The speed of cars vs. wildlife
Gutted. We hit a deer on the way to a night out in Peterborough (which was actually a great laugh – the night out, not hitting the deer) and smashed up all the front nearside of the car. Grrrrr..
If anyone knows the A605 between Thrapston and Peterborough, they’d know it’s a fast, barely lit road that runs through a large area of rural countryside. We were shooting along it at about 70 mph and this thing shot out of the darkness, slamming straight into the front nearside (the left of the car as you’re sitting in the driver’s seat) before I could react and brake (good job really as I may have over-reacted and skidded off the road, maybe..).
This prompted the car to do a tiny little bunny-hop followed by the noise of plastic and metal dragging on the road and breaking into a hundred tiny little pieces. My wife, bless her, thought it was the deer making this noise (the crunching of bones) and burst into tears!!
It was far too dangerous to stop there and then so we limped on to our destination and checked the damage (the pound signs were flashing in my head thinking about repair costs).
Little kamikaze bugger had wrecked the front of my car! A big hole under the front pumper, power steering had stopped and the washer fluid reservoir had started to drip out all its contents onto the tarmac below.
Later when telling my mum about what had happened she piped up ‘So did the deer just bounce off and limp away?’. ‘No Mum. Most definitely not!’. We were going far too quickly for it to survive.
Which got me thinking – what speed were we going in meters per second? And what chances did the deer have??
- 70 mph is about 112 kph.
- So 112 kph is 112,000 meters per hour.
- There are 3600 seconds in an hour (which is 60 secs x 60 mins)
- 112,000 meters divided by 3600 secs (or approx 112 divided by 4 if you’re doing it in your head)
- = 31.1 meters per second (28 with the rough method)
So we were travelling at approx 30 meters per second (ms^1) when we hit this deer.
So what were it’s chances of surviving?
To determine the acceleration and force on the deer, we need to know the amount of time which passed as the velocities changed.
Let’s assume that the collision occurred during a 2 second time period. So t = 2.
- Acceleration = (vf – vi) ÷ t.
Where vf is final speed (30 ms^1 in our case – I’ve disregarded the change in momentum to the car after hitting the deer as it’s so small) and vi is initial speed (0 as the deer was stationary when we hit it).
- So the acceleration of the deer = (30 – 0) ÷ 2 = 15 m/s^2
To find out the forces involved we use f = ma, where f is the force, m is the mass and a is the acceleration.
We know the m if the deer is 10kg and the acceleration as above is 15 m/s^2
So f = 10 * 15 = 150 N (or Newtons)
So what does 150 N compare to?
For example, is it more than gravity?
Gravity is acceleration, not a force. Weight is the force, which is caused by gravity.
- Weight = mass * g, g = 9.8 m/s^2
- The acceleration of the deer, 15 m/s^2
So yes, the acceleration of the deer was greater than gravitational acceleration.
They’ve investigated boxers and found they can generate up to 5,000 newtons of force with a punch. Whilst kicks can produce up to 9,000 N!
http://en.wikipedia.org/wiki/Orders_of_magnitude_(force) also has the below different orders of magnitude of force:
What does all this mean? After all 150 N seems very little compared to the examples above.
I’m not entirely sure. Please leave me an example if you can shed more light on all this.
And beware of kamikaze wildlife!