1)
An athlete runs at 10ms-1 along a straight track and someone suddenly
enters his track at 25m ahead. If he wnts to stop in front of the person, what
is his minimum deceleration? Assume the reaction time of the athele is
0.2s.
2)
A bullet is fire at a speed of 500ms-1 towards a wooden wall. The bullet is
decelerated uniformly inside the wall at 80000s-2 . To protect the bullet
from pennetrating it, what is the minimum thickness of the wall?
3)
A 1-kW heater, immersed in water (0.5kg , 20 'C), is switched on for 10
minutes. Calculate the maximum amount of water boiled away.
Physical 問題 (+20)
2009-10-27 4:14 am
回答 (2)
2009-10-27 4:51 am
I agree with physics
2009-10-27 4:39 am
1. Distance covered by the athletle before he deccelerates
= 10x0.2 m = 2 m
Using equation of motion: v^2 = u^2 +2.a.s
with u = 10 m/s, s = (25-2) m = 23 m, v = 0 m/s, a =?
hence, 0 = 10^2 + 2a(23)
a = -100/(2x23) m/s2
2. Use equation of motion: v^2 = u^2 + 2.a.s
with u = 500 m/s, a = -80000 m/s2, v = 0 m/s, s =?
hence, 0 = 500^2 + 2 x(-80000).s
s = 500^2/(2x80000) m
3. Heat produced by the heater in 10 minutes
= 1000 x (10x60) J = 600 000 J
Energy required to raise 0.5kg water from 20"c to 100'C
= 0.5 x 4200 x (100-20) J = 168 000 J
Hence, energy used to boil the water away = (600 000 - 168 000) J = 432 000 J
Let m be the mass of water boiled away
m.L = 432 000
i.e. m = 432000/L, where L is the latent heat of vapourization of water
= 10x0.2 m = 2 m
Using equation of motion: v^2 = u^2 +2.a.s
with u = 10 m/s, s = (25-2) m = 23 m, v = 0 m/s, a =?
hence, 0 = 10^2 + 2a(23)
a = -100/(2x23) m/s2
2. Use equation of motion: v^2 = u^2 + 2.a.s
with u = 500 m/s, a = -80000 m/s2, v = 0 m/s, s =?
hence, 0 = 500^2 + 2 x(-80000).s
s = 500^2/(2x80000) m
3. Heat produced by the heater in 10 minutes
= 1000 x (10x60) J = 600 000 J
Energy required to raise 0.5kg water from 20"c to 100'C
= 0.5 x 4200 x (100-20) J = 168 000 J
Hence, energy used to boil the water away = (600 000 - 168 000) J = 432 000 J
Let m be the mass of water boiled away
m.L = 432 000
i.e. m = 432000/L, where L is the latent heat of vapourization of water
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