# This may be a stupid question?

1. Nov 15, 2004

### cwestrx7

i was just watching gone in 60 seconds on television. in the movie an actor is standing on a platform and someone swings a crane's claw into him at approximately 15mph to knock him off the platform. this started an arguement between my roomates and I about whether or not the weight of the claw or large metal object is what determines how much it hurts. if it is going the same speed and has the same dimensions will the impact be the same no matter what the weight? or will increasing the weight of the object but keeping the overal size and speed make it a harder impact? also could you make it heavy enough to kill a person even if it were only going 15mph and was just knocking the person off? please give me some insight on this situation. hopefully i posted this in the correct forum, thanks in advance.

steve

2. Nov 15, 2004

### James R

What matters in this situation is the momentum of the claw.

Momentum is mass multiplied by velocity.

Double the mass, or double the speed, and the claw has double the "clout" when it hits something.

3. Nov 15, 2004

### Staff: Mentor

However, at some point the mass of the claw is so much greater than your mass that it can just as easily be considered infinite. Then all you need to calculate is how much momentum you have at 15mph and the impact is equivalent to you running at a full sprint into a brick wall.

4. Nov 15, 2004

### NoTime

It's not that simple in this case.
What would matter here is the total area of the contact and where it hit you.

For example a sword blade with << momentum would go right through you.

Chances are that you could survive this without serious injury.
Although broken bones are quite likely.
I doubt that you would stand in line for a repeat experience.

5. Nov 15, 2004

### DaVinci

Also do not forget the angle relative to the direction of motion and impact direction. :rofl:

This would be a key factor in determining the entire scope of the force and impact of the two object.

6. Nov 15, 2004

### Integral

Staff Emeritus
We need to compare apples to apples and oranges to oranges. Certainly a cutting weapon behaves differently from the shovel of a backhoe.

That being said, which would do more damage a steel sword or a paper mache sword?

I did a tour of Universal City Studios in the
60s, they showed us the foam rubber boulders used in movie landslides. You could play catch with them, try that with a real boulder, the difference was the mass or actually the momentum of the object.

Even with a sword a low velocity blow does less damage then a high velocity blow.... lower momentum (or energy if that is what you choose to measure)

7. Nov 15, 2004

### NoTime

But then it would not be hitting you at 15 mph :tongue2:

8. Nov 15, 2004

### NoTime

Not really. It all depends on the shear modulus or compressive strength of the impacted material. The force per square inch goes up as the surface area contacted is reduced, all else being equal. A sword approaches 0 for the surface area contacted.
If it was moving fast enough the shovel would cut you in half.

A tornado can drive a straw through a tree. :tongue2:

Kind of cool the tricks they play.

9. Nov 16, 2004

### theriddler876

say the crane claw hits your entire body at the same time, now using gravity and ignoring friction, you can calculate by jumping off diffrent heights to see at what velocity you would die. this would only work for original impact.

for example 15 miles per hour is about 6.7 meters per seconds if I did my math right, this would be the equivalent to falling from a distance of .683 meters, or about 2.2 feet, so falling 2.2 feet into something hard, like steel, probably won't do much damage

the weight of the object is not relevant, since a wall hitting you at 15 mph, would be the same as if the moon hit you at 15mph, or if you hit the moot at 15 mph

10. Nov 16, 2004

### NoTime

Didn't check the math, but you have the right idea.
See russ_watters post also.

11. Nov 17, 2004

### Physics is Phun

u didn't do the math right. The fall would be roughly 2.3 meters

12. Nov 18, 2004

### theriddler876

if you fall one meter, your velocity would be 9.8, so falling two meters would be twice that which 19.6 meters per second which is about 70 KM per hour which is far above 15 mph

13. Nov 18, 2004

### Chi Meson

No. From rest, you would fall 5 meters (4.9 actually) in one second. This is because after one second you would go from 0 m/s to 9.8 m/s. Your average speed would therefore be 4.9 m/s.

Falling two meters, you would be going about 6.3 m/s, or 14 mph.

14. Nov 18, 2004

### krab

2.3m. It would be like falling out of the top bunk while asleep, falling onto a concrete floor.

15. Nov 23, 2004

### Gara

15 mph does seem slow.

What if the object was going at 1 mph, how heavy would it need to be to brake bones when it swings into you?

16. Nov 30, 2004

### krab

Ignore post #2 (only momentum that matters) and read post #3. It would be like hitting a brick wall at 1 mph. That's 1/3 of a walking pace so IOW, no damage.