Work and Power questions quick

[匿名]

2009-12-15 12:24 am

The boy hits the plank with a hammer and if the metal cylinder hits the bell, he wins. The mass of the metal cylinder is 1kg and the friction between the metal cylinder and the board is 5N.
The boy strikes the plank with a hammer so that the metal cylinder leaves the plank at 4ms-2.
a) How high does the metal cylinder reach?
b) What is the min. initial speed of the metal cylinder to win the game if the bell is 3m above?
c)suggest two changes that would make it more difficult to win.

A car starts its engine and travels on a level road. The car engine then works at a cpnstant power. The frictional force acting on the car is 1600N.
a) Describe the motion of the car during the whole process.
b) If the power of the car is 80kW, find the max. speed of the car.

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Answer 1a)0.533m b)9.49ms-1 2b)50ms-1

回答 (1)

Prof. Physics

2009-12-15 2:42 am

✔ 最佳答案

1.a. It should be 4 ms^-1 instead of 4ms^-2, because it is referring to velocity.

When the metal cylinder is moving upwards, the frictional force acting on it is downwards.

Take upward direction be positive.

By Newton's 2nd law of motion,

F + mg = ma

-5 + (1)(-10) = (1)a

Acceleration, a = -15 ms^-2

By equation of motion: v^2 = u^2 + 2as

0 = (4)^2 + 2(-15)s

Maximum height, s = 0.533 m

b. By equation of motion,

v^2 = u^2 + 2as

0 = u^2 + 2(-15)(3)

Initial speed, u = 9.49 ms^-1

c. Increasing the friction between the metal cylinder and the board.

Use a heavier metal cylinder.

2.a. The car will accelerate with maximum acceleration initially, then its acceleration will decrease so that the speed of the car will increase in a slower rate. At last, the car will travel with constant speed.

b. At maximum speed, the net force acting on the car should be zero.

By P = Fv

80 000 = 1600v

Max. speed, v = 50 ms^-1

參考: Physics king

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