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1.2.2Forces and Their Effects

Speed and Velocity

5 min read 5 parts Lesson 11 of 32 1 quiz

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Section 1 of 5Speed, Velocity and Distance

1.2.2 Speed and Velocity

Speed and velocity are often used as if they meant the same thing, but only velocity includes direction.

Speed
  • A scalar that describes how fast an object moves, with magnitude only and no direction.
Velocity
  • A vector equal to speed in a stated direction, so two objects can have the same speed but different velocities.
Distance
  • A scalar giving the total length of path travelled, regardless of any change in direction along the way.
Displacement
  • A vector giving the straight-line change in position from start to finish, including its direction.

Section 2 of 5Calculating Velocity

The velocity equation uses displacement, so the direction is built into the calculation.

1
Write down the displacement in metres, remembering that it is a straight-line change in position rather than the path length.
2
Convert the time into seconds, because velocity is measured in metres per second.
3
Use v = s ÷ t to calculate the velocity, and rearrange it to s = v × t when the displacement is the unknown.
4
State the direction of motion with the answer, so the value includes both magnitude and direction.

Section 3 of 5Average Speed

Average Speed
Equation
  • Average speed equals total distance travelled divided by total time taken, using distance rather than displacement.
Non-uniform motion
  • An object that speeds up, slows down or stops has an average speed over the journey, not a single constant speed.
Round trips
  • Over a round trip the displacement is zero, so the average velocity is zero even though the average speed is not.
Units
  • Convert kilometres into metres and minutes into seconds before dividing, so the answer is in m/s.

Section 4 of 5Worked Example

Question

  • A bus travels 15 km north in a straight line in 25 minutes.
  • Calculate its average speed and state its velocity.

  1. Convert the valuesdistance = 15 000 m and time = 25 × 60 = 1500 s.
  2. Average speed = total distance ÷ total time = 15 000 ÷ 1500 = 10 m/s.
  3. The journey is a straight line, so the displacement equals the distance: 15 000 m north.
  4. Velocity = displacement ÷ time = 10 m/s north.

Answer The average speed is 10 m/s, and the velocity is 10 m/s north.

Section 5 of 5Exam Checklist & Tips

1.2.2 – Key Points for the Exam

  • Speed How fast an object is moving, a scalar quantity.
  • Velocity Speed in a stated direction, a vector quantity.
  • Displacement The straight-line change in position of an object, including its direction.
  • Distance The total length of the path travelled by an object.
  • Average speed Total distance travelled divided by total time taken.

Common Mistakes

  • Using displacement in the average speed equation; average speed always uses the total distance travelled.
  • Stating that two objects with the same speed have the same velocity; their directions may differ.
  • Leaving the time in minutes when calculating speed in m/s.
  • Giving a velocity without a direction, which loses the vector information.
Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).