Distance - Time Graphs
We can use graphs to help us describe the motion of a body. A distant time graph shows the distance of a body from a starting point (y-axis) against time take (x-axis)
The slope of the line on a distance - time graph represents speed. The steeper the slope, the greater the speed.
The speed of a body is the distance travelled each second. We can calculate the speed of a body using the equation:
speed=distance travelled ÷ time taken
The unit of speed is metres per second (m/s).
Velocity and Acceleration
Velocity is the speed of an object in a given direction. If the body changes direction it changes velocity even if the speed stays the same.
- An object moving steadily round in a circle has a constant speed. Its direction of motion changes as it goes tound so its velocity is not constant.
- Two moving objects can have the same speed but different velocities. For example, a car travelling north at 30m/s on a motorway has the same speed as a car travelling south at 30m/s. But their velocities are not the same because they are moving in opposite directions.
If the velocity of a body changes, we say that it accelerates. We can calculate acceleration using the equation :
acceleration = change in velocity ÷ time taken for the change
The unit of acceleration is metres per second squared (m/s2).
If the value calculated for acceleration is negative, the body is decelerating - slowing down.
More about Velocity - Time Graphs
A velocity time graph shows the velocity of a body (y-axis) against time taken (x-axis).
Braking: This reduces the velocity of a vehicle. The velocity is constant until the driver applies the brakes.
- The slope of a line represents acceleration.
- The steeper slpoe, the greater the acceleration.
- If the slope is negative, the body is decelerating (slowing down)
- The area under the line represents the distance travelled in a given time.
- The bigger the area, the greater the distance travelled.