Maths Help

Scarfacé · 22 Apr 2007 at 14:37

Sorry to make one of these threads but could someone confirm that I've done this the right way, it's been along time since I've done differentiation velocity/acceleration etc.

Task 1 (P5 a)

a. If s= 5 + 25t – 6t², where s metres is the distance travelled in t seconds, by a body.

Determine the velocity of the body at t = 2 seconds.

Velocity (ds/dt)

(dS/dT) = 25 – 12t

when t = 2 seconds.

Velocity = 25 – (12*2)
Velocity = 25 – 24
Velocity = 1 m/s

b. The velocity (v) of a moving body is given by v = 5 +12t – 2t², where v is in m/s and t is time in sec. Determine the acceleration when t= 3 seconds.

dv/dt = 12 - 4 t

Acceleration = 12 – (4*3)
= 12 – 12
= 0 m/s

Just got a feeling I've gone about this the wrong way about these questions.

Any help would be much appreciated.

Inquisitor · 22 Apr 2007 at 14:44

Nope they're right

p4radox · 22 Apr 2007 at 14:46

Well it looks okay to me...

georges · 22 Apr 2007 at 15:02

You want to put ms^-2 for the acceleration, or you often lose marks (not sure if you have on the paper).

Can't comment on the result as I haven't done mechanics (start that in A2), but if the method you've done is right, the answer seems to be correct.

Scarfacé · 22 Apr 2007 at 15:05

Cheers guys.

Just wasn't sure if I had to just differentiate the velocity formula to give me the acceleration formula. I know that you find the derivitive of the s formula for the acceleration.

Rebelius · 22 Apr 2007 at 15:06

do you need to put ms^-2 if it's zero? if the acceleration is zero, surely it doesn't matter what units you use?

Although I can see why it would be best to write down units of acceleration, not 0elephants. But neither is actually wrong.

georges · 22 Apr 2007 at 15:37

0ms^-2 doesn't always mean 0m/s. Just means the speed isn't changing.

Sleepy · 22 Apr 2007 at 16:32

Rebelius said:
do you need to put ms^-2 if it's zero? if the acceleration is zero, surely it doesn't matter what units you use?

Although I can see why it would be best to write down units of acceleration, not 0elephants. But neither is actually wrong.

0 m/s/s tells the reader that the property with a value of zero is the acceleration, Just stating 0 requires the property to be deduced from context and introduces more chances for error. So quote the units.

Disclaimer, I am old.

Sleepy · 22 Apr 2007 at 16:34

georges said:
0ms^-2 doesn't always mean 0m/s. Just means the speed isn't changing.

0ms^-2 never mean 0m/s

Spuds · 22 Apr 2007 at 17:26

Sleepy said:
0ms^-2 never mean 0m/s

Only if the body is compleatly stationary. If a body is moving at a constant speed of 10m/s then its acceleration is 0m/s^2 as its not accelerating, its moving at a constant speed