2 mechanics questions?

[匿名]

2016-05-31 12:06 pm

1) A small block is pulled directly up a rough slope, inclination 10 degrees, by a constant force of magnitude 20 N, acting at an angle 30 degrees above the horizontal. Find the work done by the force as the block moves a distance of 3 m up the slope.

2) A box of mass 5 kg slides over a rough horizontal surface. In sliding 5 m the speed of the body decreases from 8 m/s to 6 m/s. Find the work done against friction.
I found the deceleration is 2.8 m/s^2 but I don't know how to proceed.

Any help will be really appreciated.

回答 (1)

fooks

2016-05-31 12:28 pm

✔ 最佳答案

1)
Angle between the force and the slope = 30° - 10° = 20°

Work done by the force = F . s = (20 cos 20°) × 3 J = 56.4 J

2)
According to the law of conservation energy
Work done against friction
= Decrease in K.E.
= (1/2) × 5 × 8² - (1/2) × 5 × 6² J
= 70 J

OR :
v² = u² + 2as
8² = 6² + 2a(5)
a = 2.8 m/s²

F = m a
Frictional force, F = 5 × 2.8 = 14 N

Work done against friction = F s = 14 × 5 = 70 J

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