying 'If Q and q are of opposite signs..'? sorry to be so annoying but I'm really confused. Thanks for your help
最佳解答:
1. Let N be the normal reaction and Ff be the frictional force. Hence, consider the horizontal components, we have N.sin(5) + Ff.cos(5) = 2400 x 10^2/60 i.e. N.sin(5) = 4000 - Ff.cos(5) ------------ (1) Consider the vertical components, N.cos(5) = 2400g + Ff.sin(5) ------------ (2) (1)/(2): tan(5) = [4000 - Ff.cos(5)]/[2400g + Ff.sin(5)] solve for Ff give Ff = 1893 N The answer is option A 2. Let T be the tension in the string. The required centripetal force = Mv^2/(L.sin(theta)) Hence, T.sin(theta) = Mv^2/(L.sin(theta)) T = Mv^2/[L.sin^2(theta)] The answer is option D 3. The answer is option B. The curvature of XY shows that the force between Q and q is a repulsive force. Hence, Q and q must be of the same sign. 2014-08-11 11:51:26 補充： Your suppl question. Option A is wrong. If Q and q are of opposite sign, there is an attractive force. q will not be able to travel from X to Y. 2014-08-11 11:56:47 補充： Option C is wrong. If Q and q are of opposite signs, an attractive force exists. When the charge is released at Y, its speed at X will be higher than that at Y. 2014-08-11 11:58:20 補充： Option D is wrong. If Q is a -ve charge, the potential at Y will be higher than that at X. 2014-08-11 19:46:04 補充： Q:is it ok assume that they are of opposite signs while deciding whether the other options are correct? A: You are given the shape of the path XY in the question. Should Q and q be of opposite sign, you wouldn't be able to get the path as given in the question.... 2014-08-11 19:48:53 補充： (cont'd)...When q is released from X, it would be attracted towards the left hand side. When it is released from Y, it would follow more or less the field lines and move towards the left. 2014-08-11 23:57:46 補充： Q: you said the answer should be B? that means they should be of the same sign? A: This is the case. Given the shape of path XY, the force is repulsive. Hence, Q and a must be of the same sign, i.e. either both +ve or both -ve. An attactive force would not give the path XY as shown in the question. 2014-08-12 15:52:06 補充： As the "words limit" has exceeded, I give the answer in the 意見 column. 2014-08-12 15:55:08 補充： Q:it is known that Q and q should be of the same sign, isn't it inappropriate to explain why the other options are wrong by saying 'If Q and q are of opposite signs..'? A:The question doesn't give anything on the signs of Q and q... 2014-08-12 15:55:41 補充： (cont'd)...Hence, options A,C and D are correct if and only if the option description is valid for Q and q have the same sign and also Q and q have opposite signs. 2014-08-12 15:59:06 補充： (cont'd)...take option A for example, the description is valid for Q and q have the same sign, but NOT for Q and q have opposite signs. Hence, it is wrong.
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