Mechanics Basics

Mechanics is the branch of physics that studies motion and forces acting on objects. It's essentially about answering questions like "How fast will this fall?" or "What force do I need to move this?"

The key to mastering mechanics is learning how to turn messy real-world situations into clean mathematical problems you can actually solve. This process is called mathematical modelling.

Quick Tip: Think of mechanics as the bridge between maths and the physical world around you!

Mathematical Modelling in Mechanics

Mathematical modelling transforms complex real-world problems into simpler versions you can solve with equations. You observe what's happening, make smart assumptions, then test if your model works.

The modelling process follows a clear cycle: identify the real-world problem, set up your mathematical model with assumptions and variables, solve it, then check if your answer makes sense. If not, you refine your assumptions and try again.

Common modelling assumptions include treating objects as particles (ignoring their size), assuming no air resistance, and using gravity as 10 m/s². These simplifications make calculations manageable whilst still giving useful results.

Key models you'll use include particles (negligible dimensions), rods (length matters but not width), laminas (flat objects with area but no thickness), and uniform bodies (mass spread evenly). Each model strips away unnecessary complexity.

Remember: Every assumption you make should simplify the problem without losing the essential physics!

Quantities and Units

SI units form the foundation of all mechanics calculations. You'll work with mass in kilograms (kg), length in metres (m), and time in seconds (s). Get these wrong and your entire answer becomes meaningless.

Force is measured in Newtons (N), which equals kg⋅m⋅s⁻². Velocity uses m/s, whilst acceleration uses m/s². These compound units show how different quantities relate to each other.

Important forces include weight (gravitational pull downwards), normal reaction (surface pushing back), tension (pulling force in strings), and air resistance (opposing motion). Understanding what each force does helps you set up equations correctly.

Pro Tip: Always check your units match on both sides of an equation - it's the easiest way to spot mistakes!

Displacement and Velocity-Time Graphs

Displacement-time graphs show position changing over time. The gradient at any point gives you the velocity at that moment - steeper slopes mean faster movement.

On velocity-time graphs, the gradient represents acceleration. Horizontal lines show constant velocity (zero acceleration), whilst sloped lines indicate changing speed. The area under a velocity-time graph gives you the total displacement.

These graphs make complex motion easy to visualise. You can instantly see when objects speed up, slow down, or change direction just by looking at the shape of the line.

Average velocity equals displacement divided by time, whilst average speed uses total distance travelled. They're different because velocity considers direction, but speed doesn't.

Visual Learner Alert: Graphs often make motion problems much clearer than equations alone!

Constant Acceleration Formulae (SUVAT)

The SUVAT equations are your toolkit for solving constant acceleration problems. Each letter represents a key variable: s (displacement), u (initial velocity), v (final velocity), a (acceleration), t (time).

You've got five powerful equations to choose from: v = u + at, s = $u+v$t/2, s = ut + ½at², s = vt - ½at², and v² = u² + 2as. Pick the one that uses the variables you know and finds what you need.

The trick is identifying which three variables you have, which one you want to find, then selecting the right equation. Most problems give you three values and ask for a fourth.

Strategy Tip: Write down s, u, v, a, t and fill in what you know - the right equation will become obvious!

Vertical Motion

Vertical motion problems use constant acceleration due to gravity $a = -9.8 m/s²$. The negative sign shows gravity pulls downwards, opposing upward motion.

When objects are thrown upwards, they slow down until reaching maximum height $where v = 0$, then speed up as they fall back down. The motion is symmetrical - time up equals time down from the same height.

For projectiles launched from height, calculate the motion in stages: find maximum height above launch point, then add the initial height. When finding impact times, set displacement to the negative of starting height.

Remember to round final answers to 2-3 significant figures since gravity is approximated. Speed is always positive, but velocity can be negative (indicating downward direction).

Height Hack: Always define your coordinate system clearly - is upward positive or negative?

Forces and Motion

Newton's Second Law $F = ma$ connects forces to motion. No resultant force means objects either stay still or continue moving at constant velocity - forces are balanced.

When forces don't balance, you get a resultant force that causes acceleration. The bigger the unbalanced force, the greater the acceleration (assuming mass stays constant).

Force diagrams help visualise what's happening. Draw all forces acting on an object, then find the resultant by adding them as vectors. This resultant force determines the acceleration using F = ma.

Force Focus: Always consider every force acting on your object - missing one can completely change your answer!

Connected Particles

Connected particles move together, so they share the same acceleration. This key insight simplifies problems involving cars with trailers, masses on pulleys, or objects linked by strings.

For pulley systems, tension is the same throughout an inextensible string. The pulley changes force direction but not magnitude. Write separate F = ma equations for each mass, then solve simultaneously.

When particles are connected by strings or rigid links, their accelerations must be equal. Use this constraint along with F = ma for each particle to find unknown forces or accelerations.

The force on a pulley equals twice the tension in the string passing over it. This comes from the string pulling on both sides of the pulley with equal force.

Connection Key: Same string or rigid connection = same acceleration for all connected objects!

Variable Acceleration

SUVAT equations only work when acceleration is constant. For changing acceleration, you need calculus to connect displacement, velocity, and acceleration.

Velocity is the rate of change of displacement: v = ds/dt. Acceleration is the rate of change of velocity: a = dv/dt. You can also write acceleration as the second derivative of displacement: a = d²s/dt².

To find velocity from acceleration, integrate: v = ∫a dt. To find displacement from velocity, integrate again: s = ∫v dt. Don't forget to add constants of integration and use initial conditions.

Variable acceleration problems require you to work with functions rather than simple numbers. Set up your differential equation, integrate step by step, then apply boundary conditions to find specific solutions.

Calculus Connection: Integration undoes differentiation - use it to work backwards from acceleration to velocity to displacement!