# Backspin, and the trajectory of a cannonball vs a golf ball.

Discussion in 'MCAT Study Question Q&A' started by johnwandering, 05.05.12.

1. ### johnwandering

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BR mentions Backspin in its practice tests, but never explicitly says how it affects the ball.
1.)I know that it increases Height and Flight Time. But does it affect radius?

Also, I understand that the dimples on the golf ball allow it to create a greater pressure difference between the bottom and the top of the ball.
2.) Is this true for a regular backspin, or is this pressure effect exclusive to dimples?
Basically, do the dimples on a golf ball introduce new elements to the ball's backspin flight, or does it just exacerbate properties already there?

3.) Does anyone know what the flight path of a regular backspin cannon ball will look like compared to a normal cannon ball launch?

2. ### dmf2682

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1) are you asking if hitting the ball with backspin will change the radius? no. shouldn't.

2) dimples and backspin are two different things, so it really introduces a new effect. Dimples create turbulence, which cause the air around the ball to separate farther from the stagnation point than it would for a smooth ball. This decreases the wake caused by the ball, decreasing drag. What you have to remember for the MCAT is: dimples --> less drag --> longer, further trajectory

3) Adding backspin to a cannon ball will cause it to create lift as it flies. This is from Bernoulli's effect. The result is a longer, further trajectory but not necessarily from reduced drag, but more because of increased lift.

3. ### ipodtouch

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I never heard of backspin, crap.

So, Spinning a ball just makes it go Higher... but it lands in the same spot?
Where exactly is that extra energy coming from?

When you hit a ball with Energy=10J, and some of it is invested into spinning the ball, there should be less available for ball velocity...

Shouldn't the radius be less?

4. ### johnwandering

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Hey sorry, I meant to say RANGE, not radius. So Range increases when you add backspin?

5. ### chiddler

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6. ### Mr Spock

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As the ball moves through the air it pushes on the air in front of the ball and pulls on the air in back and this is what causes the pressure differentials. Dimples increase surface drag and make the relative airflow going around the ball to be more turbulent and stick better to its surface reducing its wake. Although dimples increase surface drag they decrease pressure drag and because this is the more prevalent drag on the ball reducing it reduces net drag.

If the ball spins while going through the air the uneven surface drag around the ball causes it to veer from the flight it would have if the ball were not spinning. This is not unlike a car tire whose rotation causes the surface drag between it and the road to make it move linearly. A spinning ball is not pushed into the ground its pushed into the air as a result of its motion through it. The result is not as dramatic but it does produce a force referred to as the Magus effect. Why if the Magnus effect were caused by surface drag any surface preperation that increases surface drag will increase the Magnus effect and that is exacly the case.

A spinning ball moveing through the air does not generate lift in the real world but does in the textbook world. Why do text incorrectly refer to this as lift or Magnus lift? Although the fact that the ball is spinning is very significant in determining that the ball is generating the Magnus effect it is totally ignored when it comes to determining the aerodynamic force that causes it. Calling it lift is based on the false premise that the ball is not spinning.