Force and pressure aren't the same!

It's not just force, though. Force alone doesn't split things in half.

If I threw a basketball in someone's face with great force, I might break their nose, teeth, etc., but I wouldn't shear their head into different pieces. If I threw an ax in someone's face using the exact same amount of force, or even less force, I might.

A .45 caliber bullet wouldn't split a cow in half, either.

Now, let's say your friend had a very large blade edge running down the front of the motorcycle. -Then- I might see slicing the cow in half and continuing on.

Do you realize what an axe would have to be made of to weigh the same amount as a baseball? If it was made of wood, which it would pretty much have to be, you aren't going to do much more damage than you would with the baseball.

And yes, the size of the point of impact makes a difference, however, the bike is going to strike with nearly 3,000 times the force. And yes, force is pressure.

Force = Mass x (Starting Velocity - Ending Velocity) / (Starting Time - Ending Time)

Pressure would be the force you're exerting over a set amount of time, but in a collision, that amount of time is going to be 1 second or less. I said 1 second, which was actually being generous, since a lot of the force would be transferred in less than that, thus increasing the force even higher.

Now, just taking the force into account, lets look at the size. The width of the head of a .45 bullet slopes from about an eighth of an inch to a quarter. So we'll even take the smallest point of an eighth. Multiply that by 3,000. That would come out to 187.5 inches. Now.. I would say that the width of a motorcycle is less than 15 feet.. wouldn't you?

So basically, if the motorcycle is even 4 inches across at the tire, it is going to be exerting more force per milimeter than the bullet by quite a lot.

Pressure is measured in force per square area. Your tire pressure is measured in psi. Force over time is work, I believe.

The bullet in question hits for 1230.7 (km/s)/in^2, which is does not at all resemble a real unit measure, but that's ok. It also might not go through as much bone as the bike needs to and still come out the other end.

The bike would need an impact area of 8 square inches to achieve the same. So, if the rider mounted a 2x4 on the front of his bike, it'd work out to the same.

F = m*a, actually, where F = force, m = mass and a = acceleration. You were probably thinking of P = m*v, where P = momentum, m = mass, and v = velocity.

Sorry to be such a stickler!

A true stickler would note that they are the same formula. Velocity is a vector, and is simply acceleration with a direction, but with as simple a thing as this, they are equal to eachother.

No.

Acceleration is a time derivative of velocity; that is, a = dv/dt, as you can see from the units of acceleration (meters per seconds-squared) as opposed to the units of velocity (meters per second). In simple terms, acceleration is the rate of change of velocity. I think you confused acceleration with speed, which is defined as the scalar value of velocity.

My rant: Thinking about physics. Maybe I shouldn't have gotten involved.

A true stickler would make note of how wrong you are, seeing as how the average American learns the difference between speed, velocity, and acceleration in sixth grade.

It works in this instance only because that's the speed at which the impact point on the cow accelerates.

Which is why they're the same when you're dealing with impact where the time of transfer is 1. In an impact, Velocity is simply acceleration with a direction, and so they are interchangeable. In a non-impact situation, they're different, but for an impact, F = mv where t = 1 works perfectly. That was what I meant. They're the same in the terms of the situation in question.

Velocity is not acceleration with a direction. At all. At. All. Velocity is speed with a direction. Speed != acceleration. At all.

You have failed today's physics quiz (while my physicist girlfriend politely reminds me of the points I got wrong over IMs). Please escort yourself to the next thread.

The math... it burns!

Math is nice and all, but why not just get a motorcycle and a cow and test it out yourselves, Mythbusters style? Sure it's reckless and dangerous (Unless you do some fancy rigs for it), but what's life without risks?

Rant: Not knowing enough about physics to actually help contribute. Ideally, if I ever need to know anything, I test it out. This is why I do not work at N.A.S.A. (Out of interest, what the is proper use of the period here? Would I be expected to put an extra period at the end, or is it considered at the end of "A."?).

Rave: Only on the Lusternian forums will people discuss if a motorcycle can cut through a cow. Not just "Ha, it so would" but actually working on the math behind it.

Yes, I know that Velocity is Speed with a direction, however, again, IN THIS SITUATION, they are pretty much the same thing in terms of results. The speed with which the bike hits the cow is the acceleration of that point, as you stated.

So, again, since in this situation a = s, then v = a, and as such velocity is acceleration with a direction. But only in terms of this situation. Mathematically its true, so I didn't fail anything.

You are correct only by accident. Moriana (and everyone else) is correct in the textbook definitions of force, acceleration, velocity, and momentum. You are thinking of the force applied on the cow by the bike, which still does not make velocity acceleration with a direction. Let's start from the top (using algebraic physics).

Force equals mass times acceleration.

Momentum equals mass times (delta/change in) velocity OR force multiplied by time of impact.

Pressure equals force divided by area.

Setting all these equal in the correct way (yay, algebra!) in the case of the motorbike hitting the cow, the force on the cow would be:

Force = mass of bike/rider * speed of bike (provided he hits the cow straight on and not at an angle) / time of impact.

I guarantee the time of impact was not one second. I also guarantee the cow did not remain stationary and the bike did not deccelerate from its maximum speed to zero in that time, therefore all of the bike's momentum is not transferred to the cow and the force does not reach the maximum shown above. So I'm going to have to go with the crowd that says ripping apart a cow with a bike is unlikely.

Severe blunt force trauma, sure. Slicing? Yeah ... no.

P.S. My roommate would like to know where you got your degree. We extend our condolences if it was ASU.

I said basketball, actually, but your objection isn't relevant.

First of all, my point was that things do not "split" because of force. There is a reason people do not chop wood with a sledgehammer, and that reason has nothing to do with force. I can generate far more force with a sledgehammer than an ax, but the ax cuts wood, and the sledgehammer does not. If a skyscraper fell on top of you, it would not split you in half.

And, incidentally, F = ma, so a basketball can generate the same amount of force as an ax, it just has to accelerate more.



A bullet will not split a cow in half. It is not a viable point of comparison. You need to find something that -will- split a cow in half and see how a motorcycle compares.

As to the rest of your use of the equation, I think that's adequately covered by everyone else.

Keep in mind, I would be delighted to be proven wrong. For some reason, this topic has really caught my fancy. Maybe we should start another thread. "Can SAW's Friend Have Split a Cow in Half with a Motorcycle?" And he very well may have. I'm just saying, from a theoretical envisioning standpoint, I have a hard time seeing a motorcyle go straight through a cow and divide it. You seem to be arguing that the motorcycle hit it really hard, which I understand. I'm just saying that hitting something really hard doesn't mean it splits.

I'll provide the bike, someone else can find a rider and a cow.

I always just write NASA.

Is this physics discussion regarding a motorcycle ripping a cow in half actually taking place?

Anyways, obviously a bullet wouldn't split a cow in half, but a motorcycle is pretty big, and I'm wondering how big the cow was. If he was going fast enough and the majority of the cow was hit by the motorcycle, I dont see why it wouldn't split in half.

I'll just say this, a bullet can't tear a cow in half, but a bullet could go through a cow. So, a motorcycle, if it achieved the proper speed, could also go through a cow. Now, given that the motorcycle is bigger, it is going to take more of the cow with it, and as such "cut" the cow in half.

Its a mathematically sound situation.

As for everything else, I wasn't wrong by accident, I was just using the simplest statement possible, and in that situation, I was correct. A stickler, or one who needles every little point, would note that in that situation, mathematically, what I said is correct. That, and I was mainly just teasing back.

People here just enjoy their own egos too much. *shrugs* doesn't matter much to me. I know what I meant and I know I was correct. You all can feel haughty all day long for all I care. No skin off my cow.

You weren't even correct in the simplified bit...

I assume you've done algebra before?

If V = S + Direction and S = A, then V = A + Direction.

Therefore, I am correct.