Forces and Free Body Diagrams Revision Notes | GCSE AQA Higher Physics

1. What Are Forces and Why Do They Matter?

Right now, as you sit reading this, multiple forces are acting on you. Gravity pulls you down, the chair pushes you up, and if you're leaning against something, it's pushing back. You don't fall through the floor or float to the ceiling because these forces are perfectly balanced.

Forces are everywhere in physics, but they're invisible. We can only see their effects - objects speeding up, slowing down, changing direction, or staying perfectly still. This is where free body diagrams become your superpower.

Think of a free body diagram as an X-ray for forces. It strips away everything except the object you're studying and shows every single force acting on it as an arrow. The length of each arrow represents the size of the force, and the direction shows which way it's pushing or pulling.

Forces always come in pairs - if you push on a wall, the wall pushes back on you with equal strength. But when we draw free body diagrams, we focus on just one object at a time and show all the forces acting on that specific object.

2. Drawing Free Body Diagrams

A free body diagram is like a force fingerprint - it shows the unique pattern of pushes and pulls acting on your chosen object. You draw the object as a simple dot or box, then add arrows pointing away from it to represent each force.

Here's the golden rule: every arrow starts from the object and points in the direction the force is acting. If gravity pulls down on a book, you draw a downward arrow. If the table pushes up on the book, you draw an upward arrow. The arrows don't show where the force comes from - they show where it's going.

Let's take a book resting on a table. The Earth pulls the book downward with gravitational force (weight). The table pushes the book upward with a contact force (normal force). These are the only two forces acting on the book, so your free body diagram shows just two arrows - one pointing up, one pointing down.

Common mistakes happen when students include forces that aren't actually acting on the object. The kick force on a football only acts while the foot is in contact with the ball. Once it's airborne, that force disappears from the free body diagram.

3. Understanding Resultant Forces

Imagine you and a friend are pulling a rope in opposite directions. If you pull with 100N to the right and your friend pulls with 80N to the left, what's the overall effect? The rope moves right with a net force of 20N. This net effect is called the resultant force.

The resultant force is like the bottom line of a force calculation. You can have five different forces pushing and pulling on an object, but they all combine to produce one overall effect. If the resultant force is zero, the object stays at constant velocity (which includes staying still). If there's a non-zero resultant force, the object accelerates.

To find the resultant force, you add up all the forces, but you must account for direction. Forces in the same direction add together. Forces in opposite directions subtract from each other. Think of it like a tug-of-war - the winning side determines the direction of the resultant force.

Worked example: Finding Resultant Force

A box experiences three horizontal forces: 50N to the right, 30N to the right, and 60N to the left. Calculate the resultant force.

1. Let right = positive, left = negative — This makes the calculation clearer
2. +50N, +30N, -60N — Right forces are positive, left forces are negative
3. 50 + 30 + (-60) = 20N — The positive result means the net force is to the right
4. Resultant force = 20N to the right — Always include the direction in your final answer

Answer: 20N to the right

This method works for any number of forces in any direction. The key is being consistent with your positive and negative directions throughout the calculation.

4. Balanced vs Unbalanced Forces

When forces are perfectly balanced, they cancel each other out completely. The resultant force is zero, and the object either stays still or continues moving at constant velocity. This is like a perfectly even tug-of-war where nobody moves.

A book on a table shows balanced forces perfectly. Weight pulls down with, say, 10N. The normal force from the table pushes up with exactly 10N. These forces are equal and opposite, so the resultant force is zero. The book stays at rest.

Unbalanced forces create acceleration. If you push a trolley with 20N and friction opposes with 5N, the resultant force is 15N forward. The trolley speeds up in the direction of the resultant force. The bigger the resultant force, the greater the acceleration.

This is where many students get confused. They think that if something is moving, there must be unbalanced forces. Actually, unbalanced forces cause changes in motion, not motion itself. A car cruising at steady speed on a motorway has balanced forces - the engine force equals the total resistance forces.

5. Real-World Applications

Free body diagrams help us understand everything from why planes fly to how buildings stand up. Consider a plane in level flight at constant speed. Four main forces act on it: weight (down), lift (up), thrust (forward), and drag (backward). For steady flight, these forces must be perfectly balanced.

When a skydiver jumps from a plane, initially only weight acts downward and they accelerate rapidly. As speed increases, air resistance builds up, acting upward. Eventually, air resistance equals weight - the forces balance, and the skydiver reaches terminal velocity, falling at constant speed.

In engineering, free body diagrams are essential for designing safe structures. Engineers must account for all forces acting on a bridge - the weight of the bridge itself, traffic loads, wind forces, and thermal expansion forces. If they miss a force or miscalculate its size, the bridge could fail catastrophically.

Sports provide excellent examples too. When a footballer kicks a ball, the free body diagram changes throughout the motion. During contact with the foot, there's a large contact force. Once airborne, only weight and air resistance act on the ball. Understanding these force changes helps players predict ball trajectories.

6. Exam Technique and Common Mistakes

Examiners love testing free body diagrams because they reveal whether you truly understand forces. The most common mistake is including forces that don't actually act on the chosen object. Remember: if you're drawing a free body diagram for a book on a table, don't include the weight of the table or the force the book exerts on the table.

Another frequent error is confusing the direction of forces. Normal force always acts perpendicular to the surface, pushing away from it. Friction always opposes motion or attempted motion. Weight always acts vertically downward toward the Earth's centre, regardless of the object's orientation.

When calculating resultant forces, students often forget to include direction in their final answer. A resultant force of 15N is incomplete - you must specify 15N to the right, or 15N upward, or whatever the direction is. The direction tells you which way the object will accelerate.

In exam questions asking you to 'describe' forces, focus on what each force is and which direction it acts. For 'explain' questions about motion, link the resultant force to the acceleration - zero resultant force means constant velocity, non-zero resultant force means acceleration in the direction of the resultant force.