Welcome to Biomechanics!
Welcome to one of the most exciting parts of your PE A Level! Biomechanics might sound like a scary word, but it is simply the "physics of sport." By understanding how forces and motion work, we can help athletes run faster, jump higher, and stay injury-free. Don’t worry if you aren't a "maths person"—we will break everything down into simple steps with plenty of sporting examples!
1. Newton’s Laws of Motion
Sir Isaac Newton came up with three laws that explain how everything in the world moves. In the OCR exam, you need to define these and apply them to sports.
Newton’s First Law: The Law of Inertia
Definition: An object will remain at rest or continue at a constant velocity unless acted upon by an unbalanced force.
Simple analogy: Think of a football sitting on the penalty spot. It is "lazy" (it has inertia). It won't move until a player applies a force by kicking it. Similarly, a ball flying through the air would keep going forever if gravity and air resistance didn't pull it down!
Newton’s Second Law: The Law of Acceleration
Definition: The rate of change of momentum is proportional to the force applied and takes place in the direction in which the force is applied.
We use this formula: \( Force = mass \times acceleration \) or \( F = m \times a \).
Sporting example: If a shot-putter wants the ball to accelerate more (go faster), they must apply a greater force. If they use the same force on a lighter ball, it will accelerate even more!
Newton’s Third Law: The Law of Reaction
Definition: For every action, there is an equal and opposite reaction.
Sporting example: When a sprinter pushes down and back into the starting blocks (the action), the blocks push back up and forward against the sprinter (the reaction). This reaction force is what actually moves the runner!
Quick Review: Remember the "Big Three": 1. Inertia (staying still/moving), 2. Acceleration (\( F=ma \)), 3. Reaction (equal/opposite).
2. Understanding Force
A force is simply a push or a pull that alters the state of motion of a body. Forces can be balanced (no change in motion) or unbalanced (causes acceleration or deceleration).
Types of Force in Sport
Net Force: The resultant force acting on a body. If the net force is zero, the object stays at a constant speed or stays still.
Weight: The force of gravity acting on a mass. Calculated as: \( Weight = mass \times gravity \).
Reaction Force: The "push back" from the ground or another object (Newton’s 3rd Law).
Friction: A force that opposes motion between two surfaces. In sport, we often try to increase friction (e.g., spikes on running shoes for grip) or decrease it (e.g., wax on skis to go faster).
Air Resistance: A force that acts in the opposite direction to a body moving through the air. A cyclist "tucking" their body is trying to reduce this.
Free Body Diagrams
In your exam, you might be asked to draw a free body diagram. This is just a simple sketch using arrows to show the forces acting on an athlete at a specific moment.
- Vertical forces: Weight (downwards) and Reaction (upwards).
- Horizontal forces: Friction/Applied force (forwards) and Air Resistance (backwards).
Did you know? Friction is affected by the surface (rough or smooth), the temperature (hotter tires have more grip), and the "normal" force (how hard the surfaces are pressed together).
3. Biomechanical Calculations
Don't panic! You only need a few simple formulas. Always remember to include your units (like kg or m/s)!
1. Force: \( F = mass \times acceleration \) (Measured in Newtons, N)
2. Momentum: \( Momentum = mass \times velocity \) (Measured in kgm/s)
3. Acceleration: \( a = \frac{(final \, velocity - initial \, velocity)}{time} \) (Measured in \( m/s^2 \))
4. Weight: \( W = mass \times 9.81 \) (Gravity on Earth is approx. \( 9.81 m/s^2 \))
Common Mistake: Students often confuse Mass and Weight. Mass is how much "stuff" is in you (kg), and it never changes. Weight is a force (N) and changes if you go to the moon!
4. Centre of Mass and Stability
Centre of Mass (CoM): The point at which a body is balanced in all directions. It is where the weight of the body is concentrated.
Factors affecting Stability
To stay stable (like a rugby player in a scrum), you should follow these rules:
1. Lower the Centre of Mass: Bend your knees!
2. Widen the Base of Support: Spread your feet apart.
3. Line of Gravity: This imaginary line must fall inside your base of support to stay balanced.
4. Mass: A heavier athlete is generally more stable because they have more inertia.
5. Levers: The Body’s Simple Machines
Our bones and muscles act as levers to create movement. Every lever has three parts:
- Fulcrum (F): The pivot point (usually the joint).
- Effort (E): The force used to move the load (the muscle contraction).
- Load (L): The weight being moved (the body part or equipment).
The Three Classes of Levers
Use the mnemonic 1-2-3, F-L-E to remember which component is in the middle:
1st Class (Fulcrum in the middle): Example: Extending the neck (nodding). L - F - E.
2nd Class (Load in the middle): Example: Plantar-flexion at the ankle (standing on tiptoes). F - L - E.
3rd Class (Effort in the middle): Example: A bicep curl. F - E - L. (Note: This is the most common lever in the body!)
Mechanical Advantage
You specifically need to know about the Mechanical Advantage of a 2nd Class Lever.
Because the Effort Arm (distance from fulcrum to effort) is longer than the Load Arm, we can move a very heavy load with relatively little effort. This is why you can lift your entire body weight just by standing on your toes!
Key Takeaway: 2nd class levers = high strength but low range of motion. 3rd class levers = low strength but high speed and range of motion.
6. Using Technology to Analyse Movement
Sports scientists use cool gadgets to help athletes improve. Here are the three you need to know:
1. Limb Kinematics
Definition: The study of the movement of body parts in space.
How it works: Using 3D motion capture (like they use for video games). It records the joint angles and velocity of an athlete's movement.
Use: To check a golfer's swing or a runner's technique to prevent injury.
2. Force Plates
Definition: Metal plates built into the ground that measure Ground Reaction Forces.
How it works: An athlete jumps or runs across the plate, and it measures exactly how much force they apply and in which direction.
Use: To analyse a sprinter's start or the power of a basketballer's jump.
3. Wind Tunnels
Definition: Large fans that blow air at controlled speeds around an object.
How it works: Athletes or equipment (like bikes or helmets) are placed inside to see how air flows around them.
Use: To improve streamlining and reduce air resistance for cyclists and F1 drivers.
Quick Review Box:
- Newton's Laws: Inertia, Acceleration, Reaction.
- Levers: 1-2-3, F-L-E (Fulcrum, Load, Effort).
- Stability: Low CoM + Wide Base.
- Technology: Kinematics (motion), Force Plates (power), Wind Tunnels (aerodynamics).