Pushing off.

[Gareth56]

Watching Spider-Man 3, who turns out to be made of very strong stuff in order for him to survive such accelerations and forces, I noted one thing that got me thinking. During one scene as a building was collapsing around Spider-Man he leaped onto a piece of falling wall and proceeded to launch himself off it. Assuming that the piece of wall was falling at terminal velocity and as such was in free fall would Spider-Man have been able to exert at force on the piece of free-falling wall in order to gain flight?

[swansont]

Terminal velocity isn't an absolute limit of speed, just the limit when the forces are limited to gravity and drag. Add in another force, snd the speed will change, so he could (in principle) do this. If the block had sufficient time and distance, it would slow to the terminal speed again afterwards.

[Gareth56]

Terminal velocity isn't an absolute limit of speed, just the limit when the forces are limited to gravity and drag. Add in another force, snd the speed will change, so he could (in principle) do this. If the block had sufficient time and distance, it would slow to the terminal speed again afterwards.

 

Does that mean that terminal velocity is the velocity at which the air resistance force of a falling object equals the weight of the object minus the acting force due to air, which halts acceleration and causes speed to remain constant? So if SM is pushing against the block of wall to provide him with the necessary acceleration to jump off it what is the block of wall pushing against in order for the wall to supply the equal and opposite force to supply SM with the force to push off the piece of wall? Is this the same scenario as if you are in a lift (elevator) that's in free fall then just before it hits the ground you could jump up and possibly save yourself from a nasty end?

[insane_alien]

being at terminal velocity is not free fall. as anyone who's jumped out a plane can tell you. at terminal velocity you can still sense down as it's like lying on a really really soft cushion, kind of, its difficult to describe accurately.

 

my advice for understanding the above is to go do it yourself. then you'll know what i mean.

[Edtharan]

So if SM is pushing against the block of wall to provide him with the necessary acceleration to jump off it what is the block of wall pushing against in order for the wall to supply the equal and opposite force to supply SM with the force to push off the piece of wall?

When SM pushes off the falling wall, the wall will move faster in one direction and SM will move in the opposite direction.

 

Imagine we are on an Ice-skating rink (and we had super ice that offered no friction - its for the sake of the example ok ) and we were stuck in the middle. If I were to"jump" off you towards one side of the rink, I would go flying off in that direction. However, you would go flying off in the opposite direction.

 

If instead of another person, it was a chunk of masonry, then because of it's mass, it would not move very fast, however, I would get nearly my full jump speed, as I am lighter (Acceleration=Force/Mass).

 

We can ignore the fact that the block of masonry is falling in the case of SM as the other forces are in equilibrium (the pull of gravity is exactly matched by the air resistance).

 

As Spider Man jumps up, he will push the masonry downwards.

 

When you jump off the ground, you do the same thing to the Earth. However, it is so massive that others don't really feel it. And when you fall back towards the ground, your gravity pulls the Earth towards you a bit, just as the Earth pulls you towards it. When you work out the maths, you will find that the amount you pushed the Earth when jumping is exactly matched by the amount your personal gravitational field pulls the Earth towards you.

[Gareth56]

How would you be able to "jump" (I'm not too sure what you mean here) off me if there were no friction for you to get a grip to do that? I appreciate the fact that if I were stuck in the middle of a frictionless ice rink and had a medicine ball (as you always have on you) I could get to the edge by simply throwing the ball in one direction and I would go in the opposite.

 

You say that SM just push the masonry downwards but what is then pushing back on the masonry in accordance with Newton's 3rd Law.

[swansont]

How would you be able to "jump" (I'm not too sure what you mean here) off me if there were no friction for you to get a grip to do that? I appreciate the fact that if I were stuck in the middle of a frictionless ice rink and had a medicine ball (as you always have on you) I could get to the edge by simply throwing the ball in one direction and I would go in the opposite.

 

You say that SM just push the masonry downwards but what is then pushing back on the masonry in accordance with Newton's 3rd Law.

 

The ice skating/medicine ball scenario is exactly what's going on in this case. The reaction force to SM pushing on the block is the block pushing on SM. Action/reaction pairs don't act on the same object (in this case, the masonry; action exerted on it, reaction exerted by it)

[insane_alien]

The ice skating/medicine ball scenario is exactly what's going on in this case. The reaction force to SM pushing on the block is the block pushing on SM. Action/reaction pairs don't act on the same object (in this case, the masonry; action exerted on it, reaction exerted by it)

 

yep but (note: the following is based purely on my own observations in highschool physics classes) people seem to have a problem relative idealised scenarios with real life especially if there is a skew in orientation(horizontal to vertical or vice versa and everything in between.)

[Edtharan]

How would you be able to "jump" (I'm not too sure what you mean here) off me if there were no friction for you to get a grip to do that?

I would be jumping off you towards the edge of the rink. I am not jumping off the rink.

 

The rink has no friction, but that is only important because it will mean that even the slightest jump off you will push both me and you towards opposite edges of the rink (I just wanted to take the surface we were sitting on out of the equation).

 

"For every Action there is an Equal and opposite reaction."

When you push something, it pushes you back with an equal and opposite force.

 

With the Medicine ball and the ice rink, you throw the ball in one direction, and the medicine ball pushes you with an equal and opposite force. That is why it goes one way and you go the opposite way.

 

Therefore if I jump off you, I give you a push in one direction (with my feet instead of my arms like with the medicine ball), and an equal an opposite force is directed at me.

 

But, why don't you go flying off just by standing on the ground (not ice) and throwing a medicine ball.

 

Well you do.

 

When you throw a medicine ball while standing on the ground (not ice), you give a force to the ball and it pushes back with an equal and opposite force.

 

However, as you are standing on the ground, there is a bit of friction. You will also have placed your feet so that you can use that friction to remain where you are on the ground (try throwing a medicine ball while standing with your feet together , you will likely stagger backwards).

 

Because of the friction with the ground, the force of the ball is transferred through you to the Earth, which then moves in the opposite direction to which you threw the ball.

 

When the ball lands. The friction with the ground slows it down. The friction is providing a force that will reduce the speed of the ball.

 

But equal and opposite again.

 

As the Earth pushes on the ball to slow it down, the ball pushes on the Earth in an equal and opposite direction.

 

Just say we threw the ball Eastwards. Then the equal and opposite force on us is in a Westwards direction. Because of friction between out feet and the ground and our stance is positioned to take advantage of that, the Westwards force is transferred through us an into the Earth, giving it a push in the Westwards direction (actually this transference of force uses this equal and opposite force thing as well - but I am just focusing on the ball and the Earth).

 

Now when the Ball lands, the friction with the Earth allows the ball to give a push to the Earth in an Eastwards direction. And the Earth Pushes on the Ball in a Westwards direction.

 

Notice that the Earth was first pushed in a Westwards Direction, and then in an Eastwards direction. Not only that, the two pushes were the same strength. So the Earth does not gain any extra speed from it.

 

So back to Spider Man:

When SM jumps off the block of masonry, he gives it a push in one direction. The masonry, because of the equal and opposite force, pushes on SM. SM goes off in one direction and the masonry goes of the exactly the opposite direction.

 

About the only thing you can say he is "pushing off" of is Inertia. It is the fact that for every action there is an equal and opposite reaction. If SM pushes the block away from him (by jumping) then the block pushes back on SM. Because there is nothing stopping the block or SM from moving in any direction, they separate from each other with the block moving in one direction and SM moving in exactly the opposite direction.

 

Because the masonry has a much larger mass than SM, it has a larger inertia. This means that the force that SM puts on the block will not make it move very fast. However, because SM has a lower mass, the same amount of force will make him move much faster than the more massive block.

 

Also, remember. they made that shot with computer generated special effects. There is no reason that the animation has to follow the real laws of physics any more than a Bugs Bunny cartoon has to.

[Gareth56]

Many thanks Edtharan for your time and trouble taken to explain this.