A basic question about Force

[Hidemons]

I'm in my beginning college physics class and I wonder why F = MA works sometimes. It get repeated over and over again from every grade since forever so I've just.... used it without question.

 

Why is it that when particle 1 is moving at a constant 1 million mph and has mass of x (whatever) and collides with particle 2, that no force is applied to particle 2? Or why is this logic flawed?

[swansont]

I'm in my beginning college physics class and I wonder why F = MA works sometimes. It get repeated over and over again from every grade since forever so I've just.... used it without question.

 

Why is it that when particle 1 is moving at a constant 1 million mph and has mass of x (whatever) and collides with particle 2, that no force is applied to particle 2? Or why is this logic flawed?

 

When 2 particles collide there is a force between them. By Newton's third law the force A exerts on B is equal in magnitude and opposite in direction to the force B exerts on A.

 

However, unless you know what the force is, and how long it is applied, it won't help you to solve for the motion of the objects, which is why momentum is a useful quantity in collisions.

[Hidemons]

When 2 particles collide there is a force between them. By Newton's third law the force A exerts on B is equal in magnitude and opposite in direction to the force B exerts on A.

 

However, unless you know what the force is, and how long it is applied, it won't help you to solve for the motion of the objects, which is why momentum is a useful quantity in collisions.

 

Right, right, all the first stuff is fine and dandy but even when using momentum to calculate force exerted, there is no force exerted right? Since velocity is constant? This logic must be wrong, but why?

 

 

Edit:

Just want to make sure we're on the same page here: F = (final momentum – initial momentum)/t

[J.C.MacSwell]

Right, right, all the first stuff is fine and dandy but even when using momentum to calculate force exerted, there is no force exerted right? Since velocity is constant? This logic must be wrong, but why?

 

 

Edit:

Just want to make sure we're on the same page here: F = (final momentum – initial momentum)/t

 

Why is velocity constant? If there is a collision they must have been on converging paths and therefore had different initial velocities. Unless they moved through each other, essentially missed or not collided, they would each have changed velocities.

[Hidemons]

Why is velocity constant? If there is a collision they must have been on converging paths and therefore had different initial velocities. Unless they moved through each other, essentially missed or not collided, they would each have changed velocities.

 

Velocity is constant because it's my hypothetical situation in which one particle is moving very fast at constant speed and the other is say.... not moving at all (doesn't matter), initially.

Are you saying that to calculate the force exerted on the second particle by the first, the only method is use information from both after and before the collision? Not just from initial information?

 

If thats the case what would be the "t = time" in that formula for deriving Force from momentum?

 

It seems kind of weird to me that a basic equations like the Force/momentum one doesn't work in some cases. In fact most cases.

 

 

EDIT:

 

Infact let me continue:

If a train were to start at a velocity of 0 mph and then accelerate to 100 mph, does that mean over longer periods of time you would be impacted with less force when hit by the train?!!?!

[J.C.MacSwell]

Velocity is constant because it's my hypothetical situation in which one particle is moving very fast at constant speed and the other is say.... not moving at all (doesn't matter), initially.

Are you saying that to calculate the force exerted on the second particle by the first, the only method is use information from both after and before the collision? Not just from initial information?

 

If thats the case what would be the "t = time" in that formula for deriving Force from momentum?

 

It seems kind of weird to me that a basic equations like the Force/momentum one doesn't work in some cases. In fact most cases.

 

 

EDIT:

 

Infact let me continue:

If a train were to start at a velocity of 0 mph and then accelerate to 100 mph, does that mean over longer periods of time you would be impacted with less force when hit by the train?!!?!

 

Starting with the bold the answer is yes. If you can collide with the 100mph train where the collision lasts a few seconds, at constant force, you could survive it quite nicely. Good luck accomplishing this if you jump in front of it.

 

For the above you can't calculate the force from initial conditions or even final conditions unless you know the time t of the collision. If you know that you can calculate the average force during that time, but you would still not know the peak force.

[Hidemons]

It seems like all of these equations only have relevance after the entirety of the contact/collision is made.

 

 

But is Force a vector?

 

Can't an object fall with a force? = ma? This would require no contact to figure this out.

 

 

If a 5000 kg train is moving from 10 mph to 30 mph in 2 seconds, its force upon impact on a wall would approx be 50,000 Kg*m/hr/s. This Force is calculated before any contact is made. Why couldn't I have used this logic in the previous example about the train, where say, it would accelerate to a speed in the course of a year, reducing acceleration to almost nothing.

[swansont]

Infact let me continue:

If a train were to start at a velocity of 0 mph and then accelerate to 100 mph, does that mean over longer periods of time you would be impacted with less force when hit by the train?!!?!

 

Yes, that's one of the ideas behind having certain safety equipment in a car, like crumple zones and airbags. Accelerating from 60 mph to zero in a short amount of time causes a lot more damage than doing it over a long period of time. So you add features that make the impact take as long as is feasible.

[J.C.MacSwell]

It seems like all of these equations only have relevance after the entirety of the contact/collision is made.

 

 

But is Force a vector?

Yes

 

 

Can't an object fall with a force? = ma? This would require no contact to figure this out.

Yes. If you know the mass, and acceleration due to gravity you can calculate the force.

 

If a 5000 kg train is moving from 10 mph to 30 mph in 2 seconds, its force upon impact on a wall would approx be 50,000 Kg*m/hr/s. This Force is calculated before any contact is made. Why couldn't I have used this logic in the previous example about the train, where say, it would accelerate to a speed in the course of a year, reducing acceleration to almost nothing.

 

If the train hits the wall and slows down from 30mph to 10 mph over 2 seconds that would be the average force during that time, but you have to be careful not to mix metric and english units. Technically it is right, if you mean 50,000 kg miles/hr second, but you should convert this to standard units in one system so that it is recognizable.

 

Before contact is made, how do you know how long the collision will last and the final speed?

[Bignose]

The more complete version of [math]\mathbf{F}=m\mathbf{a}[/math] is actually [math]\mathbf{F}=\frac{d m\mathbf{v}}{d t}[/math]. Force is the time derivative of momentum. It is usually just turned into [math]\mathbf{F}=m\mathbf{a}[/math] because most often m is just a constant, and the time derivative of velocity is acceleration.

 

But, the second way to write it shows how intimately force and momentum are related. When momentum changes, a force occurs.

[Royce]

When I think of collision, I think energy/ momentum problem.

 

The equations for energy and momentum are both derived from F=ma.

 

What I take from your example is that the object being impacted doesn't move and you want to know why.

 

The only reason that the 2nd object would not move is that if it had sufficient forces apposing this force of collision.

 

When an object is left alone on a surface, then struck. The only force that can resist the impact is the friction force.

 

If the friction force is sufficient enough to over come the collsion force the object won't budge.

 

If it is struck at a certain height it might tip, and you could calculate that with by taking moments and all that goody mechanics stuff.

 

If you push with your feet to jump off the earth does it move? The answer is yes, you have the power to move the earth. But very very very little.

 

This is because the earth is in space, and there is nothing but outside gravitational forces to appose the force you just exerted on the planet.

 

If I rammed into the planet at a million mph and didn't happen to tunnel through it then I could describe the result of this collsion by using the idea of momentum.

 

Remember momentum is only conserved in the line of impact. Even then it is still under the influence of how elastic the object is, thats why we use the modulus of elasticity.

 

A purely elastic collision will absorb all the energy from the collision and momentum is entirely conserved. e=1

 

Keep going to class and don't think to far ahead of what you are learning.

 

I don't think the earth's brightest minds would build are entire technilogical infustructure on an idea that is flawed.