Why a 30 mph vs. 40 mph Impact Makes a Difference

In summary, when a car is traveling at 30mph, the kinetic energy is 72563.63901 J, while at 40mph, the kinetic energy is 0.126445 J. The discrepancy in these values may be due to errors in converting from miles per hour to meters per second. It is important to carefully perform unit conversions in order to accurately solve problems.
  • #1
Phykid
14
0

Homework Statement


Our homework set today was to find out why is it that when you hit someone at 30mph they have a 20% chance of dying. Yet when you hit someone at 40mph they have a 90% chance of dying.


Homework Equations


KE = 1/2 M V^2


The Attempt at a Solution


First of all i found out that 1mile = 1.6km; which means 1 mile = 1600m
I then did 60x60 to find out the amount of seconds in 1 hour: which was 3600 seconds
I then divided 30 by 3600 to get the meters/second; which was 8.333 [reoccuring] m/s
Now I found out the Kinetic Energy. We were allowed to choose which car we used, and I used my fathers. This weighs 2090kg (Mazda 5)

KE = 2090 x (8.333^2) = 145127.278
145127.278 / 2 = 72563.63901 J

This is where the problem came

I then tried it with 40mph

40 / 3600 to get the m/s; which turned out as 0.0111 [reoccuring]
I now thought how the heck could that be right? But I continued & tried to do the equation.

Ke = 2090 x (0.011^2) = 0.25289
0.25289 / 2 = 0.126445

Okay now I'm very confused. Can anyone help me out?

When 30mph KE is 72563.63901 how is 40mph 0.0111?

Thanks,
Phykid
 
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  • #2
The calculation methods for 30 mph and 40 mph are identical, so recheck your arithmetic. Redo both conversions from mph to m/s.

Phykid said:
First of all i found out that 1mile = 1.6km; which means 1 mile = 1600m
I then did 60x60 to find out the amount of seconds in 1 hour: which was 3600 seconds
I then divided 30 by 3600 to get the meters/second; which was 8.333 [reoccuring] m/s

You forgot the 1.6 factor in going from miles to km.
 
Last edited:
  • #3
I'm confused, sorry :uhh:

EDIT: Ignore that, someone posted same time as me
 
  • #4
Redo your conversion of 30 mph to m/s. Do it carefully, step by step.
 
  • #5
So how would I go about including the 1.6 into it?

Thanks,
Phykid
 
  • #6
Phykid said:
So how would I go about including the 1.6 into it?

Thanks,
Phykid

Bump :O
 
  • #8
Brilliant!
Thanks for all your help
 
  • #9
I hope you realize that in your first post you correctly described how 1 mile = 1.6 km = 1600 m (approx). But then for some reason you forgot to use that bit of info when you did your conversion from mph to m/s.
 
  • #10
I didnt until you pointed it out in your first post :)
thanks
 

1. How does speed affect the impact of a collision?

Speed directly affects the amount of force generated during a collision. The higher the speed, the greater the force upon impact. This means that a collision at 40 mph will have a greater impact than one at 30 mph.

2. Why is 30 mph considered a safe speed limit in residential areas?

30 mph is considered a safer speed limit in residential areas because it allows for better reaction time and reduces the force of impact in case of a collision. It also reduces the severity of injuries in case of pedestrian accidents.

3. How does the weight of the vehicles involved in a collision affect the impact?

The weight of a vehicle is directly proportional to the force generated during a collision. This means that a heavier vehicle will have a greater impact than a lighter one, even at the same speed. This is why larger vehicles, such as trucks, require longer stopping distances.

4. How does the design and safety features of a vehicle impact the outcome of a collision?

The design and safety features of a vehicle play a crucial role in determining the outcome of a collision. Vehicles with advanced safety features, such as airbags and crumple zones, are better equipped to absorb and distribute the force of impact, reducing the risk of injuries. The overall design of a vehicle also affects its stability and maneuverability, which can impact the severity of a collision.

5. Are there any other factors besides speed that can affect the outcome of a collision?

Yes, there are several other factors that can affect the outcome of a collision. These include the angle and direction of impact, the condition of the road, the weather, and the use of seatbelts. It is important to consider all of these factors when studying the impact of a collision and developing safety measures to prevent them.

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