EK Air Resistance Problem

[MedicineForLife]

Hey guys,

I really need help on this problem:

EK says that " the force of air resistance remains constant for any mass"
but then on EK #235 in Physics 1001, the answers suggest that a more massive person parachuting would experience a greater force of air resistance than someone who has less mass (both parachuters are said to reach constant velocity once their parachutes open).

So which is it-- what's the relationship between Air Resistance and Mass?

Thanks.

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[lorenzomicron]

Hey guys,

I really need help on this problem:

EK says that " the force of air resistance remains constant for any mass"
but then on EK #235 in Physics 1001, the answers suggest that a more massive person parachuting would experience a greater force of air resistance than someone who has less mass (both parachuters are said to reach constant velocity once their parachutes open).

So which is it-- what's the relationship between Air Resistance and Mass?

Thanks.

Air resistance is proportional to the velocity of the object and its geometrical shape. In truth, the air resistance is constant for both masses. However, if the object is larger it will experience more air resistance because of its size. However, we must keep in mind that the air resistance must balance the force of the object, which will be m*g=Fg, and the air resistance will need to balance this, so simply to find the termina velocity, it will have to be bigger because it needs to balance a larger m. So, while air resistance is not a function of mass, a specific air resistance is utilized in this problem.

 

[UCB05]

Air resistance is proportional to surface area and velocity, but not mass. A 1kg 1m^3 ball falling at 10m/s will experience the same drag as a 100kg ball of the same shape and volume falling at the same speed.

The parachute question is more a force balancing question. the 1kg ball above will have a weight of 10N, so the upward drag at terminal velocity has to be 10N. The 100kg ball will weigh 1000N, so it will require 1000N of drag at terminal velocity. Drag is proportional to velocity, so it's reasonable to conclude that not only will air resistance be greater for the 100kg object to balance out its weight, but to reach that air resistance, the heavier object is also traveling at a faster terminal velocity.

However again, keep in mind during all this that the air resistance on the two balls will be the same at any equivalent velocity.