1. P3a(ii) Speed You will learn about: How to interpret simple d-t graphs Understand what acceleration is How to draw simple d-t graphs www.PhysicsGCSE.co.uk

2. Motion Road planners need to understand how cars move on the road so they can determine how safe our roads are. This graph shows a car travelling during a journey. You can see that over time its distance form where it started is increasing. It is moving away from its starting position at a constant speed. We know this because this is a straight line graph – during equal amounts of time the car increases its distance in equal amounts too. This graph shows the same car on a different journey. You can see this is a straight line (plateau) graph. This means that over time the distance is not changing. Therefore this car is stationary. Incline = constant speed Plateau = stationary Motorways are considered safer than roads because drivers tend to drive at a constant speed rather than speeding up and slowing down often. www.PhysicsGCSE.co.uk

3. Distance-Time (d-t) graph example Car moves at a constant speed away from its starting point. After 3 minutes the car reached 6 miles. Between 3 and 6 minutes the car did not increase its distance. The car is stationary. Between 6 and 10 minutes the car continued to travel at a constant speed. The car reaches 14 miles away from its starting point after 10 minutes www.PhysicsGCSE.co.uk

4. d-t graph gradients The blue line is steeper than the red line – it has a steeper gradient. It therefore covers more distance within the same time period as the red line. So it must be travelling faster. This graph shows a line which is a curve. Its gradient is constantly changing. It gets steeper and steeper with time. Therefore the speed is constantly changing – this means it is accelerating. The red line is a steeper curve therefore it accelerates at a higher rate. www.PhysicsGCSE.co.uk

5. d-t graph declines The blue line shows a straight line incline and then a decline. This means it travelled at a constant speed AWAY from the start point (incline – positive gradient) and then travelled at a constant speed RETURNING back to the start point (decline – negative gradient). So an incline = away And a decline = return The red line shows a curve going away from the start and then a curve returning back to the start. This shows an acceleration away from the start (inclining) and then deceleration back to the start point (decline) So at which point on the graph shows where the car has returned back to the starting point? When the line returns back to the x-axis. Red line returns at 9 seconds. Blue line returns at 10 seconds. You will see later in this unit that: Positive acceleration = speeding up Negative acceleration = slowing down www.PhysicsGCSE.co.uk

6. Graph interpretation in a nutshell… www.PhysicsGCSE.co.uk

7. Calculating gradients to deduce speed y 2 = 15 y 1 = 5 X 1 = 0 X 2 = 15 www.PhysicsGCSE.co.uk

8. Questions 1.Draw a d-t graph representing a car accelerating and then slowing down to a steady speed. 2.What is the significance of the x-axis in a d-t graph? 3.What is the difference between a positive and negative gradient? www.PhysicsGCSE.co.uk

9. Answers to Questions 1.Draw a d-t graph representing a car accelerating and then slowing down to a steady speed. 2.What is the significance of the x-axis in a d-t graph? It shows the time of the journey. It also indicates where the starting position is so if the line returns back to the x-axis then the journey has returned to where it started. 3.What is the difference between a positive and negative gradient? Positive is moving away from the start position, usually upwards. Negative is returning back to the start position, usually downwards. Graph shows a positive curve which plateaus out. Axis should both be labelled. Numbers are arbitrary. www.PhysicsGCSE.co.uk