Physical Education | Forces and movement

Welcome to Movement Analysis: Forces and Movement!

Hello future biomechanics expert! This chapter is the foundation of understanding why athletes move the way they do. Don't worry if words like "force" and "momentum" sound like physics class—we're going to break them down into simple, practical concepts that explain how you jump higher, run faster, and throw further.

Understanding forces is crucial because it helps us analyze and improve performance, prevent injury, and select the best techniques in sports! Let’s dive in!

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I. What Exactly is Force?

In simple terms, a Force is a push or a pull that attempts to change the state of motion of an object.

Key Characteristics of Force

• Magnitude: How strong the push or pull is (e.g., 50 Newtons).
• Direction: The path the force is traveling along (e.g., upwards, backwards).
• Point of Application: Where the force acts on the object.

Because force has both magnitude (size) and direction, it is a vector quantity.

Memory Aid: Think of a V-sign for Vector – it needs two things: Size (fingers up) and Direction (pointing).

Unit of Force: The standard unit for force is the Newton (N).

The Effects of Force

When a force acts on an object (or an athlete), it can cause one or more of the following effects:

1. Start Motion: A swimmer pushing off the starting block.
2. Change Speed/Acceleration: A cyclist pedaling harder to speed up.
3. Change Direction: A soccer player kicking the ball to curve around a defender.
4. Stop Motion/Deceleration: Applying the brakes on a bicycle, or catching a ball.
5. Change Shape: Hitting a tennis ball, causing a brief deformation of the ball.

II. Types of Forces in Sport

Forces acting on an athlete or equipment can be broadly divided into two categories:

1. Internal Forces

These forces are generated within the body (by the muscles, tendons, and bones). They cause the body segments to move relative to each other (e.g., flexing the elbow).

• Example: The force generated by your quadriceps muscle to straighten your knee during a kick.

2. External Forces

These forces act on the body from the outside. They are responsible for changing the overall motion of the body (e.g., making you jump, fall, or slow down).

We must understand four major external forces:

A. Weight (Gravity)

This is the force of attraction between the Earth and the object/athlete. It always pulls the object vertically downwards.

• Important Distinction: In PE, we often use Weight, which is a force (measured in Newtons, N), rather than Mass, which is the amount of matter (measured in kilograms, kg).
• Example: Gravity pulls a basketball back towards the ground after it has been shot.

B. Friction

Friction is the force that opposes motion when two surfaces are in contact. It acts parallel to the surfaces.

• Beneficial Friction: We need friction to start or stop motion. Examples: Cleats on a football field provide grip; chalk on a gymnast’s hands prevents slipping.
• Resisting Friction: Too much friction can slow us down. Example: Ice skaters glide because they minimize friction with the blade.

C. Air Resistance (Drag)

Air resistance is a force that opposes motion through the air. It increases significantly as speed increases.

• Minimizing Drag: Athletes try to reduce drag by adopting a streamlined shape (e.g., aerodynamic helmets in cycling, tuck position in skiing).
• Maximizing Drag: Sometimes, drag is useful, such as when using a parachute for braking or in sports like badminton where the shuttlecock's drag makes it slow down rapidly.

D. Ground Reaction Force (GRF)

This is the force exerted by the ground on the athlete's body. It is a direct result of Newton’s Third Law (explained next!).

• Example: When you push down on the ground to jump (Action Force), the ground pushes back up on you with an equal and opposite force (Ground Reaction Force), propelling you into the air.

III. Newton’s Laws of Motion: The Foundation of Biomechanics

Sir Isaac Newton's three laws explain every movement in sports. Master these three laws, and you will understand almost all biomechanical principles!

1. Newton’s First Law: The Law of Inertia

An object will remain at rest, or in uniform motion in a straight line, unless acted upon by a net external force.

Concept of Inertia

Inertia is the resistance an object has to a change in its state of motion (starting, stopping, or changing direction).

• The greater the Mass of an object, the greater its Inertia.

Analogy: Imagine trying to push a stationary empty basketball and then trying to push a stationary car. The car has far greater mass, meaning it has greater inertia and is much harder to start moving.

Sport Application: A sumo wrestler is harder to move than a lightweight gymnast because the wrestler has greater mass and therefore greater inertia.

2. Newton’s Second Law: The Law of Acceleration

The acceleration of an object is directly proportional to the net force causing it, is in the same direction as the net force, and is inversely proportional to the mass of the object.

In simpler terms: The harder you push (Force), the faster it speeds up (Acceleration). But, the heavier it is (Mass), the less it speeds up.

This law is summarized by the most famous biomechanics formula:

Force = Mass × Acceleration

\[ F = ma \]

Implications for Sport

1. To Maximize Acceleration (a): An athlete must Maximize Force (F) (e.g., throwing a shot put harder) or Minimize Mass (m) (e.g., lightweight racing equipment).
2. Catching/Stopping: To stop a fast-moving object (high acceleration change), you need a large force, or you must increase the time over which the force is applied (e.g., moving your hands backwards when catching a baseball to increase time and reduce the impact force).

Did you know? This law also introduces the concept of Momentum. Momentum (\(p\)) is the quantity of motion an object possesses: \[ p = mv \] (Momentum = Mass × Velocity). The larger the momentum, the harder it is to stop!

3. Newton’s Third Law: The Law of Action-Reaction

For every action, there is an equal and opposite reaction.

Forces never act alone; they always occur in pairs. If Object A pushes on Object B (Action), then Object B pushes back on Object A (Reaction) with the exact same magnitude but in the opposite direction.

Application: The Ground Reaction Force (GRF)

This law is fundamental for walking, running, and jumping:

• Action: Your foot pushes backwards and downwards onto the track.
• Reaction (GRF): The track pushes forwards and upwards on your foot.

Crucial Point: Sprinters must maximize their forward GRF by pushing forcefully backwards against the blocks or track. The more force they put into the ground, the more force the ground gives back to propel them forward!

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IV. Applying Force Principles in Movement

Understanding how forces combine helps us analyze movement effectiveness.

Resultant Force (Net Force)

In any situation, multiple forces are usually acting on an object (gravity, friction, muscle force, etc.). The Resultant Force (or Net Force) is the single force that represents the combined effect of all these forces.

• If the forces are balanced (Resultant Force = 0), the object is in equilibrium (it stays still or moves at a constant velocity). (Law 1)
• If the forces are unbalanced (Resultant Force ≠ 0), the object will accelerate in the direction of the resultant force. (Law 2)

Step-by-Step: Maximizing Throwing Velocity

To throw a javelin as fast as possible, we apply the principles of force:

1. Increase Force (Law 2): Use the largest possible muscle groups (legs, core, back) sequentially and quickly (kinetic chain).
2. Maximize Acceleration Distance (Law 2): Extend the path over which the force is applied. A long backswing allows the athlete to apply force over a greater distance, maximizing the final velocity.
3. Use GRF (Law 3): Push hard against the ground with the plant foot just before the throw to generate a large opposing force for maximum power transfer.
4. Minimize Air Resistance: Ensure the javelin is released at the optimal angle and position to reduce drag during flight.

V. Key Takeaways and Final Encouragement

You’ve covered the core concepts of forces in PE! Remember, biomechanics isn't just theory; it’s the science of making movement efficient and powerful.

• Force is a vector that causes acceleration or changes direction.
• We use internal forces to move our limbs, and external forces (like GRF) to move our whole body.
• Newton's Laws tell us that to improve performance, we must either increase the force applied (\(F=ma\)) or decrease the mass we are moving.
• Always consider the Ground Reaction Force—it's your source of power for sprinting and jumping!

Keep practicing applying these laws to different sports (e.g., hitting a golf ball, vaulting, catching) and you’ll find that movement analysis becomes much easier! Great work!