Register to reply

Question about fiding speed and distance

by sofiasherwood
Tags: distance, fiding, speed
Share this thread:
sofiasherwood
#1
Nov11-13, 07:58 AM
P: 17
I had a test earlier today and one question was,

A 20kg object initially at rest is accelerated at constant power of 12.0w. After 9.0s it has moved 56.0m. Find its speed at t=6.0s and its position at that instant.

I got v=6 and distance=36m are these values correct?
Phys.Org News Partner Science news on Phys.org
NASA team lays plans to observe new worlds
IHEP in China has ambitions for Higgs factory
Spinach could lead to alternative energy more powerful than Popeye
HallsofIvy
#2
Nov11-13, 08:26 AM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,310
Quote Quote by sofiasherwood View Post
I had a test earlier today and one question was,

A 20kg object initially at rest is accelerated at constant power of 12.0w. After 9.0s it has moved 56.0m. Find its speed at t=6.0s and its position at that instant.
I started to say that this makes no sense. An object accelerates at a constant acceleration measured in [itex]m/s^2[/itex]. That acceleration may be caused by a constant force, measured in Newtons, but not a constant power measured in watts.

But the "12.0 w" really has nothing to do with the problem and can be ignored. If an object accelerates from rest at constant acceleration, a [itex]m/s^2[/itex], then after time, t, it will have speed [itex]at[/itex] m/s and will have moved distance [itex](1/)at^2[/itex] m.

I got v=6 and distance=36m are these values correct?
They do not look at all right to me. How did you get them?
sofiasherwood
#3
Nov11-13, 08:56 AM
P: 17
P*t=W
12*9=108
therefore W=108joules
W=F*displacement
F=108/56
F=1.93N

F=ma
a=1.93/20
a=0.1

so at t=6
Pt=W
12*6=72joules
W=72joules

W=F*displacement
displacement=72/(20*0.1)
displacement=36m

P=Fv
v=12/(20*0.1)
v=6

russ_watters
#4
Nov11-13, 08:57 AM
Mentor
P: 22,231
Question about fiding speed and distance

Halls, it doesn't say constant acceleration. Constant power is what you might get from a car with a continuously variable transmission.
sofiasherwood
#5
Nov11-13, 08:58 AM
P: 17
I am assuming the acceleration is constant. Is that right?
sofiasherwood
#6
Nov11-13, 09:01 AM
P: 17
Have I gone wrong somewhere?
Enigman
#7
Nov11-13, 09:07 AM
Best Humor
Enigman's Avatar
P: 555
Me thinks...
P=Fv
12=20*x'*x''
3/5x'=x"
x(0)=0, x'(0)=0
http://www.wolframalpha.com/input/?i...%27%280%29%3D0
x=2(2/5)1/2t3/2
pugging t=9
x=18(6/5)^1/2
whereas according to question it should be 56m???
Either the question, me or wolframalpha is incorrect...
sofiasherwood
#8
Nov11-13, 09:12 AM
P: 17
Quote Quote by Enigman View Post
Me thinks...
Either the question, me or wolframalpha is incorrect...
This is the question given in the test word for word. The question confused me for ages, in the end I just assumed acceleration was constant. I don't know how you could work it out any other way.
Enigman
#9
Nov11-13, 09:41 AM
Best Humor
Enigman's Avatar
P: 555
If we disregard x(9) value in question { http://www.wolframalpha.com/input/?i...3D0%2Cx%289%29 }
For x(6) we get 24/(5)^0.5
http://www.wolframalpha.com/input/?i...3D0%2Cx%286%29
adjacent
#10
Nov11-13, 10:17 AM
PF Gold
adjacent's Avatar
P: 1,459
Try not to round of numbers.Use fractions.
Enigman
#11
Nov11-13, 10:24 AM
Best Humor
Enigman's Avatar
P: 555
Quote Quote by sofiasherwood View Post
I am assuming the acceleration is constant. Is that right?
Acceleration can not be constant...
P=dW/dt
P=d(∫F.dx)/dt
P=F.dx/dt
P=Fv
If a is const.
P=ma*(at)
P=ma2t
Then Power is not constant as given in the question...
f95toli
#12
Nov12-13, 04:52 AM
Sci Advisor
PF Gold
f95toli's Avatar
P: 2,241
I saw this question when it was in the other section, it looked fishy to me then and I just had a go at it. Unless I am missing something it does not have a solution.
The way the question is formulated you will -as Enigman pointed out- end up with a 2nd order ODE for the position; but then the constants are over-determined.

I wonder if this is a case of the teacher trying to add a red herring (constant power) without realizing that this implies that the accelaration can not be constant.


Register to reply

Related Discussions
Layman question about distance and speed of light Astronomy & Astrophysics 4
Hypothetical question about speed and distance across space General Physics 4
Average Speed = distance/time question. Introductory Physics Homework 4
Distance/speed trig question Calculus & Beyond Homework 3
Friction, speed & distance question Introductory Physics Homework 11