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### Course: Middle school physics>Unit 1

Lesson 1: Representing motion

# Representing motion

Review your understanding of reference frames and units in this free article aligned to NGSS standards.

## Key points:

• A reference frame is the point of view that you observe and measure things from. It is used to help define the motion or position of an object.
• Two people can have different reference frames of the same situation. If this happens, they will observe the motion and position of an object differently.
• Units are a way to describe a measurement. They allow us to describe things, like an object’s speed, mass, or position, in a consistent way.
• Scientists need to communicate data in a clear way. They do this by sharing the units and reference frame they use to take measurements.

## Want to join the conversation?

• Could a person run faster than a car.
• Maybe depending how fast the car is moving...
• did you know that if the earth stopped spinning for just 1 millisecond everything on earth would be hurled forward at 1000 miles ph because that's how fast the earth spins?
• i think the earth rotates at around 3560 kph maybe its just around 3000
• Can a human run faster than a cat?
• Well you would need to know the speed of the cat compared to the human.. it is possible a human can run faster but a cat could run faster too. Not all cats and humans are the same, for example a cat could be fat and slow where as the human racing the cat could be very fit and fast. But it could also be the same way with the human being slow and the cat being fast.
• why does the yellow truck think the blue car is going at 5mph if the blue car is ahead of them? the yellow truck should be much ahead of it if it's 5mph
• It's the perceived speed. The yellow truck can think of itself as stationary because its speed is constant and doesn't speed up or down, so to it all of the movement outside the window is the world itself moving around it.

Because the blue car is moving 5mph faster, the yellow truck that think itself stationary, will see the blue car is moving at 5mph only.

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Here's more details if you're still confused:

The yellow truck will also see the trees on the sidewalk moving backwards at 40mph! But trees don't walk?! Yes, because this is the result of the truck seeing itself as stationary.

Someone on the sidewalk will see their "real" speeds at 40mph and 45mph. But also remember that's just another reference frame, Someone from space will see them both moving at a VERY fast speed because of earth's rotation!

This is what reference frames are all about: there's no "real" speed, it's all relative to your current reference frame. From the reference frame of the milky way galaxy, we are all moving at an extra speed of 828,000 km/hr! That's 515,000mph!

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This is very relevant to real life btw, for centuries people thought that earth is stationary, and all the other planets are the ones doing all the movement. This led to a lot of messy math that couldn't account for all the planets movements correctly, because the earth IS moving relative to the other planets, and NOT at a perfectly constant velocity! It speeds up and slows down slightly as it orbits the sun, and so do the other planets.

So this is why it's important to choose your reference frames wisely, it's ok to assume earth is stationary when you're just working with cars on the road. But if you want to know how earth moves relative to other planets, you have to pick the sun as your stationary reference frame, even though you know its moving in the galaxy, it's not relevant here.

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Edit for clarity, Speed VS. acceleration. Thanks @Crystal

Speed is relative, it depends on reference frame.
Acceleration (speeding up and down) is NOT relative, it's absolute.

Example: imagine you're in a windowless car moving at perfectly constant speed (no acceleration at all) on a frictionless road. Since you can't see outside, how can you tell if the car is moving? You can't, you could be stationary or you could be moving at super high speed, as long as it's constant speed, you can't tell the difference.

Now imagine the car suddenly accelerates and speeds up, NOW you can tell that it IS moving, the acceleration will give it away, and if you had an accelerometer you can read the acceleration value. That's why acceleration is absolute while speed is not.
• does the mass increase the speed
• Mass does not affect speed directly. If two objects with different masses move at the same speed, the object with more mass will not move faster. However, when an object is more massive it becomes harder to put into motion or stop when moving than a less massive object. Because more mass means more resistance. so to answer your question, more mass will not directly affect the speed of the massive object. But the more massive object will need more force to be at the same speed as a less massive object.
• what is quantum physics and what does it do and what are its uses?
• Quantum physics is the study of matter and energy at the most fundamental level. It aims to uncover the properties and behaviors of the very building blocks of nature. While many quantum experiments examine very small objects, such as electrons and photons, quantum phenomena are all around us, acting on every scale.
• how fast is the earth moving relative to the sun
• It covers this route at a speed of nearly 30 kilometers per second, or 67,000 miles per hour.