calculating speed? without a speedometer

[kaile]

im launching another rocketboard this saturday which has 10 d-12 engines totaling 200 newtons per second. im wondering how i can calculate speed without the use of a speedometer either it be a stopwatch or plugging in variables into a formula.

for those interested about the rocketboard you can visit my site at http://rocketpowered.cjb.net/

videos:

http://putfile.com/media.php?n=rocketthrust

in this video i clustered 2 d-12 engines on the back of an oldschool board equiped with tracker b2 trucks and gumballs. This is basically a test for the ignition wires (because my freind didnt think they would ignite simultanously.)

[carveasy]

you could just use the basic- change in distance/change in time - to give you the average speed. but you wont be able to get the top speed with that. to get the top speed you need the acceleration. im not sure how to calculate the acceleration. search physics formulas on google.

[Blocko]

you could just use the basic- change in distance/change in time - to give you the average speed. but you wont be able to get the top speed with that. to get the top speed you need the acceleration. im not sure how to calculate the acceleration. search physics formulas on google.

to get the acceleration you need the velocity at the beginning of your run and the end of your run [vi and vf respectively (v=d/t)] then you can use the formula a=(vf-vi)/t


Note: t in this case is the total time of your run.

[Wheel Man]

Excuse me kind sir, have you seen my leg? I seem to have blown it off.

[kaile]

thanks. ive been having fun calculating formulas for the past hour i might mark a 50 meter run on the road and use a stopwatch to clock the time from point A to point B and then convert meters per second to miles per hour.

[Blocko]

thanks. ive been having fun calculating formulas for the past hour i might mark a 50 meter run on the road and use a stopwatch to clock the time from point A to point B and then convert meters per second to miles per hour.

if you're trying to find acceleration, it's going to be m/s^2(squared)


[edit] the units I mean. (eg 25m/s^2 or 25 meters per second squared)

[mangels]

getting back to calculating the acceleration - Newton's Law F=ma works, You know the force from the rocket motors, you know how much you and the board weigh (weight is not the same as mass - more on that later). So the acceleration a=F/m.

Your weight is the pressure you exert on the scale and is the result of gravity tugging on your mass. In the case of F=ma, F would be your weight, a rould be the acceleration due to gravity or 9.8 m/sec^2. Since you have the force of the rocket engines in Newtons, you will need to use grams for your weight and the skateboard's weight (which will be constantly changing as the motors use up their fuel - but why worry about that?).

Once you figure the total mass of your system, you'll be able to do the a=F/m thing to determine the acceleration you will experience - if you are starting from a dead stop. Obviously, if you push up to speed before igniting the motors, you will have overcome the initial rolling friction AND your initial velocity will not be zero.

Can't weight to see the videos - GO ROCKET MAN!

[mangels]

PS - I wouldn't woory to much about the difference in mass between the start of the burn and the end of the burn from the depleted feul - it will be a very small percentage of the total mass of the system. Of greater concern will be things like the friction in your bearings and the pressure of the wind in your face. You could compensate by riding down a slight hill, but then the angle of the hill plays into the whole thing too and instead of F you have F(cosA) ..... too much to think about.

Most of all - when calculating the total mass of your system, remember to include the mass of the pads and helmet that I know YOU WILL BE WEARING.

[kaile]

remember to include the mass of the pads and helmet that I know YOU WILL BE WEARING.

no question about it. I will definatley be wearing a helmet elbow pads and knee pads. I was moved emotionaly after reading the longboarding death :thumb:

[hackitysackity]

how much force is it putting out? enough to take you to the top of a hill?

bomb down a hill, then carve back up it with rocket power. mmmmm that would be tasty