f5 phy - Ans.fyi 參考答案

f5 phy

Baobao

2009-09-19 10:44 pm

1.An object of mass 0.5 kg is thrown vertically upwards with an initial
velocity 20 ms-1 . If air resistance is neglected and g=10 ms-2 ,what
is the loss of KE in the first second ?

2.A particle of mass m is thrown vertically upwards with initial speed v . When the particle returns to the starting point , what are the
change in momentum and KE of the particle?

3.A boy of mass 50kg runs up a flight of stairs 15m high in 20s. what is the power developed by the boy?

please show your step ...
thank a lot ~!

回答 (1)

Prof. Physics

2009-09-20 3:01 am

✔ 最佳答案

1. If air resistance is negligible, then the loss of K.E. in the first second is the G.P.E. gained in the first second.

By equation of motion,

s = ut - 1/2 gt2

s = (20)(1) - 1/2 (10)(1)2 = 15 m

So, the loss of K.E. in the first second

= mgs

= (0.5)(10)(15)

= 75 J

2. When it returns to its starting point, its speed is the same, but the direction is opposite.

So, the change in momentum = 2mv

Change in kinetic energy = 1/2 mv2 - 1/2 m(-v)2 = 0

3. G.P.E. gained by the boy, E = mgh = (50)(10)(15) = 7500 J

Power = E / t = 7500 / 20 = 375 W

參考: Physics king

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