A car of mass 1500kg is travelling on a level road at a constant speed of 72 km/h
a) Calculate the KE of the car.
b) A total Friction force of 500N is acting on the car. Calculate the work done against frvtion in 60s.
Work, energy and power
2007-04-15 7:55 pm
回答 (3)
2007-04-15 8:07 pm
✔ 最佳答案
a) KE = 1/2mv^2KE=1/2*1500*(72*1000/3600)^2
=300000j
=300kj
b)In 60s , displacement of the car = 72*1000/3600 *60 = 1200m
work done against friction = Fs
=500*1200
=600000j
=600kj
參考: myself
2007-04-16 6:49 am
但係constant speed唔係冇deceleration嫁咩!?@@?
2007-04-15 8:20 pm
a. 72 kmh^-1
= 72 X 1000 / 3600
= 20 ms^-1
So, kinetic energy of the car
= 1/2 mv^2
= 1/2 (1500)(20)^2
= 300 000 J
= 300 kJ
b. By Newton's 2nd law of motion
F = ma
-500 = 1500a
a = -0.333 ms^-2
So, by the equation of motion,
s = ut + 1/2 at^2
s = (20)(60) + 1/2 (-0.333)(60)^2
s = 600 m
So, the displacement of the car in 60 s is 600m.
By work done = force X displacement
Work done = 500 X 600
Work done = 300 000 J = 300 kJ
So, the work done against friction in 60 s is 300 kJ.
2007-04-15 12:21:32 補充：
樓上的朋友計b part唔記得架車有deceleration。你當咗架車係用均速行咗60s。
= 72 X 1000 / 3600
= 20 ms^-1
So, kinetic energy of the car
= 1/2 mv^2
= 1/2 (1500)(20)^2
= 300 000 J
= 300 kJ
b. By Newton's 2nd law of motion
F = ma
-500 = 1500a
a = -0.333 ms^-2
So, by the equation of motion,
s = ut + 1/2 at^2
s = (20)(60) + 1/2 (-0.333)(60)^2
s = 600 m
So, the displacement of the car in 60 s is 600m.
By work done = force X displacement
Work done = 500 X 600
Work done = 300 000 J = 300 kJ
So, the work done against friction in 60 s is 300 kJ.
2007-04-15 12:21:32 補充：
樓上的朋友計b part唔記得架車有deceleration。你當咗架車係用均速行咗60s。
參考: Myself~~~
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