Physics Q

[TawMus]

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Ok guys, I've having a hard to grasping the concept of free fall of object of different masses.

I know that in a vacuum, objects of similar surface areas but different masses fall at the same speed and hit the ground at the same time.

But in a real world situation with air resistance taken into account, which would hit the ground first? The more massive object or less massive object???

 

[Vihsadas]

Ok guys, I've having a hard to grasping the concept of free fall of object of different masses.

I know that in a vacuum, objects of similar surface areas but different masses fall at the same speed and hit the ground at the same time.

But in a real world situation with air resistance taken into account, which would hit the ground first? The more massive object or less massive object???

Okay you brought in two different parameters that could affect your answer, mass and surface area. Think about how each should affect free fall in a vacuum, and also in air.

Remember that gravity is an acceleration. That means, that regardless of mass, the acceleration of two objects disregarding any other external forces will be the same. In this situation (basically a vacuum) the objects will accelerate at exactly the same rate, and will hit the ground at the same time.

Now, think about air resistance. If the force from gravity points downward, which direction does the force from air resistance point?

Now think about a parachute that weighs 20lbs, and a rock that weighs 20lbs, which one hits the ground first?

Now think about two objects which are the same shape and size one that is 10lbs and one that is 20lbs, which one hits the ground first?

So what determines the force from air resistance? surface area, or mass?

Moving to study Q&A

 

[TawMus]

Okay you brought in two different parameters that could affect your answer, mass and surface area. Think about how each should affect free fall in a vacuum, and also in air.

Remember that gravity is an acceleration. That means, that regardless of mass, the acceleration of two objects disregarding any other external forces will be the same. In this situation (basically a vacuum) the objects will accelerate at exactly the same rate, and will hit the ground at the same time.

Now, think about air resistance. If the force from gravity points downward, which direction does the force from air resistance point?

Now think about a parachute that weighs 20lbs, and a rock that weighs 20lbs, which one hits the ground first?

Now think about two objects which are the same shape and size one that is 10lbs and one that is 20lbs, which one hits the ground first?

So what determines the force from air resistance? surface area, or mass?

Moving to study Q&A

I think you misunderstood my question.

The question I'm asking is if two objects of same shape(same surface area/ surface area is constant) but different masses where dropped in a real world situation. Which would hit the ground first? I am guessing the heavier one, because although it experiences more air resistance, because of its mass it also reaches a higher terminal velocity. Please correct me if I'm wrong.

 

[Vihsadas]

I think you misunderstood my question.

The question I'm asking is if two objects of same shape(same surface area/ surface area is constant) but different masses where dropped in a real world situation. Which would hit the ground first? I am guessing the heavier one, because although it experiences more air resistance, because of its mass it also reaches a higher terminal velocity. Please correct me if I'm wrong.

No, I got your question. I was trying to lead you to the right answer.

In a Vaccuum, two objects with the same shape and different masses, which one hits the ground first? Well as you know, gravity is an acceleration, so both will hit the ground at the same time.

Now, in a non-vacuum, let's assume that air resistance is the only other force acting on the masses. Think about what air resistance actually is. It's air molecules slamming into an object that are opposing that objects free-fall. So if you increase the surface area, you get more air molecules slamming into the object and a higher air resistance. This is why a parachute works to slow your fall, it's also why larger parachutes work better.

So now think about in a non-vaccuum, two objects with the same shape and different masses, which one hits the ground first? Try to think about if the air resistance would change, and how the forces would be different from object to object.

I'm going through this exercise because I think its important for the MCAT that we understand exactly why it happens.

 

[TawMus]

No, I got your question. I was trying to lead you to the right answer.

In a Vaccuum, two objects with the same shape and different masses, which one hits the ground first? Well as you know, gravity is an acceleration, so both will hit the ground at the same time.

Now, in a non-vacuum, let's assume that air resistance is the only other force acting on the masses. Think about what air resistance actually is. It's air molecules slamming into an object that are opposing that objects free-fall. So if you increase the surface area, you get more air molecules slamming into the object and a higher air resistance. This is why a parachute works to slow your fall, it's also why larger parachutes work better.

So now think about in a non-vaccuum, two objects with the same shape and different masses, which one hits the ground first? Try to think about if the air resistance would change, and how the forces would be different from object to object.

I'm going through this exercise because I think its important for the MCAT that we understand exactly why it happens.

Well, the more massive object would experience a greater gravitational force thus reaching higher terminal velocity than the smaller mass. Although the more massive object would probably reach terminal velocity faster, because of the larger terminal velocity it will hit the ground first. Am I on the right track???

 

[Vihsadas]

Well, the more massive object would experience a greater gravitational force thus reaching higher terminal velocity than the smaller mass. Although the more massive object would probably reach terminal velocity faster, because of the larger terminal velocity it will hit the ground first. Am I on the right track???

You are on the right track! I was really just trying to get across the point that the reason the more massive object reaches a higher terminal velocity is because the force from air resitance is equal in both cases. That is, the force from air resistance is only dependent on the number of air molecules hitting the object. If the objects are the same shape, then the same number of air molecules hit both objects, so the force from air resistance is the same.
Now, because gravity is a constant acceleration value, the force from gravity is greater with the more massive object. So if you sum the force vectors in both situations, you have a larger force vector downward for the more massive object.
I think the important point isn't necessarily to be able to "intuit" what's going on, but to realize why this happens. And if you understand air resistance, and plot a free-body diagram, you get to see why...and that kind of understanding will help you with those concept questions on the MCAT.

So as a corollary to this problem, you could figure out that the force from air resistance increases with increasing surface area, and that's why a parachute works the way it does.

GJ and good luck!