Mr Palmer's AQA GCSE Physics Notes
Motion
Velocity
Speed refers to how fast an object is traveling. Velocity refers to an object is traveling and the direction it is traveling.
Example: a girl is trying to escape from detention! She makes a break for it by pushing past Mrs Pepper at the detention room door. The girl needs to run forty-five meters from the room to the front door of the school in a minute and a half or she'll get caught! What must her speed be?
Distance-Time Graphs
Distance-time graphs show you how far an object is from a given point. Imagine an old lady with a dog on a retractable leash. If the old lady remains sitting on a park bench and the dog walks around, the length of the leash tells you how far away the dog is. If the old lady recorded the length of the leash as a function of time, she would make a distance-time graph.
A horizontal line on a distance-time graph means that an object is stationary.
The steeper the line on a distance-time graph the faster the object is moving.
Assuming that our distance-time graph is of the old lady on the park bench with her dog, this is how we would describe the dog's motion for the first 18s:
- From 0 to 3 seconds, the dog is stationary, sitting with the old lady.
- From 3 to 11 seconds, the dog is running away from the old lady.
- From 11 to 16 seconds the dog is sitting stationary 8 meters from the old lady.
- From 16 to 18 seconds the dog is running back toward the old lady. This is when the dog is moving the fastest because the line is the steepest.
The slope (gradient) of a line on a distance time graph tells you the speed of the object. The speed of the dog between 16 and 18 is calculated:
Acceleration
An underground tube car is traveling at 1 meter per second and accelerates to 5 meters per second in 6 seconds. What is the acceleration of the car?
Velocity-Time Graphs
Velocity-time graphs are a record of how fast an object was moving at a given time. On a car, they would show what the speedometer was displaying.
On a velocity-time graph a horizontal line means the object is traveling at a constant speed. (Remember that 0 is also a constant speed, but not all horizontal lines mean the object is stopped)
An upward tilting line means that the object is accelerating. The steeper the line, the greater the acceleration. A downward tilting line means that the object is decelerating. The steeper the line the greater the deceleration.
If the above graph is of a car, we would describe its motion in the first 18 seconds as such:
- Between 0 and 3 seconds the car has a constant speed of 0 m/s.
- Between 3 and 11 the car is accelerating.
- Between 11 and 16 the car is traveling at a constant speed of 8 m/s
- Between 16 and 18 the car is decelerating. This is the greatest change is velocity because the line is the steepest.
The acceleration on a velocity-time graph is calculated by finding the slope of the line. For example, to find the acceleration between 16 and 18s we would do the following:
To find the distance an object travels on a velocity-time graph, you calculate the area between the line and zero. For example, the distance travelled between 3s and 16s is calculated by adding the area of the triangle to the area of the rectangle:
The area under the line between two points tells us the distance the object traveled in that time.
Physics P1 Topics
Physics P2 Topics
- Motion
- Forces
- Energy
- Static Electricity
- Current Electricity
- Mains Electricity
- Momentum
- Nuclear Physics
Other