知识点

例题

解题过程

1. GIVEN:

   v0=20m/s (upward)

   vf=0m/s

   FIND: t=?

   SOLVE:

   by formula: vf=v0+at

   0=20+9.8t

   t=2.04s

2. GIVEN:

   v0=150m/s

   vf=0m/s (stop)

   a=-3m/s^2

   FIND:  delta x=?

   SOLVE:

   by formula: vf^2=v0^2+2a delta x

   0^2=150^2+2(-3）delta x

   delta x= 3750 m

   
   

3. GIVEN:

   v0=-25m/s

   t=2s

   FIND: vf=?

   SOLVE:

   by formula: vf=v0+at

    vf=-25+(-9.8)*2=-44.6m/s

   
   

4. GIVEN:

   v0=0m/s (accelerate from rest)

   a=25m/s^2 (upward)

   t=5s

   FIND: x

   SOLVE:

   x=x1+x2

   Before 5 s of accelerating

   by formula:  x=v0*t+(1/2)a*t^2

    x1=v0*t+1/2at^2=0+(1/2)*25*5^2=312.5 m

   v0'=v0+at=0+25*5=125 (continue to accelerate upward)

    to FIND: t

   vf=0 ( no accelerat)

   vf=v0+at

   0=125+(-9.8)t

   t=12.76 s

   After 5 s of accelerating

    x2=v0*t+1/2at^2=125*12.76+(1/2)(-9.8)(12.76^2)=797.2 m 

   x=x1+x2=312.5+797.2=1109.7 m