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Learn how to describe where an object is and how we know it is moving using reference points.
If you are sitting perfectly still reading this, are you actually moving? The answer depends entirely on who—or what—is watching you!
In physics, motion is defined as a change in position relative to another object. But here is the catch: to know if something moved, you need a fixed starting point. This stationary object is called a reference point. Think of a reference point as an 'anchor' for your observations. If the distance between an object and its reference point changes, the object is in motion. For example, if you are on a bus and you pass a mailbox, the mailbox is the reference point. Because the distance between you and the mailbox is changing, you are in motion relative to the mailbox.
Quick Check
If you are sitting in a moving car, why does your friend sitting next to you appear to be stationary?
Answer
Because if you use the car as your reference point, the distance between you and your friend is not changing.
To tell someone exactly where an object is, you need more than just a name. You need a complete description of its position. A complete description requires three specific pieces of information: a reference point, the distance from that point, and the direction. Without all three, your description is incomplete. In science, we often use the variable to represent distance. If you tell a friend, 'I am 50 meters away,' they won't find you. But if you say, 'I am meters North of the school gates,' you have provided a perfect map of your position.
Let's describe the position of a hidden treasure using the three requirements: 1. Reference Point: The Old Oak Tree. 2. Distance: meters. 3. Direction: East. Result: The treasure is meters East of the Old Oak Tree.
Quick Check
What are the three things needed to describe an object's position?
Answer
Distance, direction, and a reference point.
Whether an object is moving depends on the reference point you choose. This is called relative motion. If you use the Sun as a reference point, you are currently moving at about kilometers per second because Earth is orbiting the Sun! However, we usually choose reference points that are fixed relative to the Earth, such as buildings, trees, or signs. Choosing a moving reference point can be confusing. If you are on a plane and use a cloud as a reference point, it might look like you are moving slowly, even if the plane is traveling at km/h, because the cloud is also moving!
Imagine two trains, Train A and Train B, traveling on parallel tracks at the exact same speed of km/h. 1. If a passenger in Train A looks at the ground, they are moving at km/h. 2. If that same passenger looks at Train B, Train B appears to be standing still. 3. This happens because the distance between the two trains is not changing, even though both are moving relative to the Earth.
Suppose you are walking toward the back of a moving bus. The bus is moving forward at m/s relative to the road. You are walking backward at m/s relative to the bus. 1. To a person on the bus, your speed is m/s. 2. To a person standing on the sidewalk, your speed is the difference: m/s forward. 3. The reference point changes your observed speed!
Which of the following is the best definition of a reference point?
You tell a friend that the library is '3 blocks North of the park.' What is the reference point?
True or False: An object can be in motion and at rest at the same time.
Review Tomorrow
Tomorrow morning, look out a window and identify three objects that would make good reference points and one object that would make a poor reference point. Why did you choose them?
Practice Activity
Try to describe your current position to someone in another room using only distance, direction, and a reference point. See if they can find exactly where you are sitting!