唔該!!
可唔可以教下我??
可唔可以列equation 比我??
4. A car stopped after emergency braking. The skid mark left by the car was 22.3 m long.
Assume that the friction between the road and tyres was 0.65 times that of the weight of the car.
Estimate the speed of the car when it began to skid.
A. 5.38 ms^-1
B. 12.0 ms^-1
C. 16.2 ms^-1
D. 17.0 ms^-1
10. A space shuttle is covered by ' heat shields' over its body so as to protect the interior from getting too hot while
entering the atmosphere of the Earth. Which of the following thermal properties is/are desirable for the material of the
' heat shields' ?
(1) It should be a good conductor of heat.
(2) It should have a very high melting point.
(3) It should have high specific heat capacity.
A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
23. If the current in the circuit shown is 0.8A, what is the number of electrons passing through the point X in 1 minute?
( The charge on each electron is 1.6x 10^-19 C.)
A. 8.3x10^16
B. 1.6x10^17
C. 5.0x10^18
D. 3.0x10^20
PP phy 06
2009-04-18 8:10 pm
回答 (1)
2009-04-19 12:38 am
✔ 最佳答案
4. friction, f = 0.65mg = (0.65)(10)m = 6.5mBy Newton's 2nd law of motion,
f = ma
-6.5m = ma
Acceleration, a = -6.5 ms-2
Stopping distance, s = 22.3 m
Final velocity, v = 0
By v2 = u2 + 2as
0 = u2 + 2(-6.5)(22.3)
Initial speed, u = 17.0 ms-1
The answer is D.
10. The answer is B.
(1) whether it is a good thermal conductor or not does not matter with the protection.
(2) Has a high melting point just to protect the frame of the shuttle, but not avoid the inferior from getting too hot.
(3) Having a high specific heat capacity, that it will have a smaller temperature rise when given the same energy flow. So, this can prevent from getting too hot.
23. By Q = It
nq = It
n = It/q
= (0.8)(60)/(1.6 X 10-19)
= 3.0 X 1020
The answer is D.
參考: Physics king
收錄日期: 2021-04-19 14:10:36
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