# A bit of physics

Discussion in 'Electronics Homework Help' started by Integrator741, Sep 25, 2014.

1. ### Integrator741

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4
Jun 16, 2013
Hi, I have got this physic's question that I can't figure out, I was wondering maybe someone down here will be able to explain this stuff. Thanks!

"How long should it take a 1 ton car with a 50kW engine power to accelerate from 0-60mph."

Last edited by a moderator: Sep 26, 2014
2. ### Laplace

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Apr 4, 2010
Assume no aerodynamic drag or friction or electrical resistance, and no change in elevation. Calculate the kinetic energy of the car at 60mph. Then find how many 50kWsecs it would take to equal that kinetic energy.

3. ### Gryd3

4,098
875
Jun 25, 2014
Lets first convert these units to something more manageable.

We will need to determine acceleration with the mass, and NET force. (Account for friction or gravity if you are told to)
Once we have acceleration, we can determine the time it takes to accelerate from 0 - 60mph.

4. ### Gryd3

4,098
875
Jun 25, 2014
That's what I liked about physics, there are always numerous ways to do something.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,419
2,790
Jan 21, 2010
I would go with Laplace.

First step is to convert 60mph into metres per second

Gryd3, the force will vary with velocity, so to do it that way required some calculus. Avoiding having to calculate acceleration is the key to a simple answer.

1,417
312
Aug 31, 2014
F=ma

7. ### Colin Mitchell

1,417
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Aug 31, 2014
"the force will vary with velocity," This is NOT TRUE.

8. ### Colin Mitchell

1,417
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Aug 31, 2014
"Avoiding having to calculate acceleration is the key to a simple answer." This is NOT TRUE.

9. ### Gryd3

4,098
875
Jun 25, 2014
Well... yes and no...
In real world examples it is, as the engine output will need to compensate for increased drag.
This sounds like a Grade-11 physics Q though, in which case the Net Force on the item would remain constant.
People skills go a long way. All caps tend to rub people the wrong way... especially when quoting them and flat out calling them wrong.

Vf = Vi + A * T

Edit: Colin, you may want to remove or hide the final answer. This is homework help after all. The op didn't ask for the answer, just a way to get there.

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,419
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Jan 21, 2010
Unfortunately it is.

I'd demonstrate it to you, but it would give the answer to the OP and we don't do that in the homework section.

No, ignoring drag.

As a hint, consider the kinetic energy of a 1kg mass a 0m/s, 1m/s, 2m/s, and 3m/s. Is there a linear increase?

11. ### Laplace

1,252
184
Apr 4, 2010
Let's review the basics:

Force = Mass∙Acceleration
Work = Force∙Distance
Power = Work/Time = Force∙Distance/Time = Force∙Velocity

In this problem the power is constant, so as the velocity increases the force must decrease. And since the mass is constant the acceleration will decrease as the velocity increases.

In summary, when the power is constant the acceleration is not constant.

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12. ### Integrator741

125
4
Jun 16, 2013
Ok so what I did, I converted 60mph to meters per second (26.82 m/s); Then Using this number I calculated moving kinetic energy [0.5*m*V^2] = 0.5*1000*26.82^2=359656.23 J; and then what?

13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
OK, so you have the number of Joules of energy as kinetic energy of the moving vehicle.

What is the relationship between watts, joules, and time? Do you have two of these and want to derive the third?

14. ### Integrator741

125
4
Jun 16, 2013
P=E/T - T = P/E?

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
P = E/T

then multiply both sides by T/P

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16. ### Integrator741

125
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Jun 16, 2013
Oh.

Ok then I got T=Kinetic Energy/power of the engine = 339.66/50 = 6.79 seconds?

17. ### Colin Mitchell

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Aug 31, 2014
This is a simple year 11 Physics problem
F= ma
50kW = 36,878 ft-lb/sec
36,878 = 2240 x a
a = 16ft/sec/sec

v = u + at
60MPH = 88ft/sec
88 = 0 + 16t
t = 5.5 sec

18. ### Colin Mitchell

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Aug 31, 2014
"In summary, when the power is constant the acceleration is not constant." That is entirely untrue.
Look at F = ma.

19. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Where did you get 339.66?

20. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010

edit: Perhaps you've read this as constant force? It's constant power.

force is N = kg.m.s-2
power is W = kg.m2.s-3

so W = N.m.s-1

They're not the same.

W.kg-1 = kg.m.s-3 = N.s-1, or (as I've just discovered -- and possibly apocryphally) Yank

Last edited: Sep 26, 2014  