Dynamics: Understanding Motion and Forces

Speed and Motion

Speed is simply how fast something travels, calculated by dividing distance by time (measured in metres per second, m/s). You'll encounter two types: average speed over longer journeys and instantaneous speed at any given moment.

Speed-time graphs tell brilliant stories about motion. A horizontal line shows constant speed, whilst an upward slope indicates acceleration (speeding up) and a downward slope shows deceleration (slowing down). The steeper the line, the greater the change in speed.

Quick Tip: The area under a speed-time graph gives you the total distance travelled - super useful for exam questions!

Understanding these basics sets you up perfectly for tackling more complex motion problems. Speed graphs are your visual tool for making sense of any moving object.

Acceleration Calculations

Acceleration measures how quickly speed changes, calculated using: acceleration = (final velocity - initial velocity) ÷ time. The units are metres per second squared $m/s²$, which might look odd but makes perfect sense.

The key equation v = u + at becomes your best friend in physics. Here, v is final velocity, u is initial velocity, a is acceleration, and t is time. You can rearrange this formula to find any missing variable.

In acceleration experiments, you'll use light gates to measure precise timings. The card's width divided by the time gives you velocity at each gate, then you apply the acceleration formula using the time between gates.

Exam Success: Remember that the gradient of a speed-time graph directly gives you the acceleration - no calculation needed!

Newton's Three Laws

Newton's First Law explains that objects hate changing what they're doing. Things at rest stay put, and moving objects keep moving, unless an unbalanced force acts on them. Balanced forces mean no change in motion.

Newton's Second Law gives us the famous equation: Force = mass × acceleration $F = ma$. This tells you exactly how much force you need to accelerate any object. Heavier objects need more force to achieve the same acceleration.

Newton's Third Law states that every action creates an equal and opposite reaction. When you push against a wall, it pushes back with exactly the same force. This isn't just theory - it's why rockets work and why walking is possible.

Real-World Connection: These laws explain everything from why you wear seatbelts to how athletes push off starting blocks!

Weight, Mass and Terminal Velocity

Don't confuse mass (amount of matter, measured in kg) with weight (force due to gravity, measured in Newtons). Weight equals mass times gravitational field strength $W = mg$, where Earth's gravity pulls at roughly 10 N/kg.

Terminal velocity happens when falling objects reach maximum speed. Initially, weight pulls down whilst air resistance pushes up. As speed increases, air resistance grows stronger until both forces balance perfectly.

At terminal velocity, acceleration becomes zero because the forces are balanced. This explains why parachutists don't just keep getting faster and faster - they reach a steady falling speed.

Key Insight: Terminal velocity depends on the object's shape and size - that's why feathers fall slower than stones in air!