Movement and Position

Movement and Position

Understanding Position

  • Position refers to an object’s location in relation to an origin point.
  • In Physics, we measure position along an axis (either horizontal, vertical, or both in 2D motion studies). The direction of the axis is important when considering positive and negative values.
  • Origin is the starting reference point for descriptions of position or motion. It is usually taken to be the ‘zero’ position.
  • Distance is a scalar quantity that represents the length of the path taken from the starting point to the end point.

Understanding Displacement

  • Displacement is a vector quantity that represents the shortest route or ‘direct line’ from the starting point to the end point.
  • It is the change in position of an object, so it includes both distance and direction.
  • Displacement can be positive, negative, or zero depending on the direction of motion relative to the origin.

Interpreting Distance-Time Graphs

  • Distance-time graphs depict how an object’s position changes over time.
  • The gradient (slope) of a line on a distance-time graph gives the speed of the object.
  • A horizontal line (no gradient) shows the object is stationary, while a steeper slope indicates faster speed.
  • If the line slopes downward, this suggests the object is returning towards the starting point.

Interpreting Displacement-Time Graphs

  • Displacement-time graphs depict how an object’s position changes over time, taking into account direction.
  • The gradient of a line on a displacement-time graph gives the velocity of the object.
  • A horizontal line (no gradient) indicates the object is stationary, a steeper positive gradient indicates faster speed in the positive direction, and a negative gradient indicates motion in the opposite direction.

Calculating Speed and Velocity

  • Speed (s) is calculated using the formula: s = d / t (speed equals distance divided by time).
  • Velocity (v) is calculated using the formula: v = Δx / t (velocity equals displacement divided by time).
  • When calculating these quantities, remember to keep units consistent (e.g., metres and seconds).

Speed vs. Velocity

  • Speed is a scalar quantity, meaning it has only magnitude (size) and no direction.
  • Velocity is a vector quantity, meaning it has both magnitude and direction (positive or negative).
  • If an object’s velocity changes - either in magnitude, direction, or both - we say it has accelerated.

The Role of Vectors

  • Vectors are quantities that have both magnitude and direction, like velocity and displacement.
  • We can represent vectors graphically as arrows; the length of the arrow represents the magnitude, and the arrowhead shows direction.
  • To add vectors together, we place them ‘head to tail’ and draw a resultant from the start of the first to the end of the last. This applies to calculating the overall displacement of an object following several movements, for example.

Understanding Speed, Velocity, and Acceleration

  • The rate of change of velocity with time is called acceleration.
  • Acceleration can involve an increase in speed (positive acceleration), a decrease in speed (negative acceleration or deceleration) or a change in direction (like whirling something around at constant speed).
  • An object moving at a constant speed but changing direction is still accelerating due to the change in its velocity vector.

Interpreting Velocity-Time Graphs

  • In a velocity-time graph, the gradient at a point gives the acceleration of the object at that point.
  • A positive gradient indicates positive acceleration (an increase in speed), and a negative gradient indicates deceleration (a decrease in speed).
  • The area under the graph between two times gives the displacement of the object in that time interval.
  • If the graph crosses the time-axis, it implies a change in direction of motion.