# Newton Open Ended Question

by Jeff Weisberger
Tags: ended, newton, solved
 P: 9 1. The problem statement, all variables and given/known data According to Newton's Law of Universal Gravitation, the force exerted by the earth on an object is directly proportional to the objects mass According to Newton's Second Law of Motion, the acceleration of an object is directly proportional to the force being exerted on it. Therefore, it may be concluded that if 2 objects are dropped simultaneously from the same height, the heavier object, having a greater acceleration, will reach the ground first. Using mathematical equations, verify or refute the above conclusion 2. Relevant equations 3. The attempt at a solution I have a strong feeling the statement needs to be refuted, just not sure what equations to use in the process, besides F=MA for Newton's Second Law
 Emeritus Sci Advisor PF Gold P: 4,980 When you drop two object what force accelerates them?
 P: 9 Gravity, right? but im just unsure of whether more mass constitutes greater acceleration...
 Mentor P: 8,325 Newton Open Ended Question Do you know how to calculate the gravitational force between two objects?
 P: 9 multiply the 2 masses, then divide by the distance squared
 P: 9 ooh so the mass of the objects would not affect their acceleration, and therefore they would reach the ground at the saem height, correct?... but that doesn't seem right to me, since a feather would take longer than a bowling ball, or does that have to do w/ weight?
 Mentor P: 8,325 Close; you need the gravitational constant. So, the force of gravity exerted by the earth (mass M) on an object (mass m) at a distance r is $$F=-\frac{GMm}{r^2}$$ Now, use this in Newton's second law.
 P: 9 So, 2 objects dropped at the same time would hit the ground at the same time, since the force being exerted on the objects is the same.
Emeritus
PF Gold
P: 4,980
 Quote by Jeff Weisberger ooh so the mass of the objects would not affect their acceleration, and therefore they would reach the ground at the saem height, correct?... but that doesn't seem right to me, since a feather would take longer than a bowling ball, or does that have to do w/ weight?
A feather takes longer than a bowling ball because of the feathers relatively large surface area to weight ratio which means that air resistance has a significant effect on it compared to the bowling ball.

A rather famous experiment was done on the moon where an Apollo astronaut dropped a feather and hammer at the same time. Of course the moon has no atmosphere. Check out the vid below:

 P: 344 You don't have to use newtons law of gravity unless you want to find out how fast the 2 objects are accelerating towards earth or some other object. Keep it simple with F=ma On the surface of the earth things accelerate at 9.8 m/s or 32.2 f/s (assuming no air resistance) so input that. F=m*9.8m/s M= mass so the mass can be 1 gram or it could be a kilogram if you like. No matter what the mass is, it isn't going to change the acceleration (9.8 m/s) of an object. The only thing mass changes is the force.
Mentor
P: 8,325
 Quote by Jeff Weisberger So, 2 objects dropped at the same time would hit the ground at the same time, since the force being exerted on the objects is the same.
Sorry, I didn't read this properly first time (after reading PhantomJay's post, it was apparent that I hadn't read it correctly!) The objects will hit the ground at the same time, but the force exerted on each body is not the same.
 P: 9 alright im going to bed in 5 minutes and this is due tommrow: so, they will arrive at the same time bc they are both affected by the gravitational constant?
 P: 9 oh sweet thank you all for the help
HW Helper
PF Gold
P: 6,047
 Quote by Jeff Weisberger So, 2 objects dropped at the same time would hit the ground at the same time, since the force being exerted on the objects is the same.
No, that is incorrect. Do you see why they are different? What is that force on each object?
P: 344
 Quote by PhanthomJay No, that is incorrect. Do you see why they are different? What is that force on each object?
Newtons law of gravitation doesn't come into play unless both objects touch. What he ment was that both objects are accelerated at the same speed and will hit the groung at the same time.
HW Helper
PF Gold
P: 6,047
 Quote by hover Newtons law of gravitation doesn't come into play unless both objects touch. What he ment was that both objects are accelerated at the same speed and will hit the groung at the same time.
What does that mean? Gravity acts at a distance.....any distance, great or small. The force of earth's gravity on 2 objects of different masses are different, and in each case is equal to the objects weight.
Emeritus
PF Gold
P: 4,980
 Quote by hover Newtons law of gravitation doesn't come into play unless both objects touch.
What? This is nonsense.

The acceleration on both objects is the same but the forces are different for each one and the force is due to gravity.
P: 344
 Quote by Kurdt What? This is nonsense. The acceleration on both objects is the same but the forces are different for each one and the force is due to gravity.
I tend to only think in the form of relativity. When an object moves through space and towards a massive object like earth it won't feel anything. It will only notice that it is accelerating towards the massive object. When it comes in contact with the massive object then it will feel a force.

Am i missing something??

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