Air Resistance Question

Hey Guys, Kinda need help on this question,

Al and Betty are wearing identical mass-less parachute. Al has 3 times the mass of betty. They descend to the ground with constant velocity.

How does the force of air resistance on Al and his parachute compare with Betty and her parachute.

A. Stay the same
B. Three times greater
C. 1/3 as Great

I thought it was B, since at terminal veloctiy, mg=Air Resistance. Since Al has more mass surely he would have more air resistance. However, the answer is A, could someone explain this?

Thanks

Reply 1

Eimmanuel

Study Forum Helper

Original post by ChemBoy1

Hey Guys, Kinda need help on this question,

Al and Betty are wearing identical mass-less parachute. Al has 3 times the mass of betty. They descend to the ground with constant velocity.

How does the force of air resistance on Al and his parachute compare with Betty and her parachute.

A. Stay the same
B. Three times greater
C. 1/3 as Great

I thought it was B, since at terminal velocity, mg=Air Resistance. Since Al has more mass surely he would have more air resistance. However, the answer is A, could someone explain this?

Thanks

I would suggest you re-checked with the answer again. I believe B is the answer.

Reply 2

Eimmanuel

Study Forum Helper

Original post by BDunlop

Draw out a diagram that includes all the forces acting. You'll have mg down and Resistive forces upwards. When terminal velocity you're right mg=air resistance. Now put it into F=ma and you'll see the acceleration is the same regardless of the mass because masses cancel. Remember on the moon a feather will drop at the same rate as a 1kg mass. Air resistance is not affected any mass :smile:

I am not sure what you are trying to explain. You seem to self-contradict yourself by stating that

Original post by BDunlop

….When terminal velocity you're right mg=air resistance. …

first and then conclude that

Original post by BDunlop

…. Air resistance is not affected any mass

In between the conclusion, you are using an irrelevant example to explain why mass is cancelled.

Reply 3

Dat1Guy

Original post by Eimmanuel

I am not sure what you are trying to explain. You seem to self-contradict yourself by stating that

first and then conclude that

In between the conclusion, you are using an irrelevant example to explain why mass is cancelled.

Agreed, i meant to say acceleration not air resistance. This seems like quite a hostile comment from you, was it needed? :angelblush:

Reply 4

username2560671

Original post by BDunlop

Draw out a diagram that includes all the forces acting. You'll have mg down and Resistive forces upwards. When at terminal velocity you're right mg=air resistance. So if you increase the mass, the resistive forces must increase when at terminal velocity.

Yeahh but they are not on the moon! :s-smilie:

Reply 5

Dat1Guy

Original post by Rohit_Rocks10

Yeahh but they are not on the moon! :s-smilie:

Mass definitely doesn't affect air resistance, i'm not sure why OP said it does.

acceleration will be g regardless of the mass.

The more air molecules which the parachute collides with, the greater the air resistance force. Subsequently, the amount of air resistance is dependent upon the speed of the falling object and the surface area of the falling object. Based on surface area alone, it is safe to assume that for the same speed the two masses would encounter the same air resistance.

Reply 6

Eimmanuel

Study Forum Helper

Original post by BDunlop

It should be easy to see why air resistance can be dependent on mass in this case. When the object is falling at terminal velocity, based Newton's 2nd law,
Net force = ma
mg - air resistance = 0
mg = air resistance

So air resistance should depend on mass in this scenario.