# Launching an RC car off a ramp on a flat roof. Are these computations correct?

1. Mar 12, 2009

### denver75

Hi gang,
I've been reading for a while and now need some help of my own. Thanks in advance.

I'm currently planning a project where I'm going to demonstrate a launch of an RC car off the flat roof of a building. The height of the roof is 25'. I figure the RC car will go 25mph, and the ramp angle is about 25º. The ground below is flat and the car will be destroyed--that's OK though.

I've put together a spreadsheet that attempts to model the launch and descent, including plotting the launch trajectory. I THINK I've done everything correctly but would love to have another set of eyeballs peak at it before I distribute the datasheet. The file is attached.

Main concerns:
1. My calculated distance traveled (57.43') doesn't match up with the location on the plot where RC car hits the ground (looks to be just over 60').

2. Will it really travel 60'? That's hard to imagine.

3. I haven't included any calculations for air resistance...

Denver

#### Attached Files:

• ###### RC Car Rooftop.xls
File size:
49.5 KB
Views:
83
2. Mar 12, 2009

### cragar

ya thats what i got so the rc car is going about 36 feet per second
i used d=.5gt^2 and h=v^2(sin(x)^2)/(2g)
and range=v^2sin(2x)/g
i computed the range then divided it by two the calculated the maximun height attained and calculated how long it would be in the air from the max height
i came up with what u got

3. Mar 13, 2009

### Gnosis

Your Excel attachment is pending approval, so it can't be opened at this time.

I calculated approx 60.4 ft from the point where the car leaves the ramp however, it also depends upon how high your ramp is. You mentioned 25 degrees, but failed to mention the height of the ramp above the 25 ft rooftop. Ramp height above the rooftop's height will further increase the horizontal distance.

Additionally, it appears you are only guessing at the car's velocity. I'd lay down some markers on flat ground and time how long it takes to travel the distance marker to maker to precisely calculate its velocity. This will aid you in getting even closer to the calculated impact zone.

Why not try to have the car land on some foam or partially inflated plastic trash bags to prevent the car's obliteration? I would consider this, as I hate to see anything blatantly wasted.