Speed and Velocity Both speed and velocity tell us how far something is travelling in unit time. As velocity is a vector it must also tell us what direction the object is travelling in. Speed (m/s) = distance moved (m) time taken (s) Velocity (m/s) = displacement change (m) time taken (s) Acceleration Acceleration tells us how rapidly something is changing velocity - for instance, the change in velocity in unit time. Deceleration is the same thing, but has a negative sign as the velocity if decreasing. Question (easy one!): /**/ Displacement-time graphs These show the motion of an object very clearly and allow you to find position and velocity at any time. Any graph that you see will be a combination of these sections. Notice that the gradient = change in D (or Δ D) change in t (or Δ t) = the velocity at any time. When the velocity is changing, as on the lower two graphs, you can find the velocity at any point by drawing a tangent touching the graph at that point by drawing a tangent touching the graph at that point and working out its gradient using the same equation. Question: Label the journey shown below with the correct words by dragging them onto the graph. /**/ Velocity-time graphs These are similar to displacement-time graphs, but this time velocity is on the y-axis. Here are the only possibilities that you'll come across at A-level. Notice that the gradient = change in V (or Δ V) change in t (or Δ t) = the acceleration at any time. You also need to know that the area under the line gives you the displacement of the object up to that point. Acceleration-time graphs Last, but not least, are the acceleration-time graphs! Note: All three of the movement graphs are related to each other as the: Gradient of D/t graph gives you the points on the v/t graph. Gradient of v/t graph gives you the points on the a/t graph.