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Mass Or Weight Driven Car


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I’m posting this again as the other thread is really old and I have more questions about how to build one. This is a project for a high school physics class. Here’s the general instructiona:

 

I need to make a car powered only by gravity/a 1kg mass. The 1 kg mass must fall and power the car. No spring, electrical, chemical, elastic etc power. ONLY GRAVITY. The mass and everything else I use, must travel with the car (although I am creating it so the weight drops off once it falls the distance between the top of the car and the ground.) It must travel 3 meters (32 feet). Right now I have a car made from dowels and paint sticks which attaches to 4 “wheels” made from CDs. The axels (round dowels) are inside plastic straws which are glued to the base so that they rotate freely. We added some graphite to help with lubrication and ease of movement. Right now the car only travels about 12 feet. In order to pass the assignment it has to travel at least 25 feet. I’m stuck and don’t know what to do to improve the efficiency of the car and distance it travels. I wonder if the car just weighs too much. I find that once the weight drops and there is no more Pull on the string attached to the axel, the car stops. I’ve viewed many YouTube videos and it appears my car is very similar. I wonder also if perhaps I need to increase the distance the weight drops but it looked like from the equations used on the other posts that a 10 cm drop with a one gram weight is supposed to be enough to propel the car 9.8 meters. I am not getting anywhere close to this distance out of my car. Please note that I have limited knowledge of physics and am not planning on becoming an engineer. The other thread was super complicated when all you super smart scientists decided to have a debate about 3 vs 4 wheels, various size axels, etc. If you have any ideas please help. Thank you.

Edited by Oranjwoman
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I’m posting this again as the other thread is really old and I have more questions about how to build one. This is a project for a high school physics class. Here’s the general instructiona:

 

I need to make a car powered only by gravity/a 1kg mass. The 1 kg mass must fall and power the car. No spring, electrical, chemical, elastic etc power. ONLY GRAVITY. The mass and everything else I use, must travel with the car (although I am creating it so the weight drops off once it falls the distance between the top of the car and the ground.) It must travel 3 meters (32 feet). Right now I have a car made from dowels and paint sticks which attaches to 4 “wheels” made from CDs. The axels (round dowels) are inside plastic straws which are glued to the base so that they rotate freely. We added some graphite to help with lubrication and ease of movement. Right now the car only travels about 12 feet. In order to pass the assignment it has to travel at least 25 feet. I’m stuck and don’t know what to do to improve the efficiency of the car and distance it travels. I wonder if the car just weighs too much. I find that once the weight drops and there is no more Pull on the string attached to the axel, the car stops. I’ve viewed many YouTube videos and it appears my car is very similar. I wonder also if perhaps I need to increase the distance the weight drops but it looked like from the equations used on the other posts that a 10 cm drop with a one gram weight is supposed to be enough to propel the car 9.8 meters. I am not getting anywhere close to this distance out of my car. Please note that I have limited knowledge of physics and am not planning on becoming an engineer. The other thread was super complicated when all you super smart scientists decided to have a debate about 3 vs 4 wheels, various size axels, etc. If you have any ideas please help. Thank you.

I'm not clear what the setup is. How does the falling mass transfer a force to accelerate the car? You mention a string attached to an axle. How does the mass fall, over what distance does it drop and how is it attached to the string?  Are you using anything to multiply the length of time for which the mass pulls the car, i.e. less force but pulling the string for longer, e.g pulley or gears etc?  

Edited by exchemist
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To exchemist

“I'm not clear what the setup is. How does the falling mass transfer a force to accelerate the car? You mention a string attached to an axle. How does the mass fall, over what distance does it drop and how is it attached to the string? Are you using anything to multiply the length of time for which the mass pulls the car, i.e. less force but pulling the string for longer, e.g pulley or gears etc? ”

 

The mass is connected to one end of the string. The other end of string is connected to a small pin that is nailed into the wooden rear (dowel) axle. It is then wound tightly around the rear axel of the car, pulled up and over a wooden dowel which is 3 feet from the bottom of the car. When the weight starts to fall, the attached string rotates the axle, moving the car forward. There are videos on YouTube if you’ve not seen what it looks like. No pulleys or gears on mine as my research showed it would not really help, but just add weight to the car which would cause more resistance and slow the car more quickly.

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  • 2 weeks later...

To exchemist

“I'm not clear what the setup is. How does the falling mass transfer a force to accelerate the car? You mention a string attached to an axle. How does the mass fall, over what distance does it drop and how is it attached to the string? Are you using anything to multiply the length of time for which the mass pulls the car, i.e. less force but pulling the string for longer, e.g pulley or gears etc? ”

 

The mass is connected to one end of the string. The other end of string is connected to a small pin that is nailed into the wooden rear (dowel) axle. It is then wound tightly around the rear axel of the car, pulled up and over a wooden dowel which is 3 feet from the bottom of the car. When the weight starts to fall, the attached string rotates the axle, moving the car forward. There are videos on YouTube if you’ve not seen what it looks like. No pulleys or gears on mine as my research showed it would not really help, but just add weight to the car which would cause more resistance and slow the car more quickly.

 

I would try putting a thicker section on the rear axle for the string to wind around. This 'drum' will increase the torque (I think I said that right) and the car will accelerate faster. Think low gear in a car. In the other thread I speculated on making this thicker section cone shaped, so the weight string starts wrapping on the thickest (largest diameter) section and then works the winding down to smaller diameter. Think automatic transmission.  Hope this is more helpful than not and timely enough as I have been away. :turtle:

post-727-0-41154200-1516493725_thumb.jpg

Edited by Turtle
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  • 3 weeks later...

To exchemist

“I'm not clear what the setup is. How does the falling mass transfer a force to accelerate the car? You mention a string attached to an axle. How does the mass fall, over what distance does it drop and how is it attached to the string? Are you using anything to multiply the length of time for which the mass pulls the car, i.e. less force but pulling the string for longer, e.g pulley or gears etc? ”

 

The mass is connected to one end of the string. The other end of string is connected to a small pin that is nailed into the wooden rear (dowel) axle. It is then wound tightly around the rear axel of the car, pulled up and over a wooden dowel which is 3 feet from the bottom of the car. When the weight starts to fall, the attached string rotates the axle, moving the car forward. There are videos on YouTube if you’ve not seen what it looks like. No pulleys or gears on mine as my research showed it would not really help, but just add weight to the car which would cause more resistance and slow the car more quickly.

I get the impression from this that because the string is pinned to the axle it is stopping when all the string is wound off and tight.

 

Is that what is happening?

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