5.1 - Principles of Force
Key definitions in forces and motion
Understanding forces and motion begins with some fundamental concepts that explain how objects behave when pushed or pulled.
Force
Force is a push or pull that acts on an object, capable of changing its motion or shape. It is measured in Newtons (N).
Inertia
Inertia is the tendency of an object to resist changes to its current state of motion, whether it is at rest or moving steadily.
Mass
Mass is the amount of matter contained in an object, unaffected by its size or external forces. It is measured in kilograms (kg).
Acceleration
Acceleration is the rate at which an object's velocity changes over time. It is measured in metres per second squared (m/s2).
Newton's first law of motion
Newton's laws describe how forces influence motion, starting with the basic principle of inertia.
Newton's first law, also known as the law of inertia, states that an object at rest will remain at rest, and an object in motion will continue moving at a constant speed in a straight line, unless acted upon by an external force.
Implications of the first law
This law explains why objects remain at rest until a force acts on them, and why moving objects continue their motion unless acted upon by another force.
Newton's second law of motion
Building on the first law, the second law connects force directly to changes in motion.
Newton's second law states that an object will accelerate when acted upon by an external force. The acceleration of the object is proportional to this force and is in the direction by which the force acts.
Formula for force
Where:
- F = Force (N)
- m = Mass (kg)
- a = Acceleration (m/s2)
Worked example - Calculating force in a sprint start
A sprinter with a mass of 75 kg accelerates from rest at a rate of 4.5 m/s2 when exploding out of the starting blocks. Calculate the force applied.
Step 1: Identify the values
- Mass (m) = 75 kg
- Acceleration (a) = 4.5 m/s2
Step 2: Apply the formula
Step 3: Perform the calculation
Step 4: Interpretation
The sprinter applies 337.5 N of force to achieve this acceleration, demonstrating how the second law applies to explosive movements in athletics.
Newton's third law of motion
The third law addresses interactions between objects, emphasising that forces always come in pairs.
Newton's third law states that for every action force, there is an equal and opposite reaction force. When one object exerts a force on another, the second object exerts a force of the same magnitude but in the opposite direction on the first.
Examples in sports
This law is evident in activities like swimming, where a swimmer pushes water backwards (action) and the water pushes the swimmer forwards (reaction). Similarly, when a high jumper pushes down on the ground during takeoff, the ground pushes back upwards with equal force, propelling the athlete into the air.
Effects of force on objects
Once a force is applied, it can produce various outcomes depending on the situation.
Main effects of force
- Starting motion - A force can make a stationary object begin moving.
- Changing direction - A force can alter the path of a moving object.
- Increasing speed - A force can accelerate an object.
- Decreasing speed - A force can decelerate an object.
- Altering shape - A force can deform an object.
Types of forces in sports
Forces in physical education can be classified based on how they are applied.
Push and pull forces
- Push forces - These involve thrusting an object away, such as striking a cricket ball with a bat or tackling an opponent in rugby.
- Pull forces - These involve drawing an object towards you, like rowing a boat by pulling oars through water or performing a chin-up by pulling your body upwards.
Forces can be applied explosively for power (e.g., a weightlifter's heavy snatch) or in a controlled way for precision (e.g., a golfer's delicate chip shot or a badminton player's spin serve).
Forces and acceleration in sports scenarios
In real sports situations, multiple forces act simultaneously, affecting an object's motion through positive or negative acceleration.
How forces interact in sports
When a basketball is thrown towards the hoop, it follows a path influenced by the initial force from the player's hand.
External forces then come into play:
- Gravity pulls the ball downwards.
- Air resistance opposes the motion, gradually decelerating the ball.
- If a defender blocks it, this applies an opposing force to change its direction or stop it.
- Contact with the backboard can redirect the ball towards the basket.
Positive and negative acceleration
- Positive acceleration - Occurs when an object speeds up, like a volleyball accelerating after being spiked by a player.
- Negative acceleration - Happens when an object slows down, such as the same volleyball decelerating due to gravity and air resistance after reaching peak speed.