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# Introduction to reference frames

How the choice of reference frame is related to speed and velocity measurements.

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• I know that stationary means to not move at all, but the car and the plane is stationary and does that mean that it does not move or not? I am soo confuse about this?
• A reference frame is a like a fixed point. Properties of other objects such as: position, velocity etc. are measured using the point.
It is so because no point in the universe is stationary or static. Every point is moving depending on another 'so called' static point.
See it like this: you are going to a amusement park in a bus with your friend. When the bus starts moving you see everything outside the bus going backwards. Here you are the reference frame. But for a person standing beside the road who has just missed the bus would 'observe' your bus going onward with you and your friend. So for the pedestrian both you and your friend are moving at a certain speed. But for you, you see that your friend is just sitting beside you, according to you, he is not moving but stationary as you are.
So the summary is when you are the frame of reference you and your friend are stationary and the pedestrian is moving. For the pedestrian it is the vice versa.
• So I notice at Sal says we are taking the view point of this big giant thing called Earth when really the earth is spinning on a axis but we never see it, how does that work? The ground is stationary when you are there but when your perspective is shifted to the plane or the car the ground takes on the velocity of whatever perspective you are at. The Earth itself as a whole spins around 1042 miles per hours so why does the ground not take on an additional 250 or 50? Why is it that the Earth spins yet we cannot see it even though it goes so fast (the earth's mass is pretty large but still if you look at the sky it moves but the earth does not)? If you were in a plane staying in one spot and you looked down at the earth would it move at some point?
• The Earth, car, plane and atmosphere are all moving together with Earth's rotation. Since the motion of Earth's rotation is the same in all those frames of reference, it does not cause any change in perceived motion.
• At wouldn't you feel the G-force because 250 m/s > 9.8 m/s and couldn't you measure that?
• If you're talking about 9.8 as the acceleration due to gravity, you need to remember that its a change in speed or direction (Velocity)
so its actually 9.8m/s/s not just 9.8m/s
The plane is not accelerating, 250m/s to the right, it is maintaining its velocity, you will not feel any G-Force while it is maintaining that speed and direction(Velocity).
• yo this guy is like a library. Is there anything this guy can not teach. He basically runs khan academy all buy himself. My respects.
• Sal Khan is awesome
(1 vote)
• If there is a strong wind blowing, and the leaves and branches of the tree are moving but the trunk of the tree is still,so the tree would be in the state of rest or in the state of motion?
• Depends on your choice of reference frame.
• Hii I want to ask that the reference frames examples you've suggested is that the car moving in the same direction of the plane, thus makes the plane seems like it's moving faster for 250+50 m/s but if the car and the plane are moving in the same direction, will the plane seems to move like 250-50m/s?(in example 2) or how fast would the ground seems to move?
Thx.
(1 vote)
• If the car and the plane are moving in the same direction, the relative velocity between them would be the difference between their individual velocities. Let's consider the examples you mentioned:

Example 1: Car and Plane Moving in the Same Direction (Car Speed = 250 m/s, Plane Speed = 50 m/s)

If the car and the plane are both moving in the same direction, the relative velocity between them would be the difference between their speeds. In this case, the relative velocity would be:

Relative Velocity = Car Speed - Plane Speed
Relative Velocity = 250 m/s - 50 m/s
Relative Velocity = 200 m/s

So, from the reference frame of the car, the plane would appear to be moving with a relative velocity of 200 m/s in the same direction as the car.

Example 2: Car and Plane Moving in the Same Direction (Car Speed = 250 m/s, Plane Speed = 300 m/s)

If the car and the plane are moving in the same direction, but the plane's speed is greater than the car's speed, the relative velocity between them would still be the difference between their speeds. In this case, the relative velocity would be:

Relative Velocity = Car Speed - Plane Speed
Relative Velocity = 250 m/s - 300 m/s
Relative Velocity = -50 m/s

The negative sign indicates that the relative velocity is in the opposite direction to the car's motion. So, from the reference frame of the car, the plane would appear to be moving with a relative velocity of 50 m/s in the opposite direction as the car.

In both examples, the ground would appear to be moving at the same speed as the car since the car is in contact with the ground.
• i dont understand the first condition if we stand on a road and a car passess by it seems much faster than the airplane then how come the velocity of the car is less than that of the airplane
• Imagine what the speed of the plane would seem like if you were as close to the plane as you are to the car.
• Can someone please explain to me in depth about what are inertial and non inertial reference frames?
preferably in a way that I can inuitively understand not just a definition
• @Andrew M
thankn you for the website link that video explains it SO WELL!