Acceleration moving in the opposite direction of velocity

[collegelife101]

Hello everyone..I would really appreciate it if someone could answer this question for me.

A projectile is fired with an initial velocity v0 at an angle θ above the horizontal. If the effects of air resistance are taken into account, then the projectile will take:

less time to reach its peak than it does to descend from its peak

In the solution, it said that "Since air friction always opposes the direction of the projectile’s velocity, the projectile has a greater downward (negative) acceleration on the way up to its peak (the downward force of air resistance adds to the downward force of gravity as it rises) than on its way down from its peak (the downward force of gravity is opposed by upward force of air resistance). Since the vertical distance covered is the same up as down, a greater acceleration on the way up implies a shorter time to rise than to fall."

This doesn't make sense intuitively though. If air resistance and gravity are both opposing the projectile, then shouldn't it slow down, taking longer to reach the peak?

Thanks!

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[syoung]

Air resistance and gravity are in the same direction on the way up, and opposite on the way down.

Think about this, use a sheet of paper. If it were floating down, air resistance prevents it from falling too quickly yes? But gravity is pulling it down.

If it were to go up, air resistance would prevent it from shooting up too quickly yes? But gravity is also preventing it from shooting up too quickly by pulling it down.

 

[NextStepTutor_2]

Hey Collegelife101, I answer this question in some more depth in this thread.
http://forums.studentdoctor.net/threads/specific-science-questions.1053558/

Hope it helps!