Robocop.2160 Sy
lvl.1
Switzerland
Offline
|
Hi Paul,
I like your thinking to use math to calculate expected damage on impact but F=MA is not the right formula for this.
Also I don't follow your numbers as you stated the drone was travelling at 20 m/s but in the calculation you used 200 m/s to achieve 600 Newtons? (EDITED - I now understand that was due to decelration occuring in 1/10th of a second but that would mean you should use 89.4 in your calculation for the bike rider? i.e. 10 x 8.94? instead of just 8.94 m/s?)
The formula for kinetic energy is KE = 1/2 x Mass x Velocity squared. - but I will come back to this later.
Another factor is the surface area and hardness of the objects involved on impact.
Let me explain using two examples and I am sure you will agree.
Example 1.
Take a plastic garbage bag and fill it with 60 Kilos of sand. Now drop it from 3 meters onto a ceramic tile floor. The result will be a thud and no damage to the floor.
Now take a 60 Kg solid stainless steel ball and drop it from the same height onto the same floor. You will definitely crack at least one tile. Same weight, same speed on impact, but a totally different outcome. This is due to the way energy is dispersed on impact.
Example 2.
Again using your F=MA formula
A basketball weighs 600 grams - assume it hits you at 20 meters per second.
600g x 20 m/s = 12,000
Now compare a 10 gram bullet (e.g., 357 magnum size) - this would have to be travelling at 1,200 meters per second (Supersonic) to achieve the same numbers using the force equation. i.e. 10g x 1,200 m/s = 12,000
Take your pick, which one would you prefer to be hit by? A slow moving basketball or a supersonic bullet? Using your reasoning they will do the same damage? Do we agree that won't be the case?
As I mentioned above, the formula for Kinetic Energy will give you a more accurate idea of energy dissipated on impact
Lets run our numbers again using the correct formula.
1.
Bike rider impacting a person, 70 Kg @ 9 meters per second.
KE = 1/2 x 70 x 9 x 9 = 2,835
3 Kg drone impacting rotor blades travelling at 200 meters per second
KE = 1/2 x 3 x 200 x 200 = 60,000
The Drone impact has to dissipate TWENTY ONE times more kinetic energy (2100%)!
2.
Now lets look at the ball and bullet.
Ball 600 g at 20 m/s
KE = 1/2 x 600 x 20 x 20 = 120,000
The 10 g bullet @ 1,200 m/s
KE = 1/2 x 10 x 1,200 x 1,200 = 7,200,000
The bullet has SIXTY times the kinetic energy which is why it does significantly more damage on impact.
As mentioned earlier the hardness and size of the objects at the impact point play a large factor also.
|
|