# Work to stop

1. Dec 11, 2003

### dzem68

This should be easy...the problem asks for the amount of work needed to stop a 1000kg car traveling @ 100km/h.

I am having a hard time do to the fact acceleration,distance nor time is given.

here is how I think i need to approach this:

convert 1000km/h to m/sec

find acceleration

use Work=Mas*acceleration*Distance to find work

Here are the variables i know:

Mas=100kg
Vf = 0
V0 = 27.78 m/s

how can i find acceleration if no distance or time is given? I would think i need to find acceleration to calculate work needed to stop.

any hints?

dz

2. Dec 11, 2003

### ShawnD

convert speed to m/s:
100km/h * 1000m/km * h/3600s = 27.7777m/s

you have to remember the standard formula for all of these types of problems is energy in = energy out. expanded, it looks like this:
Wo + Ko + Po = Wf + Kf + Pf

W is work, K is kinetic energy, P is potential energy. For this equation, there is no work put in, no initial potential energy, no final potential energy, no final kinetic energy. With all the 0 terms removed, it looks like this:
Ko = Wf

solfing for equation:
(1/2)mv^2 = W
(1/2)(1000)(27.777^2) = W
W = 385800J
W = 3.858 x 10^5 J

3. Dec 12, 2003

### HallsofIvy

In order to stop any object, you need to reduce its velocity to 0. That means you need to reduce its kinetic energy to 0. Work is energy so: calculate the kinetic energy. That's the work you need to do to stop the object.