Topic Two (1.2): Velocity and Acceleration
Introduction
In this topic, you will learn definitions and distinctions between scalar and vector quantities, distance (scalar) and displacement (vector), speed (scalar) and velocity (vector). Scalar quantities do not have a direction, whereas vector quantities do. So, you could say that a car has a speed of 15 m/s, or that it has a velocity of
15 m/s to the right.
A speedometer in a car does not measure speed. It measures how many times a wheel rotates in a second and calculates speed. The same is true of acceleration, it cannot be measured directly but instead must be calculated. This topic revises and extends the basic ideas learned at (I)GCSE. Velocity is speed with direction and can be calculated from a distance-time (DT) graph. Acceleration is a vector also, and it is calculated from a velocity-time (VT) graph.
Reading
Edexcel International AS/A Level Physics Student Book 1 pages 8 to 11
Tasks
Complete the following:
- Read pages 8 to 11 of the textbook.
WATCH – Scalar and Vector Measurements (YouTube: Ninety East) Links to an external site. Focus on the difference between distance and displacement, and between speed and velocity. Take care not to forget the importance of appropriate units!
READ – NASA’s Page on Scalar and Vector Quantities (NASA) Links to an external site.
WATCH – Vectors and Scalars in 1D (YouTube) Links to an external site.
WATCH – Vectors and Scalars in 2D (YouTube) Links to an external site.
WATCH – Speed, Velocity and Acceleration Part 1 (YouTube) Links to an external site.
WATCH – Speed, Velocity and Acceleration Part 2 (YouTube) Links to an external site.
WATCH – Speed, Velocity and Acceleration Part 3 (YouTube) Links to an external site.
WATCH – Mechanics – Speed and Velocity (MASSOLIT) Links to an external site.
WATCH – Mechanics – Acceleration (MASSOLIT) Links to an external site.
- Write down points from the videos that you want to remember. This is the beginning of learning a note-taking process, a skill you will develop throughout the course. Pause the videos and copy into a notebook, there and then, anything you might think useful later. It will then be easier to find.
- Complete the questions on page 11. You will find the answers in General Resources under the heading Textbook Answers. Download Textbook Answers.
Top Tips
Make sure that you understand the following key points:
- Familiarise yourself with the ideas of Δ denoting a change, velocity as rate of change of displacement, and acceleration as rate of change of velocity.
- Average speed does not take into account variations in speed during the journey. It is simply calculated with total journey distance and time.
- Displacement is change in position; distance is ‘length along the path’.
- Displacement is the vector of distance.
- Velocity is the vector of speed.
- To specify a velocity, remember to give both speed and direction.
- Instantaneous speed (or velocity) can be calculated by taking a gradient from a certain point on a DT graph.
- An object is accelerating if either speed or direction is changing.
- Whilst accelerations can (very briefly) be extraordinarily high, no speed or velocity can ever be greater than the speed of light, which is
3×108ms−1. If you calculate a speed that is higher than this, check your calculation again as it must be wrong.
Key Terms
Add the following key terms with definitions to your word list:
- Scalar
- Vector
- Speed
- Displacement
- Velocity
- Average speed
- Acceleration