How to calculate watt

1. Apr 13, 2010

skaunejohan

ive always been interested in calculations, so i wanna try to calculate watt but im stuck

bear in mind this is a pure hobby for me and i am far from a math genious

in this example i use a car

here is how far i have come

it accelerates from 22.2m/s to 33.3 m/s in 7 seconds
to calculate acceleration you do like this

highest speed-lowest/time 33.3-22.2=11.1 11.1/7 is almost 1.58

giving the car acceleration of 1.58m/s^2 so far so good
-----------------------------------------------------------

next is calculating work
work= mass*acceleration^2

so if the car is 1200kg(newton)

then we take 1200* (1.58*1.58) = 2996 joules (this is correct i hope)
--------------------------------------------------------------------------

and this is where i get stuck because the formulas i find state that watt=work/time

2996joules/7sec is obviously wrong.....

can anybody help me?

cheers/Johan

Last edited: Apr 13, 2010
2. Apr 13, 2010

Staff: Mentor

OK, that's the average acceleration. The units should be m/s², not m/s. (But you don't really need the acceleration to find the work done.)
This is incorrect. (Note that that quantity does not have the correct units for work/energy.)

To calculate the work done, find the change in kinetic energy. Kinetic energy = ½mv².

3. Apr 13, 2010

Staff: Mentor

A couple issues. First the acceleration would be 1.58m/s^2. Velocity has units of m/s and acceleration has units of m/s^2.

Next, I do not think this is correct: "work= mass*acceleration^2" Where did you get that?

Force = mass*acceleration (F=ma), and work = Force*distance.

Does that help?

4. Apr 13, 2010

skaunejohan

sorry my mistake, like i stated above im only doing this as a hobby, i will alter the acceleration immediatley

thanks for the fast reply, im sorry but i have no idea how to calculate the kinetic energy can you help me?

5. Apr 13, 2010

skaunejohan

ahh now i see one of the problems in my calculation

it stated work=N*m/s^2

so force should be 1200*1.58??? (1896watt?)

is it possible to calculate distance from the starting speed of 22.2m/s and finish speed of 33.3m/s over 7sec

sounds over-complicated.....

6. Apr 13, 2010

Staff: Mentor

You have the mass (m) and the speed (v). Calculate the initial and final kinetic energy, then find the change.

7. Apr 13, 2010

skaunejohan

ahhh :)

starting kinetic 0,5*1200*(22.2*22.2)= 295704 joules

ending kinetic 0.5*1200*(33.3*33.3)= 665334 joules

369630 joules

(just thinking loud now)
369.6/7=53kw sounds very reasonable as a medium power

is it really that simple???

cheers/Johan

8. Apr 14, 2010

Staff: Mentor

Yes, it's that simple.

9. Apr 14, 2010

skaunejohan

thanks a lot :) this forum is great!!!