PHYSIC - Ans.fyi 參考答案

PHYSIC

bennyh1993

2010-04-03 4:53 am

18. Which of the following statements is
INCORRECT?
A. The shape of the roller coaster track in the
middle part will not affect the speed of the
cart at the lowest position.
B. A swinging pendulum eventually stops
because energy is dissipated.
C. A ping-pong ball released from a height h
can never bounce to a height higher than h
whatever the initial speed of the ball.
D. A ping-pong ball released from rest at a
height h will not bounce to a height higher
than h.
22. P and Q are two parallel straight wires carrying
equal currents flowing out of the paper. R is
equidistant from P and Q, and carries current
flowing into the paper. What is the direction of
the resultant force acting on R by P and Q?
23. A ball of mass 3 kg released from rest at a height of
2 m. It hits the ground and rebounds to a height of
1.8 m. Calculate the amount of energy dissipated
in the process.
A. 1.5 J
B. 1.2 J
C. 0.6 J
D. 0.7 J

回答 (1)

Prof. Physics

2010-04-03 6:23 am

✔ 最佳答案

18. The answer is C.

For A, the speed of the roller coaster at the bottom depends only on the total mechanical energy of the system. (Assume the track is frictionless).

For B, energy is dissipated to overcome air resistance. Total mechanical energy of the system eventually drags out.

For C and D, total mechanical energy conserved.

Initial K.E. + Initial G.P.E. = Final K.E. + Final G.P.E.

1/2 mu^2 + mgh = 1/2 mv^2 + mgh'

For a non-zero u, and v = 0, h' can be larger than h.

So, C is not correct.

22. You can draw the figure, showing an equilateral triangle.

For opposite current flow, the two wires attract.

So, the direction of resultant force on R is towards the midpoint of straight line joining P and Q.

23. Amount of energy dissipated

= G.P.E. lost

= mg(h - h')

= (3)(10)(2 - 1.8)

= 6 J

The four options are wrong.

參考: Physics king

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