Scalars and Vectors
Learn the difference when describing and using scalars and vectors. See how to use the scalars distance and speed and vectors displacement and velocity mathematically.
Basic Velocity and Speed Variables
|Velocity or Speed||v||meters per second||m/s|
|Displacement or Distance||X||meters||m|
How are scalars different than vectors?
A scalar in physics includes only a magnitude. A magnitude is a number and a unit. Two common scalars are distance, for example 5 meters, and speed, for example 5 meters per second. Speed is a scalar because it uses the scalar distance traveled per time.
A vector in physics includes a magnitude and a direction. Two common vectors are displacement like 5 meters east and velocity like 5 meters per second east.
How is distance different than displacement?
Distance (a scalar) is a measure between two points. Because distance has no direction you add up all of the segments traveled.
Displacement (a vector) is a measure of where you are from the origin. So how far in what direction from the origin or starting point.
Scalar Distance and Vector Displacement
How do you calculate distance and displacement?
Look at the difference between how distance and displacement are calculated. The stickman in the picture goes right from the origin (0 meters) all the way to 6 meters. Then he turns left and travels 4 meters back. Because direction does not matter, for distance you add every meter traveled in any direction up. So 6 meters + 4 meters.
Distance = 6 + 4 = 10 meters
For the displacement vector you turn direction into a mathematical sign and add vectors. Observe the picture to the right showing common signs to replace directions. So we call right positive and left negative. Therefore 6 meters right becomes +6 meters and 4 meters left becomes -4 meters. Now add the vectors but include the signs (+6) + (-4) = +2.
Displacement = (+6) + (-4) = +2.
Once you have your final answer turn the sign back into a direction and then include the unit. So +2 becomes 2 meters right.
Calculating Scalar Speed and Vector Velocity
Scalars go with scalars, so use distance to calculate speed as seen in the equation on the right.
Vectors go with vectors, so use displacement to calculate velocity as seen in the equation to the right.
The equation in the animation to right (v =Δx/t and also seen in two forms in the picture above it) will be used in examples to calculate speed or velocity but I will use distance if calculating speed and displacement if calculating velocity.
- In different equations x or Δx may be used interchangeably to represent displacement or position.
- Δx would represent change in position (xf – xi) and displacement is often just written as x in common equations.
- A time interval (t) can be calculated (tf – ti)
Notice in the picture to the right the stickman stops and goes at different rates.
How is instantaneous velocity different than average velocity?
- Instantaneous velocity is the velocity at that instant. For example, if stopped, his instantaneous velocity is 0 m/s but a few seconds later he is going again and its 4 m/s.
- Total displacement divided by the total time is average velocity. In the animation to the right, the total displacement was +14 m and the time it took was 4 seconds. v = +14/4 becomes 3.5 m/s forward.
What does acceleration describe?
Acceleration (a vector) describes a velocity changes over time. Therefore, the unit for acceleration has the unit for velocity over another time unit. So a meters per second change per second states as m/s/s or more commonly m/s2.
You'll learn various means to calculate acceleration in the next section.
Observe the animations below to see how distance and displacement are calculated.
Scalar Vector Example 1
Use the picture to the right for the following questions.
Thomas, on his lunch break, took 30 minutes to go to the library and then the cafe.
1. What distance did Thomas travel during lunch?
2. What was his displacement?
3. Determine the speed of Thomas during the 30 minute (1800 second) lunch?
4. What was the velocity of Thomas during the 30 minutes?