Summary of Forces, Motion, and Pressure

## Introduction Forces and pressure are fundamental ideas in physics that explain how objects move, stop, and press on each other. This guide covers key words, simple equations, everyday examples, and practical investigations you can try. Understanding these concepts helps with problems from riding a bike to inflating a tyre. ## Key words > **Acceleration:** How quickly speed increases. > **Deceleration:** How quickly your speed decreases. > **Speed:** How much distance is covered in a given time. > **Mass:** The amount of stuff in an object. Measured in kilograms (kg). > **Force:** A push or a pull that can change an object's motion. Measured in newtons (N). > **Friction:** A contact force that opposes motion between surfaces. > **Drag (Air resistance):** The frictional force caused by a fluid (a liquid or gas) on a moving object. > **Pressure:** The force per unit area exerted on a surface. Measured in pascals (Pa). > **Fluid:** Materials where particles can move freely (liquids and gases). ## Equations to remember - Speed: $$\text{speed} = \dfrac{\text{distance}}{\text{time}}$$ Example: If distance = 20 m and time = 4 s then speed = $\dfrac{20}{4} = 5$ m/s. - Force: $$F = m \times a$$ Where $F$ is force in newtons, $m$ is mass in kilograms, $a$ is acceleration in m/s$^2$. - Pressure: $$\text{pressure} = \dfrac{\text{force}}{\text{area}}$$ Or in symbols: $$P = \dfrac{F}{A}$$ Example: A force of 10 N applied to an area of 2 m$^2$ gives $P = \dfrac{10}{2} = 5$ Pa. ## Speed, friction and drag ### What is speed? - Speed tells you how fast something moves: distance covered per unit time. - Units are usually m/s or km/h. ### Friction - Friction is a contact force acting opposite to motion between two surfaces. - Rougher surfaces produce more friction than smoother ones. - Friction converts kinetic energy into thermal energy (heat). Practical example: Sliding a book across carpet requires more force than across a smooth table because carpet produces higher friction. ### Drag (air resistance) - Drag is a type of friction from a fluid (air or liquid). - Faster objects experience larger drag forces. Real-world application: Streamlined shapes (cars, airplanes) reduce drag, which saves fuel and increases speed. Fun fact: The shape of a bird's wings reduces drag and increases lift, allowing efficient flight. ## Distance–time graphs - Horizontal flat line: object is stationary (speed = 0). - Straight sloped line: object moves at constant speed; steeper slope means higher speed. - Curved line: object is accelerating or decelerating (speed changing). Example: A straight line from (0 s, 0 m) to (5 s, 10 m) shows constant speed $\dfrac{10}{5} = 2$ m/s. ## Investigating friction (Working scientifically) 1. Put an object (like a wooden block) on different surfaces (sandpaper, carpet, smooth tile). 2. Pull the block with a newton meter until it just starts moving. 3. Record the reading on the newton meter; this is the frictional force. 4. Repeat and compare results. Observations you should expect: - Rough surfaces give larger newton meter readings. - Lubricants (oil) reduce friction between surfaces. ## Pressure and gases ### Pressure basics - Pressure depends on how large a force is and how small the area is. - Smaller area with the same force gives greater pressure. Example: A knife cuts easily because its sharp edge has a very small area, producing high pressure for the same force. ### Pressure in gases - Gas pressure comes from gas molecules colliding with surfaces. - You can increase gas pressure by: - Increasing temperature (molecules move faster), - Reducing volume (molecules collide more often), - Adding more gas particles (more collisions). Did you know that atmospheric pressure is lower at high altitude because there are fewer air particles above you pressing down? Practical application: Bicycle tyres are pumped to a certain pressure to support the rider; too low pressure makes pedalling harder