# velocity and speed

• Jun 24th 2006, 04:52 AM
jacs
velocity and speed
Equation is: x = 12t - t².
Calculate the average velocity and the average speed.

So far I have this.
v = 12 - 2t

Can the answer be determined from just this info? Or is smt missing? (My teacher is fond of making errors in questions....grrrr)

thanks
• Jun 24th 2006, 05:09 AM
Quick
Quote:

Originally Posted by jacs
Equation is: x = 12t - t².
Calculate the average velocity and the average speed.

So far I have this.
v = 12 - 2t

Can the answer be determined from just this info? Or is smt missing? (My teacher is fond of making errors in questions....grrrr)

thanks

What is your equation above supposed to represent?

the area in red isn't right becuase $x-x^2$ can't be simplified.
• Jun 24th 2006, 05:17 AM
jacs
x is the equation of rectilinear motion, displacement x, in terms of t, time

usually we use this kind of thing to find instantaneous velocity or acceleration. I have not seen a question before on average speed or velocity without a specific time given.
• Jun 24th 2006, 05:29 AM
topsquark
Quote:

Originally Posted by jacs
Equation is: x = 12t - t².
Calculate the average velocity and the average speed.

So far I have this.
v = 12 - 2t

Can the answer be determined from just this info? Or is smt missing? (My teacher is fond of making errors in questions....grrrr)

thanks

v = 12 - 2t is the instantaneous velocity.

Given a specific time interval:

Average velocity = $\frac{1}{t-t_0} \int_{t_0}^t \, dt (12 - 2t)$

Average speed = $\frac{1}{t-t_0} \int_{t_0}^t \, dt |12 - 2t|$

-Dan
• Jun 24th 2006, 05:59 AM
jacs
thanks Dan, i understand the general expression, it makes sense, cheers.