When does mass matter?

[05med05]

Ok,

I know I should know this already, but other than when air resistance is considered, when does the mass matter in accelleration due to gravity or projectile motion problems?

First scenario

If a ping pong ball and a ball filled with lead (both exactly the same size) are either:

a) dropped and accellerate due to a force of mg,
b) role down a ski jump and therefore experience projectile motion,

both will land in the same place at the same time if air resistance is ignored.

If air resistance is considered, then the lead ball will travel farther than the ping pong ball, and hit the ground ever so slightly before the ping pong ball does.

Correct?

2nd scenario

If two people jump out of a plane with identical parachutes, will the heavier person or the lighter person hit the ground first? I would think the heavier person, but if you consider upward force due to the air resistance in the parachutes, then wouldn't the lighter person reach the ground first since the upward tension of the parachute would be greater for the heavier person?

What basic rule am I missing or misinterpreting here to be so easily confused???

Thanks!

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[Wrigleyville]

The acceleration due to gravity is always 9.8m/s/s. Mass does not enter into the equation. The forces on objects of different masses are different, but the acceleration is always the same.

 

[willthatsall]

Some say size doesn't matter at all, others say it does. But I think they are usually talking about length and girth, not mass necessarily. I guess it depends on who you are dealing with whether mass matters or not.

 

[FutureDocDO]

Some say size doesn't matter at all, others say it does. But I think they are usually talking about length and girth, not mass necessarily. I guess it depends on who you are dealing with whether mass matters or not.

LOL

 

[rcd]

ermm how do you get a parachute problem that tells you to ignore air resistance .

 

[sdnstud]

i will try to answer this one:

Air resistance depends on the object's shape and size, NOT on the object's mass!!!

1) if a ping pong ball and a lead ball are dropped, both balls experience the SAME air assistance. However, the terminal velocity of the lead ball is greater than the terminal velocity of the ping pong ball (terminal velocity is proportional to mass). That explains why teh lead ball lands before the ping pong. (refer to TPR TEST A)

2) If the balls roll down an inclined plane and launched, the lead ball will travel farther due to greater inertia and great initial velocity (assuming there is friction between the balls and the inclined plane). Again, the air resistance depends on the balls' shape and size and therefore the balls experience the same air resistance. (EK fulllength 2)

3. Similar logic as #1, heavier skydiver will land before the lighter skydiver. The two experience teh same air resistance (same shape and size), butthe heavier skydiver achieves a greater terminal velocity.

 

[willthatsall]

Everyone is right, but simply put: if air resistance is not considered, mass doesn't matter. But like the other guy said, how can you have a parachute problem and not consider air resistance?

 

[05med05]

Sorry,

The parachute problem would obviously incorporate air resistance....

I guess I didn't make myself clear.

(Does this mean I need help with the writing sample on how to write more clearly???)

 

[05med05]

2) If the balls roll down an inclined plane and launched, the lead ball will travel farther due to greater inertia and great initial velocity (assuming there is friction between the balls and the inclined plane). Again, the air resistance depends on the balls' shape and size and therefore the balls experience the same air resistance. (EK fulllength 2)

QUOTE]

Then, if there is no friction between the balls and the inclined plane, then both balls would leave the inclined plane at the same speed, but the lead ball would still travel farther (horizontally) due to greater inerta?

 

[willthatsall]

I'm pretty sure you don't need to know about rolling balls and rotational kinetic energy.