# Mechanics - Energy, work and power

• Jun 10th 2010, 02:05 AM
Cathelyn13
Mechanics - Energy, work and power
The driving force exerted by D's engine is 4 times as great when D is at the top of the hill as it is when D is at the bottom. Find the ratio of the power developed by D's engine at the top of the hill to the power developed at the bottom.

• Jun 10th 2010, 02:33 AM
Ackbeet
You're more likely to get more accurate help if you post this question on the physics help forum, our sister site. I say that for your benefit.
• Jun 10th 2010, 10:17 AM
ebaines
Since power is defined as in work per unit time, and work is force times distance, then power = force time velocity. You tell us the engine develps 4 times as much force at the top of the hill as at the bottom, but you don't tell us about the velocity - specifically whether the velocity of D is the same at the top of the hill as at the botttom. Without that information it's impossible to answer.
• Jun 10th 2010, 10:24 PM
Cathelyn13
Opps, sorry, I forgot the previous parts (Sweating)

A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal. The driving force exerted by C's engine has magnitude 1800N. Another car D, also of mass 1200kg, climbs the same hill with increasing speed. The speed at the bottom is 8m/s and the speed at the top is 20m/s. It is assumed that the resistance to the motion of D is constant and has magnitude 700N. The driving force exerted by D's engine is 4 times as great when D is at the top of the hill as it is when D is at the bottom. Find the ratio of the power developed by D's engine at the top of the hill to the power developed at the bottom.

• Jun 11th 2010, 04:53 AM
BabyMilo
Quote:

Originally Posted by Cathelyn13
Opps, sorry, I forgot the previous parts (Sweating)

A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal. The driving force exerted by C's engine has magnitude 1800N. Another car D, also of mass 1200kg, climbs the same hill with increasing speed. The speed at the bottom is 8m/s and the speed at the top is 20m/s. It is assumed that the resistance to the motion of D is constant and has magnitude 700N. The driving force exerted by D's engine is 4 times as great when D is at the top of the hill as it is when D is at the bottom. Find the ratio of the power developed by D's engine at the top of the hill to the power developed at the bottom.

Find what ratio of power? Thrust to weight ratio?
• Feb 18th 2013, 08:33 PM
Re: Mechanics - Energy, work and power
Net work done, W by the engine = 1800x500 = 9x10^5 J
W =change in potential energy of car, W1 + work done against resistance, W2
W1 = 1200x9.8x500x sin 6 = 6.1463x10^5 J
W2 = 2.8537x10^5 J is the answer to th first problem.
In the second problem, we have to find out the net external work done, WE, by the engine.. It has three components. Change in kinetic energy of car, KE, change in potential energy, PE and work done against resistance.

WE = 0.5x1200(20^2-8^2) + 6.1463x10^5 + 700x500
WE = 2.016x10^5+6.1463x10^5+3.5x10^5 = 1.1623x10^6 J is the second answer.

lol, its been 2 years since you asked this question :P anyways..
• Feb 18th 2013, 08:52 PM
LanellePalmer
Re: Mechanics - Energy, work and power
A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal.

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• Feb 19th 2013, 05:18 AM
ebaines
Re: Mechanics - Energy, work and power
Quote:

Originally Posted by LanellePalmer
A car C of mass 1200kg climbs a hill of length 500m at a constant speed. The hill is inclined at an angle of 6 degrees to the horizontal.

Do you have a question?
• Feb 19th 2013, 02:17 PM
topsquark
Re: Mechanics - Energy, work and power
Quote:

Originally Posted by Ackbeet
You're more likely to get more accurate help if you post this question on the physics help forum, our sister site. I say that for your benefit.

Point 1. Yes, this thread is two years old. :)

Point 2. LanellePalmer is a spammer.

Point 3. PhysicsHelpForum no longer exists.

-Dan
• Feb 20th 2013, 04:23 AM
ebaines
Re: Mechanics - Energy, work and power
Quote:

Originally Posted by topsquark
Point 3. PhysicsHelpForum no longer exists.

-Dan

PhysicsHelpForum is alive and kicking, although traffic is a bit slow:

Physics Help Forum
• Feb 23rd 2013, 07:13 AM
topsquark
Re: Mechanics - Energy, work and power
Quote:

Originally Posted by ebaines
physicshelpforum is alive and kicking, although traffic is a bit slow:

physics help forum

HOLY CRAP!! I just figured that the Physics forum fell under the "mash hammer" and disappeared. (Doh)

Gee, I'm a mod on that forum too. Maybe I ought to visit it once in a while.

Thanks for the info!

-Dan