How to Calculate Watt: Understanding the Basics

[skaunejohan]

ive always been interested in calculations, so i want to try to calculate watt but I am 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

 

[Doc Al]

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 so far so good

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.)

-----------------------------------------------------------

next is calculating work
work= mass*acceleration^2

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.

 

[berkeman]

ive always been interested in calculations, so i want to try to calculate watt but I am 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 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

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?

 

[skaunejohan]

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.

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

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

 

[skaunejohan]

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?

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...

thanks for the fast reply

 

[Doc Al]

im sorry but i have no idea how to calculate the kinetic energy can you help me?

Kinetic energy = ½mv².

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

 

[skaunejohan]

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

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

 

[Doc Al]

Yes, it's that simple.

 

[skaunejohan]

thanks a lot :) this forum is great!