1	Chapter 3 Getting a Move On
2	Sprint Start Demonstrates Newton’s Third Law of Motion: The Law of Action-Reaction “For every action there is an equal and opposite reaction.”
3
4	"Muscular Force overcomes the inertia..." Muscular Force overcomes the inertia of the sprinter He begins to accelerate Gravity works against the sprinter Acceleration is proportional to the Force the sprinter applies to the blocks – in the horizontal direction Besides Gravity, there is Friction and Air Resistance acting on the body
5	Momentum Mass X Velocity Depends on the athlete’s mass and his/her velocity Momentum plays a role in impacts Example: an SUV and a VW bug collide going head on at 30 mph Example: a 300 lb. lineman hits an oncoming 190 lb. halfback each going 6 m/s Collisions are common in some sports
6
7	Momentum Force = the change in momentum time F = m (Dv / Dt); since a = Dv / Dt F = m Dv / Dt; or Force is the change in momentum divided by time
8	Momentum Some sports require maximum momentum, and some require optimum controlled momentum Examples: Football punt, Soccer pass, jump shot in Basketball, Shot put
9	Impulse The Force applied to an athlete to get him/her moving requires a certain amount of time. The amount of time that it takes to impart the force to a golf ball, or tennis ball, or baseball takes very little time. The amount of time it takes to impart a force to the body for the high jump takes much longer time
10
11	Impulse Is the application of a force for a certain period of time F = ma = m Dv / Dt or mv/t So, if F = m Dv /t you can multiply each side by t Then, Ft = m Dv = mv_f – mv_i
12	Impulse A flexible athlete can apply the force for a longer distance and thus a longer time than an inflexible athlete Some sports require very large amounts of Force over very short periods of time Examples: Karate blow, hitting a baseball
13	Ball hitting bat
14	Graph of ball hitting bat
15	Impulse in the Javelin Throw Force is applied over a much longer time than the Karate blow or hitting the golf ball A javelin thrower first increases the velocity of the javelin by running forward This increases the momentum of the body The thrower brings the javelin backwards behind his body. He throws forward as his legs block his body’s forward momentum
16	"This transfers some of his..." This transfers some of his body’s momentum up through his body to the arms
17
18	Impulse of the High Jump The objective of the take off in the high jump is to generate as much vertical velocity as possible. The body is a projectile Unlike the javelin throw the high jumper applies force for a much shorter time Instead of jumping from a full squat the jumper may bend the knees only 20-30º
19
20	"The velocity of the approach..." The velocity of the approach run in the high jump does not give the jumper much time to apply force on his take off foot Leaning backward as they plant the foot causes a blocking motion in the take off leg, and gives more time for the forces to be absorbed into the leg muscles (elastic stretch) Vertical jumping in VB, Basketball, Soccer is similar.
21	Impulse and Cadence in Sprinting, Speed Skating, and Rowing The sport is a repetition of a certain skill in a cyclic fashion throughout the race. The javelin and high jump are discrete skills in which the phases of the skill do not keep repeating The runners, rowers and skaters will apply forces to the ground, water, or ice during one or two parts of the cycle.
22	"During acceleration there are more..." During acceleration there are more cycles per minute than when they reach full speed. Runners and skaters use short, choppy strides in order to get as much force X time as they can When they reach full speed the cycle rate drops Unbalanced force produces acceleration, and the athlete can only handle so much speed Maintenance of speed is the objective
23	Impulse to Slow Down What are sports examples in which an object or the body is slowed down or stopped? Examples: catching a ball, running to a base, stopping after 100m race, landing in a pit. Small frictional forces and air resistance are applied to the golf ball during flight and upon landing. Forces applied for a period of time slow down the object
24	Landing from a jump Examples: high jump, pole vault, basketball dunk The basketball player lands stiff legged and receives a sudden force which suddenly stops his body; the time is very short But the shock is very great By flexing the hips, knees and ankles the time of application of the force is increased
25	Reducing the Force of the Impulse If an object is moving at a certain velocity and then is stopped, the change in momentum is the same no matter how long it takes to stop it. The longer it takes to stop it the smaller the force; and conversely, the shorter the time it takes to stop it the greater the force.
26	Cushioning the Force of Impact Sometimes one cannot increase the time of impact Cushioning the impact increase the time It also spreads out the force of impact over a larger area Examples of cushioning of impact: high jump pit, baseball glove, shoes with cushioned soles and heels, football pads
27	Area of Impact If a baseball is pitched and it hits the batter on the shoulder, the area of impact is small. If the same pitch his a padded shoulder, where the pad is 20 X greater area than the ball, the force per unit area is reduced 20 X. Example: Stepping on a nail vs. lying on a bed of nails
28	Marshall Arts Examples If one goes into a blow, the impact force is increased, but if one goes with the blow the time of impact is extended and the force is reduced. Example: Mohammed Ali’s rope-a-dope; Judo
29	Work Physics definition of work is: Force X distance If one applies a force through a distance they are doing work. In the vertical direction when we lift something we are working against gravity. If we move a 100# box up 6 feet, we have done 600 ft-lbs of work.
30	Work Examples: Javelin thrower, baseball pitch, lifting a barbell Isometric situation may generate a lot of force, but no work is done in a mechanical sense
31	Power This is the amount of work done per unit of time: Power = Work / time Horsepower is the ability to move 550 pounds over a distance of one foot in one second Example: A 200 lb. person walks up a flight of stairs (10 feet up) in 10 seconds and then runs up in 3 seconds. (20 lb/sec vs 67 lb/sec)
32	Power 1. Power = Work / time 2. Or Power = (force X distance) / time 3. Or Power = force X (distance/time) All three are the same, however in the 3^rd one we can simplify it to read: Power = force X velocity Power is the application of force with velocity
33	Power Lifting is not Power When one does a 1 RM, a one repetition maximum lift, it is done very slowly. A weight around 30-50% of a 1 RM done as fast as possible generates the greatest POWER
34	Sports that require POWER Some sports it is necessary to apply a great amount of force with high speed Examples: throwing events, jumping events, gymnastic events
35	Energy Means the capacity to do work Three kinds of mechanical energy Kinetic Energy KE = ½ mv² Potential Energy PE = weight X height Strain Energy SE = Force X distance
36	Kinetic Energy Is the energy exhibited by an object by virtue of its speed and mass The object has momentum (mv) and it has Kinetic Energy
37	Potential Energy Is energy stored in an object by virtue of its elevation above something, and its weight When this energy is released it transforms into kinetic energy Example: Ski Jumper is at the top of a slide (has PE) and goes down the slide picking up speed (KE)
38	Strain Energy When deformed an object has a certain ability to return to its original shape. The energy of deformation, that is, the energy left to return it to its original shape is Strain Energy This is also called elastic recoil or rebound Muscles are often stretched to produce this elastic recoil and produce greater force
39	Pole Vault An example of KE, SE and PE in one event The vaulter sprints full speed down the runway increasing KE The vaulter plants the pole in the box and converts the KE to SE After the pole reaches full bend it recoils and throws the vaulter vertically upward generate KE in order to increase PE
40	"When the vaulter is over..." When the vaulter is over the bar, the KE is near zero and PE is maximum As the vaulter falls toward the pit, the PE decreases and the KE increases As the vaulter hits the pit, the KE decreases, the SE increases, and the PE decreases slightly until the Vaulter is deepest into the pit. The pit returns the SE to KE as it recoils pushing the vaulter back up in a rebounding fashion.
41	Other Sports that Use Strain Energy Shaft of a golf club and elasticity of the ball Strings of the tennis racket and elasticity of the ball Trampoline bed with bungey cords The string of the Bow in archery
42	How Kinetic Energy and Momentum are Related All moving objects have both momentum and KE KE is a form of energy that is capable of doing work on itself, or on another object. The two components of KE are mass and velocity Since Velocity is squared (½ mv²) small increases in velocity result in large increases in KE With Momentum small increases in velocity make small increases in momentum (mv).
43	"To stop a mass that..." To stop a mass that has momentum it takes a force. That force is applied to the moving object and can do damage to the object, or to the object applying the force It is better to increase the velocity of a baseball bat than the mass of the bat. A heavier bat slows down and the KE is reduced A lighter bat moves faster thus increasing the KE
44
45	Rebound When objects impact each other the momentum of each is applied to the other object. The momentum is slowed to zero and a large reaction force is created between the two objects. This force causes a deformation in the objects. As the momentum slows to zero, the force causing the deformation accelerates the object in the direction of the applied force.
46	Ball hitting bat
47	Graph of ball hitting bat
48	Rebound One object can be moving and the other is stationary; like a bowling ball. The bowling ball has a great deal of momentum. When it strikes the pins, the pins fly away from the ball and the ball slows down somewhat. The pins exert a force to slow down the ball, and the ball exerts a force to increase the momenta of the pins. The angle at which objects collide effects the direction of the rebound.
49	Rebound and Elastic Recoil Recoil is the force at which the objects push back on each other as they move back to their original shape Elasticity is the degree to which the energy stored in the deformation of the object is returned in the recoil. The term for this is Coefficient of Elasticity
50	Elasticity If the Coefficient of Elasticity is one, the ball will bounce forever and not stop. If it is zero, it will not bounce once. Golf balls have a high degree of elasticity. Energy not returned to the ball is dissipated as heat. The Strain Energy that is released does work on the ball and increases its velocity until the two objects are not touching. This is called release velocity.
51	Elasticity The elasticity of the two objects must be taken into consideration Example: dropping a tennis ball on a hard court, or grass court, or clay, or astroturf, changes the Coefficient of Elasticity. Different athletic playing surfaces result in different recoils. Example: a soccer ball bounces differently off of artificial turf than off of natural grass.
52
53	Rebound and Temperature Heat causes the air inside of a ball to expand and increase the pressure inside the ball. This causes the ball to recoil more, or bounce higher. A pole vaulting pole when hot bends more and recoils slower than when colder. Vaulters change poles depending on the bending responses to varying temperatures.
54	Angle, Velocity, and Frictional Forces in a Rebound The rebound of a ball depends upon the angle of the path of the ball relative to the surface at impact. The amount of spin on the ball is dependent upon the friction between the ball and the surface Example: table tennis, golf drive, golf chip shot Effect of spin: backspin or top spin
55
56	Friction Depends on two things 1. The force pushing the two objects together 2. The nature of the two surfaces sliding over one another. Example: Leather soled shoes on tile floor, vs. Rubber soled shoes on tile floor. The force pushing the two objects together is the force component perpendicular to the two surfaces.
57
58	Static Friction If one pushes on the box with a small effort, the box does not slide. This is static friction (box does not move) Static Friction, F = m N, where m is the coefficient of friction, and N is the normal (perpendicular) force. m is based on the nature of the two surfaces placed together.
59	Sliding Friction As the person pushes on the box the resistance to the push increases (action-reaction) until the maximum static friction is reached. Then the box begins to slide. The coefficient of sliding friction is much lower than the coefficient of static friction. It takes less force to keep the box sliding than to get it to slide.
60	Friction If you had two boxes weighing the same, but they had different sized bases the frictional force would still be the same. In the case of the smaller box, you have more weight per square inch pressing against the floor. In the case of the larger box, you have more surface being pressed together even though the force per square inch is less.
61	Friction The nature of the types of materials that are in contact is what determines the coefficient of friction. One type of shoe will produce different m’s on different surfaces. Example: tennis court shoe on hardcourt, grass, clay, cinders, sand, or ice
62	Rolling Friction Occurs when a round object, such as a ball or wheel, rolls across a contacting or supporting surface. Example: bowling, golf, billiards, field hockey, baseball, cycling, soccer. Rolling friction is much less than sliding friction. RF based on force pushing two objects together, nature of the surfaces, diameter of the rolling object. The amount of inflation effects the friction of wheels.
63	Rolling Friction Bicycle races use narrow tire wheels that are inflated to around 120 psi (pounds per square inch). Very little surface contacts the track which minimizes rolling friction, but knobby, fat tires used in mountain biking to get traction in rough terrain are very sluggish on the track, and narrow, thin tires are useless in the rough terrain.