Physics (9203) | Forces and terminal velocity

Welcome to Forces and Terminal Velocity!

Hello future physicist! This chapter is all about motion, stopping things, starting things, and the amazing physics behind skydiving. Forces are fundamental to how the universe works, and understanding them will unlock the secrets of why things move the way they do.

In this section, we will explore how forces combine, why resistance is important, and the step-by-step process of achieving terminal velocity. Don't worry if concepts like 'drag' or 'resultant force' seem complex right now—we'll break them down step-by-step!

1. Understanding Force and Its Effects

A Force is simply a push or a pull. Forces are measured using the unit Newton (N), named after Sir Isaac Newton.

Forces are vector quantities, meaning they have both a magnitude (size) and a direction.

The Three Main Effects of a Force

When a force acts on an object, it can cause one of three main things to happen:

1. Change in Speed (make it faster, slower, or stop).
2. Change in Direction (make it turn).
3. Change in Shape (squash it, stretch it, or twist it).

Analogy: Think of kicking a football. You change its speed, its direction, and momentarily, its shape (it compresses a tiny bit!).

2. The Resultant Force (Net Force)

Usually, objects have several forces acting on them at the same time. The Resultant Force (or Net Force) is the single force that represents the combined effect of all the individual forces.

Calculating Resultant Force

Case A: Forces acting in the same direction: You add them.
Example: Two people pushing a heavy box forward with 40 N and 60 N. Resultant Force = 40 N + 60 N = 100 N forward.

Case B: Forces acting in opposite directions: You subtract the smaller force from the larger one. The direction of the resultant force is the direction of the larger force.

Example: A tug-of-war team pulls right with 500 N, and the other pulls left with 450 N. Resultant Force = 500 N – 450 N = 50 N to the right.

Balanced vs. Unbalanced Forces – What Does Resultant Force Tell Us?

This is one of the most important concepts in motion!

1. If the Resultant Force is zero (0 N), the forces are balanced. This means:
    a) The object is stationary (not moving).
    b) OR the object is moving at a constant velocity (steady speed, steady direction).

2. If the Resultant Force is not zero (unbalanced), the object will accelerate. This means it will speed up, slow down, or change direction.

3. Resistive Forces (Friction and Drag)

Forces that oppose motion are called Resistive Forces. They always act in the opposite direction to the object’s movement.

Friction and Drag

Friction occurs when two solid surfaces rub against each other (e.g., tires on a road).
Drag is the resistance felt when an object moves through a fluid (a gas, like air, or a liquid, like water).

When an object is moving through the air, we call drag Air Resistance.

The Factors Affecting Drag (Air Resistance)

Drag is not constant; it changes based on three key factors:

1. Speed: The faster the object moves, the greater the drag force. (This is why high speed is so hard to maintain in cycling or racing.)
2. Cross-sectional Area: The bigger the front surface area that hits the fluid, the greater the drag.
3. Shape (Streamlining): A smooth, pointed, or streamlined shape reduces drag by allowing the air/water to flow easily around it. A blunt, non-streamlined shape increases drag.

Memory Trick: To reduce drag, think of making yourself smaller, smoother, and slower!

4. Weight, Mass, and Terminal Velocity

Before we discuss falling, we must be clear on two forces:

Prerequisite Check: Weight vs. Mass

Mass: This is the amount of matter in an object (measured in kilograms, kg). Mass stays the same no matter where you are (Earth, Moon, or space).

Weight: This is the force of gravity acting on the object's mass (measured in Newtons, N). Weight changes depending on the gravitational field strength (e.g., your weight on the Moon is much less than on Earth).

Weight always pulls the object downwards towards the centre of the planet.

The Journey of a Skydiver: Reaching Terminal Velocity

Terminal velocity describes the maximum, constant speed an object can reach when falling through a fluid (like air). It occurs when the forces acting on the object become balanced.

Let's look at the stages of a skydiver's fall (before the parachute opens):

Stage 1: The Start of the Fall

• Forces: Weight (downwards) is the only major force. Drag (Air Resistance, upwards) is zero or very small because the speed is zero or low.
• Resultant Force: Large and downwards.
• Motion: The skydiver accelerates rapidly due to the unbalanced force of Weight.

Stage 2: Increasing Speed, Increasing Drag

• Forces: As the skydiver speeds up, the Drag force increases quickly (remember: drag depends heavily on speed!). Weight stays constant.
• Resultant Force: Still downwards, but getting smaller because Drag is catching up to Weight.
• Motion: Acceleration slows down, but the skydiver is still speeding up.

Stage 3: Reaching Terminal Velocity

• Forces: Drag (upwards) has increased until it is exactly equal to Weight (downwards).
• Resultant Force: Zero (0 N).
• Motion: Acceleration stops. The skydiver falls at a maximum, constant speed called Terminal Velocity.

Did you know? The typical terminal velocity for a human in a spread-eagled position is about 55 m/s (around 125 mph)!

The Parachute and a New Terminal Velocity

When the skydiver pulls the cord, the process starts again, but with a massive change:

1. The parachute opens, massively increasing the cross-sectional area.
2. This causes a sudden, huge increase in Drag (Air Resistance). Drag is now much greater than Weight.
3. The Resultant Force is now large and upwards (unbalanced). The skydiver decelerates (slows down rapidly).
4. As the speed decreases, the Drag also decreases until it balances the Weight force again. A new, much lower Terminal Velocity is reached (safe for landing).

Key Takeaways

You have successfully mastered the basics of forces in motion!

The movement of any object is dictated entirely by the Resultant Force. If you see an object moving at a constant speed, you know the forces are balanced.

Remember that Drag depends on speed, shape, and area, which is how skydivers control their fall speed and, crucially, how parachutes work.

Keep practising calculating the resultant force, and soon you'll find these physics concepts simple!