I have a doubt in this Physics question.

[Milo]

Pls help me. When a mouse and an elephant are dropped from the same height. Which animal will reach the ground first? Why?

[Shull]

Pls help me. When a mouse and an elephant are dropped from the same height. Which animal will reach the ground first? Why?

 

Ignoring the difference in air resistance due to surface area, both will hit the ground at the same time because acceleration due to gravity is always 9.81m/s^2.

[Relagsingh]

Pls help me. When a mouse and an elephant are dropped from the same height. Which animal will reach the ground first? Why?

 

Ignoring external forces, height and hence in still air, both will hit the same terminal velocity and hit the ground at the same time. Gravitational forces for both are the same.

[Yewheng]

Ignoring the difference in air resistance due to surface area, both will hit the ground at the same time because acceleration due to gravity is always 9.81m/s^2.

 

Hmm.. so if there is air pressure, that means heavier weight will land on ground 1st due to the weight being able to act downward pressure more than a lighter weight that will be more affected by air pressure?

[SimonTan]

This test was done on top of the leaning tower of pisa!!

 

You want to put in air pressure? Then the bigger surface object viewed from the direction of movement(fall) will have higher air resistance acting on it to slow down its acceleration!

Yes in theory larger surface area object falls slower compared to a smaller surfaced object.

 

By conventional physics theory, the density(aka weight/mass) as no effect on the acceleration in a free fall situation.

The formula V=U + a*t

The V is final velocity

The U is initial velocity (can be zero if drop from a stationary position)

The a is acceleration, in thsi case is gravity, g= 9.82m/s

The t is the time duration of the fall.

 

So you see...in this formula...there is no relation to mass/weight/density!!

 

 

[Yewheng]

This test was done on top of the leaning tower of pisa!!

 

You want to put in air pressure? Then the bigger surface object viewed from the direction of movement(fall) will have higher air resistance acting on it to slow down its acceleration!

Yes in theory larger surface area object falls slower compared to a smaller surfaced object.

 

By conventional physics theory, the density(aka weight/mass) as no effect on the acceleration in a free fall situation.

The formula V=U + a*t

The V is final velocity

The U is initial velocity (can be zero if drop from a stationary position)

The a is acceleration, in thsi case is gravity, g= 9.82m/s

The t is the time duration of the fall.

 

So you see...in this formula...there is no relation to mass/weight/density!!

 

K thanks.. but what about a big styrofoam and a piece of wood of same surface area.. aren't the piece of wood land on ground 1st?

Edited July 12, 2011 by Yewheng

[Milo]

Thank you.

[Yewheng]

Theoretically they would both hit the ground at the same time, especially if the test was being done in a vacuum.

 

So what is the cause of the wood to land on the ground 1st as compared to styrofoam of same surface area on real example.

 

Not air pressure, not weight, not mess than what is the thing that causes the difference?

Edited July 12, 2011 by Yewheng

[SimonTan]

Assume we are talking about a styrofoam and wood of exactly the same physical dimension. But of course the wood piece will be heavier.

 

1) In a windless still air situation, a perfect situation....both will reach the ground exactly the same time!

 

2) But in our natural environment...there are wind factors....which will 'rotate'(tumble) the lighter material....causing air turbulance around the lighter falling object. We are talking subjectively here. The heavier material will 'rotate' less.

But it does not mean that the lighter object will reach the ground slower. It might come to a situation where the lighter object can rotate to a plane position, thereby reducing its air resistance surface while falling down, so it will reach the ground faster than the heavier object.

Very subjective as there are many variables not controllable during the free fall situation!

 

 

K thanks.. but what about a big styrofoam and a piece of wood of same surface area.. aren't the piece of wood land on ground 1st?

 

[Yewheng]

Assume we are talking about a styrofoam and wood of exactly the same physical dimension. But of course the wood piece will be heavier.

 

1) In a windless still air situation, a perfect situation....both will reach the ground exactly the same time!

 

2) But in our natural environment...there are wind factors....which will 'rotate'(tumble) the lighter material....causing air turbulance around the lighter falling object. We are talking subjectively here. The heavier material will 'rotate' less.

But it does not mean that the lighter object will reach the ground slower. It might come to a situation where the lighter object can rotate to a plane position, thereby reducing its air resistance surface while falling down, so it will reach the ground faster than the heavier object.

Very subjective as there are many variables not controllable during the free fall situation!

 

I see thanks.. now I understand.. Thank you. [thumbsup]

[Ender]

So what is the cause of the wood to land on the ground 1st as compared to styrofoam of same surface area on real example.

 

Not air pressure, not weight, not mess than what is the thing that causes the difference?

Inertia.. for your case will be mass... Low inertia will changed(slow down) easily by air resistance (from atmosphere).

[Chrispie]

so is air resistance a function of only surface area and not mass at all?

[Saycheese78]

So what is the cause of the wood to land on the ground 1st as compared to styrofoam of same surface area on real example.

 

Not air pressure, not weight, not mess than what is the thing that causes the difference?

 

Wood will reach 1st.

 

Assuming both has the same surface area, the air resistance will create an upward force to counter the gravitational force caused by the mass.

 

The downward force caused by gravity is by the formula F= MA where A is 9.81m/s2. Wood having larger M will reach 1st.

 

The experiment on the eiffel tower was down on onjects with little air resistance (small surface area) and that is why they both reach at same time.

 

--------------------------------------------------------------------------------------

Example, consider the mass of wood to be 2kg, mass of styrofoam to be 1KG, A = 10m/s2 for simplicity.

 

Assume the air resistance (upward force) to be 5 newtons or 5kg m/s2.

 

Wood: Downward force = 2kg * 10m/s2 = 20N. Upward force = 5N. Resultant force = 20N -5N = 15N downward.

F=MA

15N = 2kg * A

A = 7.5m/s2

 

Styro: Downward force = 1kg * 10m/s2 = 10N. Upward force = 5N. Resultant forcce = 10N -5N =5N downward

F = MA

5 = 1kg*A

A =5m/s2

 

Wood accelerate faster.

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Edited July 17, 2011 by Saycheese78

[Ender]

so is air resistance a function of only surface area and not mass at all?

Surface area is a factor. Being aerodynamically shape is another factor.

 

But the if you assume the air resistance to be the same for both wood and styrofoam (both same shape and area), the wood will still land 1st, becoz of inertia. Inertia is just resistant to change.

 

The 'ideal' acceleration for both objects is 9.82 m/s2 . Air resistance is going to change the 'ideal' acceleration of both of them to 'non-ideal' acceleration. If the same air resistance is applied to both the Wood and Styrofoam, the acceleration of the wood will change lesser due to its higher inertia(mass in this case). For mathematically incline, can look that the e.g from saycheese78

Edited July 17, 2011 by Ender

[Saycheese78]

Assume we are talking about a styrofoam and wood of exactly the same physical dimension. But of course the wood piece will be heavier.

 

1) In a windless still air situation, a perfect situation....both will reach the ground exactly the same time!

 

2) But in our natural environment...there are wind factors....which will 'rotate'(tumble) the lighter material....causing air turbulance around the lighter falling object. We are talking subjectively here. The heavier material will 'rotate' less.

But it does not mean that the lighter object will reach the ground slower. It might come to a situation where the lighter object can rotate to a plane position, thereby reducing its air resistance surface while falling down, so it will reach the ground faster than the heavier object.

Very subjective as there are many variables not controllable during the free fall situation!

 

The wind acts as another force. Heavier material rotate less as F=MA, the rotation is an A.

 

I cannot agree with the rotation theory as this creates a new question altogether. Both may be drop from a position of highest air resistance or from position with lowest are resistance. The rotation may result it moving to a position with lowest resistance , for example a shuttle c--k.

[Yewheng]

Wow.. thanks.. did't know actually it is just quite simple to calculate it out.. and thus free fall in natural environment is not 10m/s at all.

 

What about difference in air pressure.. does it make a difference in free fall? Assuming both object (same material, same size and surface area) are to fall from same height distance example the distance of fall is 10m. One is drop from a height of 3000m ( at mountain ) another one which is drop from elevation of 50m. Which one will land on the 10m free fall 1st? The one on elevation of 50m?

[Balaballs]

Very educational.....thank you to all. nice post.

[Sabretan]

In line w the New A n O level direction of setting morally correct n environmentally correct questions w view of senstivity to animals n living objects, such questions have already been phased out of A level/ O level GCE papers.

 

WHO in e right mind wld think of dropping a mouse n elephant? Who wld be able to drop an elephant w/o much effort in e first place? Ridiculous n stupid assumptions in e first place.

 

The wood n styrofoam qtn wld be of a better quality as it involves much more achievable assumptions. However do bear in mind that the relationship between surface area, velocity of object dropping, viscoscity of the fluid in which the object is travelling all contributes to the retardation force due to air resistance and this force is always trying to slow down the object n hence the object will reach terminal velocity needing longer time. The equation to solve this problem is not as straight forward as retardation force F is proportional to kvsq (for high velocity object) or kv (for low velocity object). Those who did Mechanical engineering @ Uni level will know that u need a modelling equation which involve at least a 2nd order differential equation to solve real life simulation.

 

but for TS qtn assuming the commonly seem level of this qtn, the standard kinematics eqn will hold as long as u assume air resistance is negligible PROVIDED it is reasonable to do so. HOWEVER, for a piece of wood n styrofoam expt, unless u do it in a vacuum or near vacuum environment, it is NOT reasonable to assume that air resistance is negligible, which reflect on the logical skill of the question setter.

 

Ok very lor soh n academic but it is the truth.

Edited July 17, 2011 by Sabretan