Pearson Edexcel GCSE (9-1) Physics revision notes
Motion and forces Revision Notes
Review revision notes for Motion and forces in Edexcel Physics.
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Motion and forces
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Understanding Distance-Time Graphs and Speed Revision Notes
Interpreting Distance-Time Graphs
A distance-time graph plots distance (y-axis) against time (x-axis). The shape of the line reveals information about an object's motion. A straight line indicates constant speed, while a horizontal line signifies the object is stationary. A curved line means the speed is changing, indicating acceleration or deceleration. The steeper the gradient, the faster the speed.
Calculating Speed from the Gradient
The speed of an object is determined by the gradient of its distance-time graph. Gradient is calculated as the change in distance (rise) divided by the change in time (run). Mathematically, speed = (change in distance) / (change in time). For a non-linear graph, the instantaneous speed at any point can be found by calculating the gradient of the tangent to the curve at that point.
Common Graph Features
A line sloping upwards from left to right implies movement away from the starting point. A line sloping downwards from left to right implies movement back towards the starting point. If the line passes through the origin (0,0), it means the object started at the reference point at time zero. A sudden vertical line is impossible in reality as it would imply infinite speed.
Understanding Acceleration Revision Notes
Defining Acceleration
Acceleration quantifies the rate at which an object's velocity changes. It is specifically a change in velocity, not just speed. Velocity, unlike speed, includes direction. Therefore, an object can accelerate by changing its speed, changing its direction, or both. The standard unit for acceleration is metres per second squared (m/s²).
The Acceleration Equation
The formula used to calculate acceleration is given by: acceleration = (final velocity - initial velocity) / time taken. This is often written in symbols as a = (v - u) / t. Here, 'a' represents acceleration, 'v' is the final velocity, 'u' is the initial velocity, and 't' is time. 'v - u' represents the change in velocity. It's crucial to ensure all units are consistent before performing calculations: velocities should be in m/s and time in s to yield acceleration in m/s².
Interpreting Positive and Negative Acceleration
A positive value for acceleration means that the object's velocity is increasing in the direction of motion. This is typically what we associate with 'speeding up'. A negative value for acceleration, also known as deceleration or retardation, means the object's velocity is decreasing, or it is accelerating in the opposite direction to its current motion (slowing down). For example, if a car is moving forward and accelerates at -2 m/s², it is slowing down.
Understanding the Equation of Motion: v² - u² = 2ax Revision Notes
The SUVAT Equation: v² - u² = 2ax
This equation relates final velocity (v), initial velocity (u), acceleration (a), and displacement (x). It's one of the key kinematic equations used to analyse motion under constant acceleration, particularly when time is not given or required. Remember that 'x' in this context refers to displacement rather than distance, although for motion in a straight line without a change in direction, they can be numerically equal.
Applying the Equation and Units
To use v² - u² = 2ax, ensure all values are in SI units: velocities in m/s, acceleration in m/s², and displacement in m. When calculating, carefully substitute the known values and rearrange the equation to find the unknown. For example, if you need to find acceleration, it becomes a = (v² - u²) / (2x). Always double-check your units throughout the calculation process.
Interpreting Results and Common Pitfalls
A negative value for 'a' indicates deceleration (slowing down). If the initial velocity 'u' is 0, the equation simplifies to v² = 2ax, indicating motion starting from rest. Be careful with squaring velocities and correctly handling square roots when finding v or u. Always consider the physical meaning of your answer; for instance, a car cannot accelerate to an impossibly high speed over a short distance.
Understanding Momentum Revision Notes
Definition of Momentum
Momentum is a measure of the 'quantity of motion' an object has. It depends on an object's mass and its velocity. The greater the mass or the greater the velocity, the greater the momentum. It is a vector quantity, so direction is important.
Momentum Equation and Units
Momentum (p) is calculated by multiplying an object's mass (m) by its velocity (v). The equation is: p = m × v. The standard unit for mass is kilograms (kg), and for velocity is metres per second (m/s). Therefore, the unit for momentum is kilogram metre per second (kg m/s).
Calculating Momentum
To calculate momentum, ensure all values are in their standard SI units: mass in kg and velocity in m/s. If units are provided differently (e.g., mass in grams or velocity in km/h), they must first be converted before applying the formula p = m × v. For example, 1 kg = 1000 g, and 1 m/s = 3.6 km/h (or 1 km/h = 1000/3600 m/s).
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