TBR physics question

[sundaydress03]

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I'm having a ton of trouble with TBR physics. This, specifically, is the question I'm having problems with (Passage IV Q23)

Basically, it talks about how students are launching spherical balls from a slingshot and they're varying the angle, initial potential energy and mass of projectile.

Q23: If the ball is placed too high in the cavity, it will leave the slingshot with backspin. How does this backwards rotation affect flight time and the max height?

A. Both flight time and max height increase.
B. Flight time incrases and max height decreases
C. Flight time decreases and max height increases.
D. Both flight time and max height decrease.

Apparently, the answer is A, which goes against all my intuition. I understand that backspin can cause lift, but I had no idea that lift is the result of unequal air pressure above and below the ball, causing air molecules to collide more violently with the bottom half of the ball than the top half.

My logic was that if backspin is happening, the initial velocity is going to be less "effective" and result in a decrease in flight time (since vertical velocity will be less and acceleration constant) and max height (since it just doesn't have the power going up).

Can anyone explain how to get to the right answer with just the information given in the chapter??? They don't say a single thing about air resistance or lift, rendering it pretty difficult to come to the right answer. Is this an example of a question I'd truly see on the MCAT??? It seems to deal more with trivialities than real concepts 🙁

 

[Morsetlis]

http://en.wikipedia.org/wiki/Magnus_effect
http://en.wikipedia.org/wiki/Backspin

Is it that hard to integrate a random fact into your life? 🙂

Without the entire passage, I can't tell you exactly how to deduce the backspin lift velocity from just the passage. If you posted it, maybe somebody can reason it out for you without wikipedia.

 

[minombre]

Put yourself in the reference frame of the ball: the air around the ball is moving towards it while the ball is moving forwards (so if the ball were moving towards the right, then the air would be moving towards the left). If you have a backspin, then you have to take into account the interactions between the rotational velocity of the ball and the velocity of the air: the vector sum of the two velocities (the ball's rotational and the air's) are going to be additive on top of the ball (b/c they're in the same direction) and destructive under the ball (b/c they're in opposite directions). Therefore, the net velocity is higher on top of the ball then under it. By Bernoulli's Equation, the pressure must be lower on top then under, so there's a net force pushing the ball up.

Since there's a net force pushing it up, it can resist the pull of gravity for longer, thereby giving it both a longer time of flight and a higher maximum height.

Hope that helps!

 

[costales]

There is a graphical illustration of this kind of phenomenon under "Why are golf balls dimpled?" at http://math.ucr.edu/home/baez/physics/index.html

 

[UrshumMurshum]

Put yourself in the reference frame of the ball: the air around the ball is moving towards it while the ball is moving forwards (so if the ball were moving towards the right, then the air would be moving towards the left). If you have a backspin, then you have to take into account the interactions between the rotational velocity of the ball and the velocity of the air: the vector sum of the two velocities (the ball's rotational and the air's) are going to be additive on top of the ball (b/c they're in the same direction) and destructive under the ball (b/c they're in opposite directions). Therefore, the net velocity is higher on top of the ball then under it. By Bernoulli's Equation, the pressure must be lower on top then under, so there's a net force pushing the ball up.

Since there's a net force pushing it up, it can resist the pull of gravity for longer, thereby giving it both a longer time of flight and a higher maximum height.

Hope that helps!

This is an excellent response.

 

[FishyTheFish]

Some energy is required to make the ball spin, this energy is received from the potential energy in the sling-shot; so shouldn't the ball leave the sling shot with a lesser velocity if it leaves spinning as opposed to not spinning?

 

[MT Headed]

Is this an example of a question I'd truly see on the MCAT???

No. MCAT physics topics are more about seeing the obvious physics in a complicated situation. This question is more about trying to find complicated physics in a simple situation. Notice how the previous post ^^^ gives a really good reason why the ball should go less far.

If you learned something, even if it's just to be more paranoid about your gut feelings, consider it time and money well spent. TBR bio was equally full of ambiguous and unfair questions, but they still taught me a lot.