Inertial Frame of Reference: Definition & Example

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• 0:00 Inertial Frame of Reference
• 1:05 Newton's Law & Frames…
• 2:50 Fictional Forces
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Lesson Transcript
Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

Learn how to tell the difference between an inertial frame of reference and a non-inertial frame of reference. Then find out what these frames of reference have to do with Newton's first law and fictional forces.

Inertial Frame of Reference

At some point in your life, you've probably been on a form of public transportation. It might have been a bus, metro, train, plane, or even something else. Often we share these rides with other people. From your point of view in the vehicle, these people are sitting or standing still. However, to a bystander standing still outside the vehicle, you and your fellow passengers are moving at a faster velocity. We can call these two points of view two different frames of reference. In physics, we can classify frames of reference by two main types: inertial and non-inertial.

Inertia is the property of matter in which an object that is at rest wants to remain at rest, and an object that is moving wants to remain moving in a straight line unless another force acts upon it. Likewise, an inertial frame of reference is a reference frame in which an object stays either at rest or at a constant velocity unless another force acts upon it. When a body does not seem to be acting in accordance with inertia, it is in a non-inertial frame of reference or accelerating.

Newton's Law & Frames of Reference

Newton's first law states that a body at rest will remain at rest, and a body in motion will remain in motion at a constant velocity in a straight line unless acted upon by an external force. The external force in this law being a force that does not originate from the body itself. Looking at the previous definitions for inertia and an inertial frame of reference, we can see that their definitions are almost the same as Newton's first law. Another way to define an inertial frame of reference is a frame of reference in which Newton's first law remains true. So then you might be wondering, when could Newton's first law ever not appear to be true? Well you've actually probably experienced a time when it doesn't seem true yourself, and it has to do with non-inertial frames of reference.

Let's think back at our example of the bystander outside a vehicle and the passenger inside from the first section of the lesson. I'm going to change the example a little bit first. For one, we will simplify it down to one passenger inside the vehicle. Secondly, let's imagine the vehicle the passenger is in is a soundproof box he or she cannot see out of. This way the passenger will have no way of knowing if it is moving or stationary. In fact, to them all that matters is that they are standing still. In our example, the bystander outside the vehicle will always be in an inertial frame of reference since he or she is standing still (i.e. at rest). For the passenger, it depends on the vehicle's movement. Let's assume the vehicle is moving in a straight line. If the vehicle is not accelerating, the passenger is in an inertial frame of reference. This is because no acceleration means your velocity is constant. If the vehicle is accelerating, the passenger is now in a non-inertial frame of reference.

Continuing with this same scenario, try and put yourself in the role of that passenger. From your perspective in the vehicle, you are standing still and nothing is acting on you, but you experience what feels like a force pushing you in the opposite direction of the vehicle's movement. You're probably familiar with this feeling. It's that feeling of being pushed back into your seat when a vehicle first starts moving or accelerates quickly.

Fictional Forces

So where does that feeling of pushing you back in your seat come from? Let's assume the vehicle was stopped and just started accelerating. From the bystander's perspective, he or she can see that the vehicle started moving. Since the passenger was at rest, he or she wants to remain at rest. This feeling is your body resisting the movement due to inertia. From the passenger's perspective there is no movement, and it feels like a force has just suddenly been applied to his or her body. We call this kind of phenomenon from the passenger's perspective a fictional force.

A fictional force, also known as a pseudo-force, is defined as a force that is physically apparent to be acting on an object, but also does not appear to exist in a non-inertial frame of reference. This can seem like quite a confusing concept, so let's look at another example of a fictional force.

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