1	Chapter 2: Basics of Mechanics
2	Gravity . Pull of the earth on body and vice versa Newton’s law of gravity: F a M₁M₂ -------- d²
3	"2." 2. Body weight – earth’s gravitational force on the body – depends on how much mass of the earth and the body. The closer to the center of the earth the more the body weighs, and conversely the farther from the center of the earth the less the body weighs. 3. Mass - substance or matter – inertia The mass stays the same throughout the universe
4	Inertia Newton’s First Law of Motion Law of Inertia: an object at rest will stay at rest and an object in motion will stay in motion at the same velocity in a straight line for ever unless acted upon by an unbalanced force. On earth this seldom occurs because of other forces; like gravity, air resistance, friction
5	Examples of Inertia in Sport 300 pound football lineman gets up and runs at the snap of the ball 180 pound half back tries to block a 300 pound defensive tackle Sumo Wrestlers The bigger they are the weaker they are, relatively 200 m runner going around a curve F = ma
6	"In which sports does a..." In which sports does a person: Change the velocity of their body? Change the direction of the movements? Change the velocity of an object?
7	Speed, acceleration, and velocity Speed is the rate of change of distance The change in distance in a certain amount of time: speed = Dd / Dt Speed is velocity without direction Velocity is speed with a direction; like 20 ft/second North. Acceleration is the rate of change of speed: a = Dv / Dt ; where v = velocity
8	Average Speed and Acceleration A driver covers 200 miles in 4 hours. What is her average speed? Was she going this speed the entire time? How do we know that?
9	Speed and Velocity Speed has just magnitude; like the wind is blowing at 5 mph. Velocity has magnitude and direction; like the wind is blowing at 5 mph from the North.
10	Vectors A vector is represented by an arrow The vector has four important characteristics: Magnitude Direction Point of application Line of action Vectors are velocities, accelerations, and forces
11	Vector diagram
12	Vector diagram
13	Vector diagram Parallelogram law
14	Vector diagram
15	Acceleration Acceleration is the rate of change of speed: a = Dv / Dt ; where v = velocity Acceleration varies usually during an activity. Example: sprinting, drag racing. Acceleration of gravity does not vary. It is a constant or uniform acceleration of: 32 ft / second² or 9.8 m / s²
16
17	Center of Gravity Pull of gravity centered on a theoretical point Body made up of parts of varying density/mass CG changes with distribution of this mass CG changes with implements in the hands, i.e. barbells
18
19
20
21
22
23	Center of Gravity in Flight Diver - gravitational pull concentrated on CG a. body position and body mass distribution has no effect on CG during flight b. Path is established at the moment of take-off c. Athlete can change body position around CG but path is not changed in flight d. PV, high jumpers, divers, long jumpers, change body position to take advantage of CG path
24
25	Variations in density Density is weight per unit of volume: Metric: 1.1 grams / cm³of muscle 0.90 gram / cm³of fat Muscle is denser than fat Muscle sinks in water, but fat floats An object that floats displaces its weight in water and still has more volume Water’s density is 1.0 grams / cm³
26	Density Variations and Center of Gravity Body shape and movement of the segments of the body cause changes in CG Variations in the volume and mass of the individual segments causes differences in the location of the CG Body builders who work the upper body more increase mass in their chest, shoulders and arms shift their CG upwards. Implements held in the hands of the athlete shift the CG of the combined CG of the athlete and the implement.
27
28	Reaction Forces of the Earth Acts on Athletes Newton described the law of gravitation Weight is just the manifestation of the gravitational attraction for the mass of the object Weight is a force. The force of the gravitational pull of the earth. The earth pushes back with an equal and opposite force to the direction of the weight. The standing person is in equilibrium with the earth. The force pushing up against the body is the Ground Reaction Force (GRF).
29	Law of Action-Reaction This the law of action-reaction. If you push against an object the object pushes back with an equal and opposite force. The Ground Reaction Force can be greater or less than the athletes weight. If the athlete stands motionless, the GRF is equal and opposite the weight of the athlete. If the athlete tries to push downward so that he/she can jump upward the GRF exceeds the body weight of the athlete and the athlete is propelled into the air.
30
31	GRF and Friction The amount of GRF also influences the amount of friction between the shoes and the earth. Traction is another name for this frictional force. The amount of frictional force is equivalent to the nature of the two surfaces and the force pushing them together. Different shoes for different surfaces In order to turn in skiing the frictional force is best when it is least. The skier unweights, or drops down as he/she turns. This reduces the friction between the skis and the snow and the skis are then turned easier
32	Projectiles When an object is in the air there is no external force acting on it except gravity. In some conditions, air resistance, too. No other GRF is acting The path of the object is predetermined from: Velocity of take-off Angle of take-off Height of CG Horizontal distance from mark at release
33
34	Forces and Force Vectors Internal and external forces Internal forces cause motion to be initiated within the body, by muscles and tendons applying force to the bones. An external force refers to a force not generated from within the object or the body that acts on the body A force is a push or a pull that tends to change the state of rest of an object or the state of motion of the object. Example: Weight lifter lifting a barbell. If it is too heavy there is no motion of the bar. But force has been exhibited. If the force is greater than the weight of the object it begins to move. Forces cause motion, or change of motion.
35
36	Types of Motion Linear Translation – All of the points on an object go in the same direction and the same distance Rectilinear Curvilinear Circular
37
38
39
40
41	Angular Rotation All of the points on an object go through the same angle but NOT the same distance also called spinning, swinging, circling, turning, rolling, pirouetting, somersaulting, and twisting
42
43
44
45
46