Projectile Motion Question...Again

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GeorgianCMV

MCAT Studyaholic
10+ Year Member
5+ Year Member
Guys, I'm having issues understanding this. Someone on 4/10/08 posted the question of: if you throw a small ball vertically upward in real air with air resistance, does it take longer to go up or come down?

See, if I write the equations out like BloodySurgeon did...

Mg + Air resistance = ma (Ascent)

Mg - Air resistance = ma (Descent)

...it makes sense. However, when I draw a picture, I can't convince myself why the descent would take longer. If on the ascent the direction of motion is being opposed by two forces, whereas on the descent it's only being opposed by one, air resistance, and is in the same direction of gravity, then why wouldn't the ASCENT take longer?

It's seriously making me nuts that I can't grasp this.

Wouldn't you want as many as forces as possible in the same direction of motion???

Kaustikos

Antibiotics 4 Lyfe
10+ Year Member
• Guys, I'm having issues understanding this. Someone on 4/10/08 posted the question of: if you throw a small ball vertically upward in real air with air resistance, does it take longer to go up or come down?

See, if I write the equations out like BloodySurgeon did...

Mg + Air resistance = ma (Ascent)

Mg - Air resistance = ma (Descent)

...it makes sense. However, when I draw a picture, I can't convince myself why the descent would take longer. If on the ascent the direction of motion is being opposed by two forces, whereas on the descent it's only being opposed by one, air resistance, and is in the same direction of gravity, then why wouldn't the ASCENT take longer?

It's seriously making me nuts that I can't grasp this.

Wouldn't you want as many as forces as possible in the same direction of motion???
The way I look it at is that the acceleration upwards is larger initially compared to the acceleration downwards. The force upwards takes into consideration the air resistance plus the weight of the ball because you overcome both of those with the throw so you throw it with an acceleration overcoming the two. The downwards fall only works with the acceleration of gravity in play and so takes longer.

GeorgianCMV

MCAT Studyaholic
10+ Year Member
5+ Year Member
Ok that definitely helps. Thanks!