DescentBB

A go-kart, weighing 550 lbs total needs to travel at 50 mph. how much horsepower is needed to attain and hold this speed?

the only thing it has to deal with is wind resistance and it's own mass.

Wind resistance is 1/2*rho*v^2*A

where rho is the air density, v is the speed, and A is the frontal area.

This gives you a constant force opposing it at 50 MPH. But to find the HP, you need a power. However, thinking about the units is useful here.

Power = Force x Distance / Time

You have a force from the wind resistance. You don't have a specific distance or time, but you have a speed, which is Distance / Time.

So if you multiply the force by the velocity, you get the power required.

ok, force turned out to be 8.8 lbs

Try this for a question,

A white 1993 pontiac grandam is driving on a road facing NNW about 32 mph, the wind is moving SE at about 12 mph differeing between 12-15 mph every 3 minutes or so, the question is this.. How many times will a 4\" long toothpick rotate after being thrown out of the car and hit the road before it eventually stops.

bleh, you're all noobs

You are helping your friend's family put new asphalt shingles on the roof of their winter cabin. The 60 ft x 20 ft roof is made of 1-in pine board covered with asphalt shingles. There is room for 2 in. of roof insulation, and your friend's family is wondering how much of a difference it would make to their energy bill if they were to install the two inches of insulation

calculate the r factor of the roof

ccb056 wrote:bleh, you're all noobs

You are helping your friend's family put new asphalt shingles on the roof of their winter cabin. The 60 ft x 20 ft roof is made of 1-in pine board covered with asphalt shingles. There is room for 2 in. of roof insulation, and your friend's family is wondering how much of a difference it would make to their energy bill if they were to install the two inches of insulation

calculate the r factor of the roof

What kind of pine is it and what kind of insulation are you talking about?

hoo boy. zero to offtopic in one post.

the question i was posting would help answer questions of a project that i'm thinking of undertaking sometime during the summer or fall.

yes. I want to build an electric go kart. for shizs and giggles. and to burn around the offroad tracks around here.

paul here was the only one who actually posted anything productive. for the rest of you.. pfft.

You'd think that, being a physics major, I'd be able to help, but we physics majors usually get to deal with pleasant little \"idealized\" situations, in which things like wind resistance and friction don't necessarily exist.

Ok, first imagine that the gocart is a sphere.........

Ferno wrote:A go-kart, weighing 550 lbs total needs to travel at 50 mph. how much horsepower is needed to attain and hold this speed?

the only thing it has to deal with is wind resistance and it's own mass.

none, just drop it from a high enough altitude

...is it on a road that is designed to move in the opposite direction of the car and match it's speed?

how far would ccb056 fly if he got hit by a speeding train?

Miracles include curing the sick by touch, healing the blind, and staying on topic.

Be warned Ferno, the frontal area stuff isn't anything more than a (very) rough approximation. Fluid dynamics is a beast all by itself.

[edit]Why not go for a gas one? If you can get your hands on a small engine of some type (maybe like a lawn mower engine) you can probably get more power/weight than if you try to make an electric. Plus, I'b be willing to bet that you would dramatically increase your run time. Before you get too deep into it, I would select a motor as do some battery life calculations. (Most batteries are rated by amp hours, divide the motor's current draw by the Ah rating of the battery, and you get yourself an approximate battery life.- I'd multiply by a factor of 2/3 to get a realistic battery life.)

[edit2] Sry, you need hours to be on top, so divide battery amp hours by motor's amps.

You're all noobs

REDNECK ENGINEERING EXAM

1. Calculate the smallest limb diameter on a persimmon tree that will support a 10 pound possum.

2. Which of the following cars will rust out the quickest when placed on blocks in your front yard? 66 Ford Fairlane, 69 Chevrolet Chevelle, 64 Pontiac GTO.

3. If your uncle builds a still that operates at a capacity of 20 gallons of shine per hour, how many car radiators are necessary to condense the product?

4. A pulpwood cutter has a chain saw that operates at 2700 rpm. The density of the pine trees in a plot to be harvested is 470 per acre. The plot is 2.3 acres in size. The average tree diameter is 14 inches. How many Budweiser Tallboys will it take to cut the trees?

5. If every old refrigerator in the state vented a charge of R-12 simultaneously, what would be the decrease in the ozone layer?

6. A front porch is constructed of 2x8 pine on 24-inch centers with a field rock foundation. The span is 8 feet and the porch length is 16 feet. The porch floor is 1 inch rough sawn pine. When the porch collapses, how many hound dogs will be killed?

7. A man owns an Tennessee house and 3.7 acres of land in a hollow with an average slope of 15%. The man has 5 children. Can each of the children place a mobile home on the man's land?

8. A 2-ton pulpwood truck is overloaded and proceeding 900 yards down a steep grade on a secondary road at 45 mph. The brakes fail. Given the average traffic loading of secondary roads, what are the chances that it will strike a vehicle that has a muffler?

9. A coal mine operates a NFPA Class 1, Division 2 Hazardous Area. The mine employs 120 miners per shift. A gas warning is issued at the beginning of 3rd shift. How many cartons of unfiltered Camels will be smoked during the shift?

10. At a reduction in gene pool variability rate of 7.5% per generation, how long will it take a town that has been bypassed by the interstate to breed a country-western singer?

LAWL

You know....some of you folks could actually try to help instead of being spamming morons.

Back on topic:

First thing is to seperate the known factors from the unknowns. This doesn't take into account the free-moving wheels on the vehicle (assumes that it is a dead mass) so take the final figure and take atleast 1/4 of it as a good estimation of what you need. Calculating air resistance would be next to futile since we have everything with the exception of the entire frontal area of the vehicle.

weight (w) = 550lbs
initial velocity (Vo) = 0mph
final velocity (Vf) = 50mph
acceleration due to gravity (g) = 32ft/s^2

Now you've got to make some conversions to get your mass, and your velocities down in divisions per second. Figuring out your mass is easy. Divide the weight by the acceleration due to gravity. (550lbs / 32ft/s^2) which gives you just over 17 slugs (17.2 sl to be exact). Now to convert your velocity figure from miles per hour to feet per second. (This MUST be done, or your final answer will be thrown way off.) Now, one mile per hour is the equivilent of 1.47 feet per second. To convert, simply take your 50mph figure and multiply it by 1.47. That comes out to 73.5ft/s.

Now that our conversions are done, lets get down to the REAL fun. The dreaded algebra formulas! *mock horror* The formula we have to mess with is:

2as = Vf^2 - Vo^2

Typing out the forumula in ASCII format is a pain in the ass, so I'll just post a quick photoshopping. For the sake of this calculation, I'm going to assume that the distance (s) from start to finish is 100ft. Long story short, this is the formula you eventually get to:



Since your intial velocity is zero, it makes no sense to include that in the formula since squaring a zero is still zero, so you'd end up taking nothing out of your final velocity. This is not always the case, but for this situation it is. After plugging in all of the data, that gives us 27ft/s^2 for an accleration constant. This is needed to calculate the force so you can get your horsepower figure. Force is equal to mass times acceleration. (F = m * a) We already have the mass figure, and we just got acceleration, so this is a peace of cake. Ends up being roughly 464 pounds of force (464.4 lbs to be exact).

Finally, we come to our power calculation. Power is force (F) times velocity (v). You want it to hit and maintain 50 miles per hour, so we'll use the final velocity figure. So take the 464.4 lbs of force and multiply it by 73.5 ft/s and you get your answer in lb ft units. (34,133.4 lb ft) We're not done yet. You have to convert that figure to horsepower. 1 hp is equal to 550 lb ft, so the conversion is simple. Divide by 550 lb ft and you have your answer in horsepower. (62.1hp) Again, keep in mind this calculation assume that your 550 lbs of weight is a DEAD MASS. It doesn't take into account friction, nor the free-moving wheels.

A 15 - 20hp motor is a good upper limit for what you want to be able to pull. If its too much, you can always govern it down, or replace it with a slower motor.

MD-2389 wrote:You know....some of you folks could actually try to help instead of being spamming morons.

Oh sorry, I thought the question had already been answered. But I guess if you are too fukin stupid to understand it, but all means answer it again.

snoopy wrote:Be warned Ferno, the frontal area stuff isn't anything more than a (very) rough approximation. Fluid dynamics is a beast all by itself.

already considered that, thanks

[edit]Why not go for a gas one? If you can get your hands on a small engine of some type (maybe like a lawn mower engine) you can probably get more power/weight than if you try to make an electric. Plus, I'b be willing to bet that you would dramatically increase your run time. Before you get too deep into it, I would select a motor as do some battery life calculations. (Most batteries are rated by amp hours, divide the motor's current draw by the Ah rating of the battery, and you get yourself an approximate battery life.- I'd multiply by a factor of 2/3 to get a realistic battery life.)

how does a DC motor capable of 15 hp in a 25lb package grab ya?

Dedman wrote:Oh sorry, I thought the question had already been answered. But I guess if you are too fukin stupid to understand it, but all means answer it again.

Not entirely, or are you too lazy to read?

Ferno wrote:how does a DC motor capable of 15 hp in a 25lb package grab ya?

I'm talking about batteries, mostly. I assume you are planning on using lead acids- if you use more than one or two they start to add up really quickly. Looks like you run at somewhere around 1lb per Ah. (Maybe I'm talkin out of my butt.)

Oh, a real-world problem, huh? Yeah, in that case you'll have to consider inefficiencies, also... but that should be good as a rough approximation.

snoopy wrote:I'm talking about batteries, mostly. I assume you are planning on using lead acids- if you use more than one or two they start to add up really quickly. Looks like you run at somewhere around 1lb per Ah. (Maybe I'm talkin out of my butt.)

some 40aH batteries are a little over 25 lbs a piece.

\"Slugs\"? \"Pounds\"? \"Feet/second\"? What are these heathenistic units you're employing, MD? Such English notation has no place in the world of almighty physics.

Kidding aside, MD's post covers the basic kinematics of the situation, but a lot of the finer details are a real pain to work with. You could always go with the tried-and-true \"Let's see if this works...\" method, provided you don't kill yourself in the process.