The Trampoline Effect

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The Trampoline Effect

• Amilcah Gomes
• February 2, 2005
• PHY3091 - 01

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The Trampoline Effect

• Introduction
• The Trampoline Effect in Baseball
• The Trampoline Effect in Tennis
• The Trampoline Effect in Golf

3
Introduction

• The trampoline effect refers to pronounced
  elasticity in the impacting object (baseball bat,
  tennis racquet, golf club, etc.) such that it
  acts like a trampoline.
• It is also referred to as the spring-like effect
  because of the degree to which the object
  depresses, then springs back into shape when
  striking a ball.

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The Trampoline Effect in Baseball

• The trampoline effect in baseball refers to the
  elasticity of a bat upon impact with a baseball.
• When a ball hits a wood bat, it compresses to
  nearly half its original diameter, losing up to
  75 of its initial energy to internal friction
  forces.
• However, in a hollow bat such as an aluminum bat,
  the bat barrel compresses somewhat like a spring.
  This means that the ball is not compressed as
  much and loses less energy to internal friction
  forces.
• Furthermore, most of the energy temporarily
  stored in the bat is returned to the ball in a
  metal bat. The energy which is lost in the bat
  compression is much smaller than that lost
  without compression.

Figure 1. Velocity diagram describing the swing
of a baseball bat before impact, upon contact,
and after impact with a pitched ball.
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Wood or Metal Bats?

• The Crisco-Greenwald study compares batted ball
  speeds for balls hit with a wood bat and the
  highest performing metal bat used in their study.
  
• Figure 2 shows that for a given swing speed, the
  aluminum bat can potentially hit the ball 5-7 mph
  faster than the wood bat.
• This can be explained if the metal bat has a
  trampoline effect which returns more of the
  energy to the ball.
• Thus, the study offers some evidence of an
  enhancement in performance for metal bats due to
  an elastic property of the bat .

Figure 2. A comparison between batted ball speeds
for balls hit with a wooden bat (orange) and an
aluminum bat (blue). The horizontal axis
represents the bats swing speed. Plotting the
data as such normalizes the results, removing the
effect of different moments-of-inertia.
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The Trampoline Effect in Tennis

• The trampoline effect in tennis refers to the
  elasticity of a tennis racquet upon impact with a
  tennis ball.
• In science, power is the rate of doing work. The
  player/racquet system has power, with the player
  providing the effort and the racquet providing
  the interface with the ball to deliver that
  player effort. 
• So if, consistent with this scientific meaning,
  we consider a powerful racquet to be one can
  achieve a certain ball speed with the least
  player effort per unit time, and we limit the
  time of the stroke, what power then becomes is
  the inverse of Work low Work means high power.
  This concept can be best understood as
  efficiency.   

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The Trampoline Effect in Tennis

• The strings of the racquet are the major
  component in racquet bounce. The strings act as a
  medium that absorbs much of the ball's kinetic
  energy and returns some of that energy back to
  the ball. Anecdotally, stiff frames with large
  heads are known to be bouncy, with a pronounced
  trampoline effect. 
• In the ball-racket interaction, it is good to
  have most of the energy stored in the strings,
  which can give back 95 of it. Tighter strings
  produce lower ball speeds because of energy loss
  when the strings start to move and rub within the
  string frame. If the strings are looser rather
  than tighter, it will lead to slightly higher
  rebound velocities (more efficiency). The
  elasticity of the strings is a very important
  factor for storing energy.
• Control, however, suffers as bounce increases,
  particularly with large heads.  Expert players
  tend to prefer lower efficiency racquets in order
  to maintain control during a match.

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The Trampoline Effect in Tennis

• Figure 3. Typical force at the base of index
  finger (IF) and onlittle finger side (G) of the
  hand in an off-center impactof a tennis forehand
  with an eastern grip. Note that the forces of
  frame vibrations are smaller than the pattern of
  impulsive loading and are damped out in less
  than 1/10th of a second.

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The Trampoline Effect in Golf

• The trampoline effect in golf refers to the
  elasticity of a golf club face upon impact with a
  golf ball.
• Similar to the player/racquet system in tennis,
  the player/club relationship in golf also has
  power, with the player providing the effort and
  the golf club providing the interface with the
  ball to deliver that player effort. 

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The Trampoline Effect in Golf

• Over the years of golf, driving distance has
  improved. This has changed from 255.0 yards (the
  average on the Tour in 1968) to 278.5 yards in
  2001, which is a 23.5-yard increase in 33 years.
  14.3 of these yards (60.8 of the increase)
  within the last six years. The average driving
  distance has increased at a rate of only 1.0 foot
  per year from 1968 to 1995. Using this as a base
  rate, the sudden jump to 7.2 feet per year, from
  1995 to 2001 is quite interesting. (See Figure 4)
• It was over this period of time that Titanium
  drivers with enhanced rebound velocity "Spring
  Like Effect" (SLE) were introduced. Test data
  indicates that this type of club (at the USGA
  limits) will significantly increase the ball
  velocity and this may be sufficient to increase
  distance from 10 to 15 yards over clubs without
  SLE.
• The scoring average has only changed from 71.9
  strokes per round in 1968 to 70.88 in 2001. This
  change is 1.12 strokes per round in more than
  thirty years. But in the last ten years the
  average score has changed almost 0.4 of a stroke.
  (See Figure 5)

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The Trampoline Effect in Golf
Figure 4. PGA Average Driving Distance
(1968-2001). Note the sudden increase from 1.0
foot per year from 1968 to 1995 to 7.2 feet per
year from 1995 to 2001.
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The Trampoline Effect in Golf
Figure 5. Actual vs. Adjusted Scores PGA Tour
(1968-2001). Note the major decrease from 1995 to
2001.
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The Trampoline Effect in Golf

• In 1998, the US Golf Association (USGA) announced
  a test that would determine if the face thickness
  (or other properties of the face) in some manner
  launched the ball too quickly, thus producing a
  noticeable distance advantage to players. The
  USGA concluded that as the face thickness
  decreases, the resultant trampoline effect
  increases.
• What most players dont realize is that although
  a ball hit longer that goes straight is a
  distinct advantage one that goes longer in the
  wrong direction (i.e., hooked or sliced) becomes
  a greater disadvantage.