Physics Mechanics(S4)

CHU

2015-01-15 7:38 am

1. A stone of mass 0.01kg is thrown straight up from the ground with a velocity of 10m/s. Find the stone's
(a) kinetic energy when it is thrown from the ground,
(b) potential energy when it is at the highest point,
(c) maximum height above the ground,
(d) kinetic energy on hitting the ground again.

2. A block of mass 5kg is pulled by a force of 60N along a smooth horizontal surface.
(a) What is the work done by the force in moving the block through 2m?
(b) What is then the kinetic energy of the block if it starts from rest?

3. The block is now raised by a string passing over a smooth pulley.
(a) What is the potential energy gained by the block in moving a distance of 2m?
(b) Explain why this is not equal to the work done by the force.

4. A car of mass 1500kg is traveling at 20m/s.
(a) What is the kinetic energy of the car?
(b) Brakes are applied to stop the car. If the braking force is 7500N, what is the braking distance?

回答 (1)

天同

2015-01-15 7:25 pm

✔ 最佳答案

1(a) Kinteic energy = (1/2).(0.01).(10^2) J = 0.5 J
(b) Potential energy = 0.5 J (By conservation of energy)
(c) Max height = 0.5/0.01g m = 5 m
where g is the accleration due to gravity, taken to be 10 m/s^2
(d) Kinetic energy = 0.5 J (conservation of energy)

2(a) Work done = 60 x 2 J = 120 J
(b) Kinetic energy = 120 J (By conservation of energy)

3.(a) Potential energy = 5g x 2 J = 100 J (Take g = 10 m/s^2)
(b) Part of the work done is acquired by the block as kinetic energy.

4(a) Kinetic energy = (1/2).(1500).(20^2) J = 3 x 10^5 J
(b) Braking distance = 3x10^5/7500 m = 40 m

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