The Trampoline EffectThe Trampoline Effect

Amilcah GomesAmilcah Gomes

February 2, 2005February 2, 2005

PHY3091 - 01PHY3091 - 01

The Trampoline EffectThe Trampoline Effect

• IntroductionIntroduction

• The Trampoline Effect in BaseballThe Trampoline Effect in Baseball

• The Trampoline Effect in TennisThe Trampoline Effect in Tennis

• The Trampoline Effect in GolfThe Trampoline Effect in Golf

IntroductionIntroduction

• The The trampoline effecttrampoline effect refers to pronounced elasticity in the refers to pronounced elasticity in the impacting object (baseball bat, tennis racquet, golf club, impacting object (baseball bat, tennis racquet, golf club, etc.) such that it acts like a trampoline. etc.) such that it acts like a trampoline.

• It is also referred to as the It is also referred to as the spring-like effectspring-like effect because of the because of the degree to which the object depresses, then springs back into degree to which the object depresses, then springs back into shape when striking a ball. shape when striking a ball.

The Trampoline Effect in The Trampoline Effect in BaseballBaseball• The trampoline effect in The trampoline effect in

baseball refers to baseball refers to the elasticity the elasticity of a bat upon impact with a of a bat upon impact with a baseballbaseball. .

• When a ball hits a wood bat, it When a ball hits a wood bat, it compresses to nearly half its compresses to nearly half its original diameter, losing up to original diameter, losing up to 75% of its initial energy to 75% of its initial energy to internal friction forces. internal friction forces.

• However, in a hollow bat such However, in a hollow bat such as an aluminum bat, the bat as an aluminum bat, the bat barrel compresses somewhat barrel compresses somewhat like a spring. This means that like a spring. This means that the ball is not compressed as the ball is not compressed as much and loses less energy to much and loses less energy to internal friction forces. internal friction forces.

• Furthermore, Furthermore, most of the most of the energy temporarily stored in energy temporarily stored in the bat is returned to the ball the bat is returned to the ball in a metal batin a metal bat. The energy . The energy which is lost in the bat which is lost in the bat compression is much smaller compression is much smaller than that lost without than that lost without compression.compression.

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

Wood or Metal Bats?Wood or Metal Bats?• The Crisco-Greenwald studyThe Crisco-Greenwald study

compares batted ball speeds compares batted ball speeds for balls hit with a wood bat for balls hit with a wood bat and the highest performing and the highest performing metal bat used in their study. metal bat used in their study.

• Figure 2 shows that for a Figure 2 shows that for a given swing speed, the given swing speed, the aluminum bat can potentially aluminum bat can potentially hit the ball hit the ball 5-7 mph faster5-7 mph faster than the wood bat.than the wood bat.

• This can be explained if the This can be explained if the metal bat has a trampoline metal bat has a trampoline effect which returns more of effect which returns more of the energy to the ball. the energy to the ball.

• Thus, the study offers some Thus, the study offers some evidence of an enhancement evidence of an enhancement in performance for metal bats in performance for metal bats due to an elastic property of due to an elastic property of the bat .the bat .

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

The Trampoline Effect in The Trampoline Effect in TennisTennis• The trampoline effect in tennis The trampoline effect in tennis

refers to refers to the elasticity of a the elasticity of a tennis racquet upon impact with tennis racquet upon impact with a tennis balla tennis ball. .

• In science, power is the rate of In science, power is the rate of doing work. The player/racquet doing work. The player/racquet system has power, with the system has power, with the player providing the effort and player providing the effort and the racquet providing the the racquet providing the interface with the ball to deliver interface with the ball to deliver that player effort.  that player effort. 

• So if, consistent with this So if, consistent with this scientific meaning, we consider scientific meaning, we consider a powerful racquet to be one a powerful racquet to be one can achieve a certain ball speed can achieve a certain ball speed with the least player effort per with the least player effort per unit time, and we limit the time unit time, and we limit the time of the stroke, what power then of the stroke, what power then becomes is the inverse of Work: becomes is the inverse of Work: low Work means high power. low Work means high power. This concept can be best This concept can be best understood as understood as efficiencyefficiency.   .   

The Trampoline Effect in The Trampoline Effect in TennisTennis

• The strings of the racquet are the major component in racquet bounce. The strings of the racquet are the major component in racquet bounce. The strings act as a The strings act as a medium that absorbs much of the ball's kinetic energy and returns some of that energy medium that absorbs much of the ball's kinetic energy and returns some of that energy back to the ballback to the ball. Anecdotally, stiff frames with large heads are known to be bouncy, with a . Anecdotally, stiff frames with large heads are known to be bouncy, with a pronounced trampoline effect.  pronounced trampoline effect. 

• In the ball-racket interaction, it is good to have most of the energy stored in the strings, 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 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 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 rather than tighter, it will lead to slightly higher rebound velocities ((more efficiencymore efficiency)). The . The elasticity of the strings is a very important factor for storing energy. elasticity of the strings is a very important factor for storing energy.

• Control, however, suffers as bounce increases, particularly with large heads.  Expert players 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.tend to prefer lower efficiency racquets in order to maintain control during a match.

The Trampoline Effect in The Trampoline Effect in TennisTennis

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

The Trampoline Effect in The Trampoline Effect in GolfGolf

• The trampoline effect in golf refers to The trampoline effect in golf refers to the elasticity of the elasticity of a golf club face upon impact with a golf balla golf club face upon impact with a golf ball..

• Similar to the player/racquet system in tennis, the Similar to the player/racquet system in tennis, the player/club relationship in golf also has power, with player/club relationship in golf also has power, with the player providing the effort and the golf club the player providing the effort and the golf club providing the interface with the ball to deliver that providing the interface with the ball to deliver that player effort. player effort. 

The Trampoline Effect in The Trampoline Effect in GolfGolf• Over the years of golf, driving distance has improved. This Over the years of golf, driving distance has improved. This

has changed from 255.0 yards (the average on the Tour in 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 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) in 33 years. 14.3 of these yards (60.8% of the increase) within the last six years. The average driving distance has within the last six years. The average driving distance has increased at a rate of only increased at a rate of only 1.0 foot per year1.0 foot per year from 1968 to from 1968 to 1995. Using this as a base rate, the sudden jump to 1995. Using this as a base rate, the sudden jump to 7.2 feet 7.2 feet per yearper year, from 1995 to 2001 is quite interesting. (See , from 1995 to 2001 is quite interesting. (See Figure 4)Figure 4)

• It was over this period of time that Titanium drivers with It was over this period of time that Titanium drivers with enhanced rebound velocity "Spring Like Effect" (SLE) were enhanced rebound velocity "Spring Like Effect" (SLE) were introduced. Test data indicates that this type of club (at the introduced. Test data indicates that this type of club (at the USGA limits) will significantly increase the ball velocity and USGA limits) will significantly increase the ball velocity and this may be sufficient to increase distance from 10 to 15 this may be sufficient to increase distance from 10 to 15 yards over clubs without SLE.yards over clubs without SLE.

• The scoring average has only changed from 71.9 strokes The scoring average has only changed from 71.9 strokes per round in 1968 to 70.88 in 2001. This change is 1.12 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 strokes per round in more than thirty years. But in the last ten years the ten years the average scoreaverage score has changed almost 0.4 of a has changed almost 0.4 of a stroke. (See Figure 5)stroke. (See Figure 5)

The Trampoline Effect in The Trampoline Effect in GolfGolf

Figure 4. PGA Average Driving Distance (1968-2001). Note the sudden increase Figure 4. PGA Average Driving Distance (1968-2001). Note the sudden increase from from 1.0 foot per year1.0 foot per year from 1968 to 1995 to from 1968 to 1995 to 7.2 feet per year7.2 feet per year from 1995 to 2001. from 1995 to 2001.

The Trampoline Effect in The Trampoline Effect in GolfGolf

Figure 5. Actual vs. Adjusted Scores: PGA Tour (1968-2001). Note the Figure 5. Actual vs. Adjusted Scores: PGA Tour (1968-2001). Note the major decrease from 1995 to 2001.major decrease from 1995 to 2001.

The Trampoline Effect in The Trampoline Effect in GolfGolf• In 1998, the US Golf In 1998, the US Golf

Association (USGA) Association (USGA) announced a test that would announced a test that would determine if the face determine if the face thickness (or other properties thickness (or other properties of the face) in some manner of the face) in some manner launched the ball too quickly, launched the ball too quickly, thus producing a noticeable thus producing a noticeable distance advantage to distance advantage to players. The USGA concluded players. The USGA concluded that as the face thickness that as the face thickness decreases, the resultant decreases, the resultant trampoline effect increases. trampoline effect increases.

• What most players don’t What most players don’t realize is that although a ball realize is that although a ball hit longer that goes straight hit longer that goes straight is a distinct advantage; one is a distinct advantage; one that goes longer in the wrong that goes longer in the wrong direction (i.e., hooked or direction (i.e., hooked or sliced) becomes a greater sliced) becomes a greater disadvantage.disadvantage.