Speed and Velocity

Defining Speed and Velocity

  • Speed is a measurement of how fast an object is moving. It is a scalar quantity, meaning it only has magnitude and no direction.
  • Velocity is a vector quantity. It includes both the speed of an object and its direction of travel.

Calculating Speed and Velocity

  • Speed is calculated using the formula speed = distance/time. The units used are metres per second (m/s).
  • Velocity is calculated as velocity = displacement/time. Like speed, its units are also m/s, but with a specified direction.
  • Displacement is the distance in a specific direction, so it’s different from a straightforward distance.

Average Speed and Velocity

  • The average speed of an object is the total distance travelled divided by the total time taken.
  • The average velocity of an object is the total displacement divided by the total time taken.
  • If an object returns to its starting point, its total displacement is zero, thus making its average velocity zero.

Graph Interpretation

  • Distance-time graphs and Displacement-time graphs can be used to deduce information about an object’s velocity and acceleration.
  • The gradient of a distance or displacement-time graph gives the speed or velocity.
  • A horizontal line on these graphs shows the object is stationary.
  • On a velocity-time graph, the gradient represents the acceleration.
  • The area under the curve in a velocity-time graph represents the distance or displacement travelled.

Changing Velocity

  • Acceleration is the rate of change of velocity. It includes speeding up, slowing down and changing direction.
  • Deceleration or negative acceleration means the object is slowing down.
  • Even if an object is moving at a constant speed in a circular path, it is accelerating because its direction is continuously changing.

Acceleration Due to Gravity

  • All objects in free fall near the Earth’s surface accelerate downwards at approximately 9.81 m/s^2, which is known as the gravitational field strength.
  • This acceleration is often rounded to 10 m/s^2 for calculation purposes in exam questions.