Motion and Speed

Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor)


Speed or Velocity?

It's very important to understand the difference between these terms, because Physicists are very fussy about the precise definition of words.

  1. 1
    Speed is the rate of movement.
    Speed = distance / time
    Was this step helpful? Yes | No | I need help

  2. 2
    'Velocity is not just the rate of movement, but the rate of movement in a certain direction.
    Velocity = displacement / time. The diagram below shows displacement. See how it can have positive and negative values.
    diagram showing what displacement is
    Was this step helpful? Yes | No | I need help


Acceleration is when the speed of an object is increasing - similar to when you put your foot on the accelerator to go faster on the motorway. It does not go straight to 130 km/h, but must speed up to reach it. The equation for acceleration is

Was this helpful? Yes | No | I need help

equation of acceleration

Speed - Time Graph

a speed time graph

From the start to point A the train is travelling at a constant speed of 10ms-1

Between points A and B it is accelerating because the line goes up, on a distance time graph this would appear as a curve. It accelerates at: a = (v-u)/t a = (30 - 10)/100 a = 0.2ms-2

From B to C it is at a constant speed of 30ms-1

To find the distance you calculate the area under the line using simple laws of geometry. The main difference with a velocity / time graph, is when the line goes down, it may mean the object is decelerating or it is moving in the opposite direction, so it is possible to have a negative velocity.

Was this helpful? Yes | No | I need help

Stopping Distance

Stopping distance is the distance a vehicle travels from the point the driver should stop, to when he or she actually becomes stationary. It combines thinking distance and deceleration distance. Here it is represented in a speed/time graph.

Was this helpful? Yes | No | I need help

graph showing stopping distance

Factors that can increase stopping distance are:

  1. 1
    A faster vehicle will travel more distance (d = s x t).
    Was this step helpful? Yes | No | I need help

  2. 2
    A heavier vehicle may travel further because it has greater momentum.
    Was this step helpful? Yes | No | I need help

  3. 3
    Road Condition.
    If the road is wet, for example, the vehicle will move more because there is less friction to stop it. That is why we should drive slower when it rains, the DSA advises that stopping distance is doubled in the rain and increases by 10 times on icy roads.
    Was this step helpful? Yes | No | I need help

  4. 4
    Reaction Time.
    The thinking distance will be lengthened if the driver isn't concentrating or their reaction has been inhibited by alcohol or sleep deprivation for example.
    Was this step helpful? Yes | No | I need help



ScienceAid welcomes all comments. If you do not want to be anonymous, register or log in. It is free.

Featured Author
166 Articles Started
1,267 Article Edits
42,900 Points
Jamie is a featured author with ScienceAid. Jamie has achieved the level of "Captain" with 42,900 points. Jamie has started 166 articles (including this one) and has also made 1,267 article edits. 9,600 people have read Jamie's article contributions.
Jamie's Message Board
Jamie: Hi, my name is Jamie.
Jamie: Can I help you with your problem about "Motion and Speed"?

Article Info

Categories : Forces

Recent edits by: Jamie (ScienceAid Editor)

Share this Article:

Thanks to all authors for creating a page that has been read 69 times.

Do you have a question not answered in this article?
Click here to ask one of the writers of this article

Thank Our Volunteer Authors.

Would you like to give back to the community by fixing a spelling mistake? Yes | No