describes the location of an object according to a choice of zero point and positive direction. The zero point is called the and upwards is commonly used as the positive direction when analyzing vertical motion. For example, with upward positive, a skydiver in a stationary balloon at an altitude of 12,000 ft would have a position of 12,000 ft, if we called the ground the origin. If we chose 12000 ft as the origin then the position of the skydiver would be zero. If we chose 24,000 ft as the origin, the skydiver would have a position of -12,000 ft. It doesn’t matter where you put the origin or which direction is positive as long as you keep them both consistent throughout your analysis.
Let’s say we placed the origin at the ground and chose upwards as positive, as in the diagram above. If we are analyzing the motion of the skydiver starting just as they jump to just as they land, then their (xi) would 12,000 ft and their (xf)would be 0 ft. The change in position would be -12,000 ft because they moved 12,000 ft downward, which is the negative direction. We call the change in position the (Δx) and we calculate the displacement as:
For our skydiver example we have:
As we analyze motion we are beginning to see that it’s important to keep track of directions for different quantities of motion like and , just like we do for forces. Just as with , we will make the symbols for these vectors bold when writing equations to remind ourselves that these quantities include directions.
Distance and Displacement
It may seem odd that we have introduced as a new word for distance that something travels, but there is actually an important distinction between the two terms. The distance and displacement are sometimes equal, but not always. For example, the distance our skydiver traveled from balloon to ground was 12,000 ft, but their displacement was -12,000 ft. If we analyze the motion of the skydiver starting from when they got into the balloon on the ground to when they landed after the jump then the distance traveled by the skydiver would be 24,000 ft. However, the would be 0 ft because their and were the same. The distance traveled can be greater than, or equal to the displacement, but it can never be less. This distinction arises because direction matters in calculating displacement, but not in measuring distance.
Instantaneous Speed and Velocity
The maximum speed reached by a body (or any object) falling under the influence of both and is often called terminal velocity or . In everyday life we often use speed and velocity to mean the same thing, but they actually have different meanings in physics. is the rate at which the position is changing and is the rate at which distance is covered. Objects cannot travel negative distances so the speed will always be positive. However, position can become more negative, as was the case for our example skydiver, so velocity can be negative. The speed at any instant in time is known as the instantaneous speed. The velocity is just the instantaneous speed with a direction included. For example, if at some point our skydiver reached a of 89 MPH, then their terminal velocity would be 89 MPH downward or -89 MPH for our choice of downward as the negative direction.
Initial and Final Velocity
Just as we defined and for the section of an object’s motion that we are analyzing, we can also define and . For example, if we analyze the skydiver’s motion from jump until they reach an example of 180 MPH, then the initial velocity of our skydiver was zero and the final velocity was -180 MPH.
Sometimes we are interested in the over some amount of time rather than the at a single time. To calculate the average velocity for a section of an objects motion we need to divide the change in position () by the time interval (Δt) over which the it occurred.
Velocities will be negative when the is negative, as was the case for our skydiver’s trip from balloon to ground. The negative displacement of our skydiver would result in a negative during their trip from balloon to ground. This makes sense, as we should be expecting a negative velocity for our skydiver because downward was chosen as our negative direction and the skydiver was moving downward.
Average Speed and Velocity
Sometimes and are the same, but sometimes they are not. Speed is the rate at which distance is traveled so to calculate average speed we divide the distance traveled by the time required for the travel. Remembering that we use rather than distance in calculating average velocity, we can see that speed and velocity are different. For example the velocity of the skydiver in our example is negative on the way down because displacement is negative, however we cannot say the diver actually traveled a negative distance, so the average speed is positive.
Let’s imagine the skydiver in our example rode a hot air balloon upward for 21 minutes, then jumped and fell for 2.0 minutes, then opened their parachute and drifted downward for 5.0 minutes before landing. Let’s calculate the and for the entire trip in feet per minute.
The average speed is the total distance covered divided by the total time, which would be 24,000 ft divided by 27 minutes for an average speed of: 860 ft/min.
The average velocity would be the total divided by the total time. The skydiver started and ended the trip on the ground, so the total displacement for the round trip was zero, therefore the average velocity for the trip was zero! Comparing this average velocity to the average speed of 860 ft/min we can really see why its important to distinguish between instantaneous vs. average and speed vs. velocity.
- Balloon over Straubing, Germany by Runologe, via wikimedia commons ↵
- "Gabriel Skydiving" By Gabriel Christian Brown [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons ↵
- "EOD parachute jump" Petty Officer 3rd Class Daniel Rolston (https://www.dvidshub.net/image/1465626) [Public domain], via Wikimedia Commons ↵
- Parachute precise landing by Masur [Public domain], via Wikimedia Commons ↵
location in space defined relative to a chosen origin, or location where the value of position is zero
location where the position is zero
position at the start of the time interval over which motion is being analyzed
position at the end of the time interval over which motion is being analyzed
change in position, typically in reference to a change away from an equilibrium position or a change occurring over a specified time interval
any interaction that causes objects with mass to change speed and/or direction of motion, except when balanced by other forces. We experience forces as pushes and pulls.
attraction between two objects due to their mass as described by Newton's Universal Law of Gravitation
a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid
the speed at which restive forces such as friction and drag balance driving forces and speed stops increasing, e.g. the gravitational force on a falling object is balanced by air resistance
a quantity of speed with a defined direction, the change in speed per unit time, the slope of the position vs. time graph
distance traveled per unit time
existing or measured at a particular instant
the value of velocity at the start of the time interval over which motion is being analyzed
the value of velocity at the end of the time interval over which motion is being analyzed
the average of all instantaneous velocities that occurred within a certain time interval, equal to the displacement divided by the time interval
average rate at which distance was traversed, equal to total distance traveled within a time interval, divided by the time interval