牛頓第二運動定律

Question

1.一架車剎掣後,衝行了80m才停定.假設剎掣後,氣車受路面的摩擦力為10000N.氣車的質量為1000kg,,氣車剎掣前的速率為多少kmh^-1?



需要列明公式!!!!!

Answer 1

F = ma
10000 = (1000)a
a = 10 ms^-2
or a = -10 ms^-2 [-10 因為是減速，a的方向和v相反]
v^2 = u^2 + 2aS
0 = u^2 + 2(-10)(80)
u^2 = 160
u=40 ms^-1

Answer 2

We can use the principle of work done , because the car finanlly stopped, the work done by the frictional force of the road is equal to the kinetic energy of the car .

F.S = 1/2mv^2
10000(80) = 1/2(1000)v^2 ,where v is the speed of the car just before started to brake
v = 40