Imagine you are standing at the plate, staring down a major league baseball pitcher. You hold your bat up behind your head and you think "bring it on...". The pitcher winds up and hurls the ball toward you. You swing your bat around, contacting the ball with a loud crack. It soars, spinning through the air and out of the park for a home run. You win the World Series!

(Ok, enough fantasy for the moment). Though from pitch to hit is less than half a second, this is possibly the most important half-second in baseball. It's also where lots of physics come to play in the game.

The collision between bat and ball is a good example of the principle of Conservation of Momentum at work. Momentum is a vector, so it has a direction as well as a number value. Right before the collision, the bat will have momentum in a forward direction, and the ball in the opposite direction. When they hit, momentum lost by the bat is gained by the ball, which is propelled forward, hence conservation.

The law of "conservation of momentum" states that the total momentum of two objects before the collision is equal to the total momentum of the two objects after the collision. So, in other words, in an elastic collision (in this case, the bat hitting the ball), the total momentum of the two objects before the collision will equal the total momentum after the collision.

Momentum depends on two things though: mass and velocity. For example, in a pitch, a 145-gram baseball is travelling at a certain velocity. Multiply the mass times the velocity, and you get momentum. The average baseball bat, at more than 900 grams, has more mass than the ball, though less velocity than a blistering pitch. More on this later...

Did you know? Little energy is lost in the brief bat-ball collision to heat and friction as the baseball compresses and decompresses slightly on impact.

"Because it's such a small impact period of time, then essentially what you've got is a conservative system," says Carol Putnam, a who teaches upper-level biomechanics at Dalhousie University.

But not all collisions result in homeruns. Putnam says a major factor in an effective swing is the sweet spot, or node of the bat. It is the spot where the vibrations in the bat from the impact cancel each other out. At this spot, less energy is lost to vibrations and the bat makes most effective contact with the ball.

"When you don't hit that sweet spot of the bat, you get that stinging feeling in your hands and the ball is not going to go as far," says Putnam.

Aluminium vs. Wood Bats

Players in U.S. college leagues have used aluminium bats since 1974, but they are illegal in Major League Baseball. Unlike wooden bats, aluminium bats are hollow. They are generally a few ounces lighter than wooden bats, though this depends on the bat's length and the thickness of aluminium.

So does a lighter aluminium bat produce a better swing? Because they have less mass, aluminium bats have a shorter moment of inertia (i.e. shorter tendency of a bat at rest to remain at rest), making them easier to swing quickly. Shorter swing times allow batters to judge the ball's flight in the air for longer. With some major league pitchers able hurl a ball up to 160 km/hour, a split second can mean the ball has travelled several metres further, and revealed itself to be a nice straight fastball, or a sinking slider.

Also, a hollow aluminium tube is less rigid than a solid wooden bat. So, when the ball hits the bat, the bat compresses more. While the ball still transfers its energy to the bat, the compressed aluminium acts like a spring full of potential energy. When the bat expands again, most of the energy is given back to the ball, which is propelled forward, generally at a greater velocity than with a wooden bat.

Did you know? The measure of how quickly an object returns to its natural shape when it's squished is called the coefficient of restitution.

Aluminium bats are lighter and less rigid than wooden bats making them easier to swing and giving a ball more velocity when hit. Because the aluminium bat is more elastic, the ball is not squished as much on impact so less energy and momentum are lost.

But Putnam says wooden bats do have their advantages.

"Lighter bats are easier to swing in terms of getting lots of velocity," she says "but there's a trade off because there is less mass."

If you remember the discussion of momentum, mass times velocity, then you can see how mass plays into being a power hitter. According to Daniel Russell, a physics professor at Kettering University in Michigan, home run-champion Barry Bonds used a 900-gram bat to hit his 73 homers in 2001, while the legend Babe Ruth, who hit 60 in 1927, liked his 1.5 kg hickory slugger.

Essentially, a lighter bat will help a hitter contact the ball more often, but a ball can still be hit effectively with a larger piece of lumber.

And finally... the corked bat controversy

Former homerun king Sammy Sosa was suspended for eight games in 2003 for using a "corked bat." His illegal bat broke on a hit to reveal that it had been drilled and filled with cork to make it lighter.

Many players think, as discussed with aluminium bats, that corked bats, being lighter, will produce higher swing velocity and better hits. But because there is less mass, all swings being equal, the batter will get less momentum than with a full weight bat.

Filling the bat with cork instead of leaving it hollow also negates what benefit would have been gained by making the bat springier, like the aluminium bat. In 1974, it was discovered that Graig Nettles had tried to put some spring in his bat when it broke and six super bouncy balls spilled out onto the field. In the end, it may be that corking a bat just makes it less structurally sound, and more likely to break like Sosa's and Nettles'.

Learn More!

On the Web

ESPN.go.com, "Doctored Bat infractions"

http://espn.go.com/mlb/s/2003/0603/1562807.html

"The Physics of Baseball." Last revised March 3, 2006

www.npl.uiuc.edu/~a-nathan/pob/

"Physics and Acoustics of Baseball and Softball Bats" www.kettering.edu/~drussell/bats.html

The Exploratorium, "Science of Baseball"

www.exploratorium.edu/baseball/

ThinkQuest "The Physics of Baseball: Momentum."

http://library.thinkquest.org/11902/physics/momentum.html

In Books

Hay, James G. the biomechanics of Sports Techniques 4th Edition. Prentice-Hall: Englewood Cliffs, New Jersey, 1993.

Ainslie MacLellan is a freelance journalist from Halifax, Nova Scotia. She graduated from the University of King's College school of journalism in May ('06). She had a keen interest in science and loves keeping up with sports by playing for her club soccer team, and for the Nova Scotia's provincial Women's Rugby team.

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