# Why don't heavy objects fall more SLOWLY?

by chudd88
Tags: fall, heavy, objects, slowly
P: 25
 Quote by chudd88 This is a simple question, and I'm sure it has a trivial answer, but this thought occured to me just now. How is it that gravity is able to accelerate things at the same rate regardless of its mass, while one definition of mass is the measure of resistance to acceleration? If two objects are 100 miles from Earth, I would expect that the Earth's gravity would be able to accelerate the lighter object more quickly, while the heavier object would resist the acceleration and fall more slowly. Why is this not the case?
Well - it can't be so. Just think of the situation if heavier objects really had fallen slower, what would have happened if you divided the heavy object in two parts? Should those parts start falling quicker then? Why? Or divide it in 1000 small light parts! Should speed depend on if those parts where held together or not? How tight do the parts need to sit together to fall slower than they would individually? It obviously could not work that way.

Best way to think about it is that gravity acts equally on each atom in every object. No matter how many atoms there are, or if they are bound together or not.
P: 490
 If you are in a rocket ship out in space undergoing acceleration and you "drop" a marble and a bowling ball in your ship, they will "fall" with the same acceleration. Einstein's equivalence principle is saying that gravity is the same thing as the "force" that pushes you back in your car seat when you step on the gas.
The marble and the ball only fall with the same acceleration if you "drop" them.If you placed the marble and ball on the floor of the spaceship when it was accelerating and then decelerated or stoped the craft, the bowling ball and marble would accelerate away from the floor at different rates.Gravity is similar to the force that pushes you back in car seat when you step on the gas but not the same.
P: 755
 Quote by Buckleymanor If you placed the marble and ball on the floor of the spaceship when it was accelerating and then decelerated or stoped the craft, the bowling ball and marble would accelerate away from the floor at different rates.
If what you mean by "stop the craft" is to simply turn off the thrusters then that's not true. If you disregard the elasticity of the objects then both balls will remain on the floor. To bring them off the floor you would need to reverse the thrusters. And even then they would accelerate away from the floor at the same rate. That's because the balls will not really accelerate at all. The ship will accelerate from the balls.
P: 490
 Quote by TurtleMeister If what you mean by "stop the craft" is to simply turn off the thrusters then that's not true. If you disregard the elasticity of the objects then both balls will remain on the floor. To bring them off the floor you would need to reverse the thrusters. And even then they would accelerate away from the floor at the same rate. That's because the balls will not really accelerate at all. The ship will accelerate from the balls.
No it would not, the craft would just have to be accelerating or travelling at a rate less than maximum speed the bowling ball had attained when it was first accelerated by the spacecraft.The objects are elastic I don't see how you can disregard this unless they are made of unobtanium.
You might as well conclude that the balls are stationary and the craft is doing all the movement and acceleration around them.
If the balls were placed on your car seat and accelerated and the car crashed the marble would be accelerated faster and travell further than the bowling ball if it's exit through the winderscrean was unristricted.
P: 102
 Quote by Buckleymanor .If you placed the marble and ball on the floor of the spaceship when it was accelerating and then decelerated or stoped the craft, the bowling ball and marble would accelerate away from the floor at different rates.
How do you manage to get this? Ignoring elastic rebound, they would appear to accelerate in the opposite direction as the rocket, with the same magnitude.

 Quote by Buckleymanor Gravity is similar to the force that pushes you back in car seat when you step on the gas but not the same.
And how would you tell the difference between constant accelerating and gravity in a soundless, windowless car?

 Quote by Buckleymanor No it would not, the craft would just have to be accelerating or travelling at a rate less than maximum speed the bowling ball had attained when it was first accelerated by the spacecraft.
Unless the direction of acceleration were to change, then the objects would not come off of the floor, even if the magnitude of acceleration were to decrease. That is because the spaceship will always be moving faster at a time t+dt and the only way the object will do that is if the floor of the spaceship is in contact with it.

 Quote by Buckleymanor The objects are elastic I don't see how you can disregard this unless they are made of unobtanium.
Ah, I see how you managed to get that earlier. It's an idealization, used to ignore whatever is irrelevant to the situation. And it's irrelevant since once the object loses contact with the floor, its elasticity doesn't matter; it will continue with constant velocity because of Newton's First Law.

 Quote by Buckleymanor You might as well conclude that the balls are stationary and the craft is doing all the movement and acceleration around them.
That's pretty much the entire point of the equivalence principle, that there is no difference between being on a spaceship with constant acceleration, or being on the surface of a planet with the gravitational acceleration.

 Quote by Buckleymanor If the balls were placed on your car seat and accelerated and the car crashed the marble would be accelerated faster and travell further than the bowling ball if it's exit through the winderscrean was unristricted.
No, they would continue on with the same velocity they had right before they lost contact with the seat.
 P: 755 Buckleymanor, I agree with Jasso's objections to your comments. An analogy to ignoring the elasticity of the objects is in the thought experiment for the universality of free fall. In that scenario we commonly ignore the effects of air resistance and buoyancy when we say that two objects will accelerate at the same rate regardless of there difference in mass. We do that because, as Jasso pointed out, those effects are irrelevant to the point of the thought experiment. To demonstrate, here's another thought experiment. Imagine two balls of different mass sitting on a table in an accelerating spacecraft (or sitting on the surface of earth). Because of the elasticity of the objects, the two balls, and the table they are sitting on will deform from the force of acceleration. When the thrusters are turned off (or earths gravity is turned off) the balls and the table will resume their normal shape (rebound), causing the balls and the table/spacecraft (or earth) to accelerated away from each other. But once the balls and the table are no longer in contact, the acceleration stops, and they continue on away from each other at a constant speed. This is the effect that we want to ignore. If we ignore the elasticity of the objects then the balls will remain on the floor when the thrusters are turned off. The only way to bring them off the floor would be to reverse, or change direction of, the thrusters. If we reverse the thrusters, then the spacecraft will accelerate away from the balls.
P: 490
Quote by Jasso
How do you manage to get this? Ignoring elastic rebound, they would appear to accelerate in the opposite direction as the rocket, with the same magnitude.
 That's the point you can't ignore elastic rebound.

And how would you tell the difference between constant accelerating and gravity in a soundless, windowless car?
 By using elastic rebound to note the difference speeds of acceleration upon different objects.

Unless the direction of acceleration were to change, then the objects would not come off of the floor, even if the magnitude of acceleration were to decrease. That is because the spaceship will always be moving faster at a time t+dt and the only way the object will do that is if the floor of the spaceship is in contact with it.
 Would that not depend on the speed the magnitude of acceleration decreased.

Ah, I see how you managed to get that earlier. It's an idealization, used to ignore whatever is irrelevant to the situation. And it's irrelevant since once the object loses contact with the floor, its elasticity doesn't matter; it will continue with constant velocity because of Newton's First Law.

That's pretty much the entire point of the equivalence principle, that there is no difference between being on a spaceship with constant acceleration, or being on the surface of a planet with the gravitational acceleration.
 There is.

No, they would continue on with the same velocity they had right before they lost contact with the seat.
Which would be different for the bowling ball and marble.Substitute car and seat for catapult.
Fire bowling ball, then marble, with the same amount of force, which travells furthest and fastest.
 Mentor P: 22,295 You're confusing acceleration and force.
P: 490
 Quote by russ_watters You're confusing acceleration and force.
Don't forces come into play when onboard accelerating spacecraft or cars.
I ain't sure if it's just a question of blanking out the windows and making them soundproof.
P: 490
 Quote by TurtleMeister Buckleymanor, I agree with Jasso's objections to your comments. An analogy to ignoring the elasticity of the objects is in the thought experiment for the universality of free fall. In that scenario we commonly ignore the effects of air resistance and buoyancy when we say that two objects will accelerate at the same rate regardless of there difference in mass. We do that because, as Jasso pointed out, those effects are irrelevant to the point of the thought experiment. To demonstrate, here's another thought experiment. Imagine two balls of different mass sitting on a table in an accelerating spacecraft (or sitting on the surface of earth). Because of the elasticity of the objects, the two balls, and the table they are sitting on will deform from the force of acceleration. When the thrusters are turned off (or earths gravity is turned off) the balls and the table will resume their normal shape (rebound), causing the balls and the table/spacecraft (or earth) to accelerated away from each other. But once the balls and the table are no longer in contact, the acceleration stops, and they continue on away from each other at a constant speed. This is the effect that we want to ignore. If we ignore the elasticity of the objects then the balls will remain on the floor when the thrusters are turned off. The only way to bring them off the floor would be to reverse, or change direction of, the thrusters. If we reverse the thrusters, then the spacecraft will accelerate away from the balls.
Completly agree as to ignoring the air resistance and buoyancy effects on different objects in free fall.To enable us to come to the conclusion that they accelerate at the same rate regardless of there different mass.
The difference between that and the anology you are making is that it is possible to conduct an experiment with two different objects free falling within a vacuum to check your results.
Ignoring elasticity in a thought experiment is not possible experimentally so might as well substitute it for green cheese.
Mentor
P: 22,295
 Quote by Buckleymanor Don't forces come into play when onboard accelerating spacecraft or cars.
Of course - whenever you have one (net force, that is) you have the other. But you defined your scenarios such that in one case the accelerations were equal (thus, forces different) and in the other the forces were equal (thus accelerations different).
P: 490
 Quote by russ_watters Of course - whenever you have one (net force, that is) you have the other. But you defined your scenarios such that in one case the accelerations were equal (thus, forces different) and in the other the forces were equal (thus accelerations different).
If I were to hit a bowling ball with a tennis bat and then a marble, both with the same amount of force won't one travell further than the other.
The marble and the bowling ball are seperated on the floor of the craft.
 Mentor P: 22,295 Yes....
Mentor
P: 11,869
 Quote by Buckleymanor If I were to hit a bowling ball with a tennis bat and then a marble, both with the same amount of force won't one travell further than the other. The marble and the bowling ball are seperated on the floor of the craft.
In your spaceship you are accelerating them both at the same time to the same speed. Both of them are accelerating at the same rate and once you reach your maximum velocity both will be traveling at that velocity. Each one takes a different amount of force to accelerate, but since they are both on the floor of the ship at the same time they both have to accelerate at the same rate.

This is kind of similar to hitting both the bowling ball and the marble with the bat at the same time, except that hitting them with the bat has much more complicated physics come into play such as elasticity and such. Imagine you had them both sitting on a catapult at the same time and simply launched them into the air with it. Both would accelerate at the same rate to the same speed.
P: 490
 This is kind of similar to hitting both the bowling ball and the marble with the bat at the same time, except that hitting them with the bat has much more complicated physics come into play such as elasticity and such. Imagine you had them both sitting on a catapult at the same time and simply launched them into the air with it. Both would accelerate at the same rate to the same speed.
Yes if they were both sitting in or on the catapult together the bowling balls mass and the marble mass would each have an influence on each other and the catapult, so they would accelerate at same rate.
If they were separated and on the floor of the spacecraft, or placed on seperate seats in the front of car when the craft decelerated or the car crashed and elasticity was 'allowed' to do what it does.
They would still accelerate at the same rate to the same speed.The less massive marble would not have as much elastic effect when first accelerated as the more massive bowling ball.
Thanks.
Mentor
P: 11,869
 Quote by Buckleymanor If they were separated and on the floor of the spacecraft, or placed on seperate seats in the front of car when the craft decelerated or the car crashed and elasticity was 'allowed' to do what it does. You would be able to tell that the objects are being accelerated and not in a gravitational field.
No, the floor of the spacecraft is like the catapult, accelerating both equally. Same for a car.
 P: 490 Yes I must have been editeing at the same time you were posting thanks.
P: 490
 Quote by Drakkith In your spaceship you are accelerating them both at the same time to the same speed. Both of them are accelerating at the same rate and once you reach your maximum velocity both will be traveling at that velocity. Each one takes a different amount of force to accelerate, but since they are both on the floor of the ship at the same time they both have to accelerate at the same rate. This is kind of similar to hitting both the bowling ball and the marble with the bat at the same time, except that hitting them with the bat has much more complicated physics come into play such as elasticity and such. Imagine you had them both sitting on a catapult at the same time and simply launched them into the air with it. Both would accelerate at the same rate to the same speed.
To be honest I am haveing trouble trying to understand this.
If you just had the the bowling ball on the floor of the spaceship and you accelerated it to your maximum velocity and then applied the brakes or reverse thrust and measured the distance and the speed it travelled at.
Then you repeated the same experiment with just the marble won't the distances and speeds of the two objects be different.

With respect to this question I posted.
 If I were to hit a bowling ball with a tennis bat and then a marble, both with the same amount of force won't one travell further than the other.
And the answer I recieved from russ_watters
 Yes....
If that is the case then won't it be possible to distinguish between acceleration due to gravity and acceleration due to acceleration.

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