picture:http://yfrog.com/0x51353673p
Two blocks X and Y are connected by a light string passing over a smooth pulley as shown above. The mass of
X is greater than that of Y. The two blocks are released from rest. Which of the following statements is/are
correct?
(1) The potential energy lost by X is equal to the potential energy gained by Y
(2) The kinetic energy gained by X is greater than the kinetic energy gained by Y
(3) The potential energy lost by X is equal to the sum of kinetic energy gained by X and Y
A. (2) only B. (3) only C. (1) and (2) only D. (1) and (3) only E. (1),(2) and (3)
請講埋解釋!!!!!!!!!!!
phy mc
2010-01-17 2:39 am
回答 (2)
2010-01-17 3:01 am
✔ 最佳答案
Statement (1) is wrongwhen X falls a distance d, say, Y will rises the same distance d. But mass of X is greater than mass of Y, hence loss of PE of X is larger than that for Y.
Statement (2) is correct.
since both masses X and Y must be of the same speed because they are connected together by a string, but the mass od X is greater than the mass of Y, thus the KE of X is higher than that of Y.
Statement (3) is wrong
By conservation of mechanical energy,
PE lost by X = KE gained by X + KE gained by Y + PE gained by Y
2010-01-17 4:53 am
Let m1 = the mass of X
m2 = the mass of Y
(1) Since the string is inextensible, the distance travelled by X = that by Y.
PE lost by X = (m1)gh
PE gain by Y = (m2)gh
Since m1>m2 (given)
PE lost by X > PE gain by Y
(2) Since the string is inextensible, the speed of X = speed of Y.
KE of X = 1/2 (m1) v^2
KE of Y = 1/2 (m2) v^2
Since m1>m2
KE of X > KE of Y.
(3) Since there is no external force, energy is conserved.
Thus, PE lost by X = (PE gain by Y) + (KE gain by X) + (KE gain by Y)
Obviously, the answer is A.
m2 = the mass of Y
(1) Since the string is inextensible, the distance travelled by X = that by Y.
PE lost by X = (m1)gh
PE gain by Y = (m2)gh
Since m1>m2 (given)
PE lost by X > PE gain by Y
(2) Since the string is inextensible, the speed of X = speed of Y.
KE of X = 1/2 (m1) v^2
KE of Y = 1/2 (m2) v^2
Since m1>m2
KE of X > KE of Y.
(3) Since there is no external force, energy is conserved.
Thus, PE lost by X = (PE gain by Y) + (KE gain by X) + (KE gain by Y)
Obviously, the answer is A.
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