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## High school physics - NGSS

### Course: High school physics - NGSS > Unit 1

Lesson 2: Introduction to momentum# Introduction to momentum

Momentum is defined for a particular frame of reference; it is the mass times the velocity of the object. Created by Sal Khan.

## Want to join the conversation?

- If you drove the truck at 20 m/s into a stationary formula one car, assuming all the momentum was transferred from the truck into the car, would the car have a momentum of 600,000 kg*m/s just after the time of impact?

If so, could you calculate the velocity of the car?(2 votes)- yes the standard unit of velocity magnitude (also known as speed ) is the meter per second (m/s). ... If you are driving the car, the velocity of the car relative to your body is zero. If you stand by the side of the road, the velocity of the car relative to you is 20 m/s northward.

hope this helped :/(3 votes)

- what is the difference between mass, inertia, and momentum?(2 votes)
- 1. Mass is the quantity of matter which an object contains.

2. Inertia is the tendency to move or stay at rest.

3. Momentum is kind of how much difficult to stop an object.

The relationship between these is if an object has more matter then it will have more inertia. So according to p=mv, it will have greater momentum.

Think about a train and a normal car. Apparently, the train has much more mass than the car. So, it will have more inertia. So when you push the breaks of these two vehicles at the same time, it will be more difficult to stop the train.(1 vote)

- How was momentum as mass-times-velocity discovered as a significant quantity? Why not, say, mass-velocity ratio or some other quantity? How did physicists arrive at this specific product as a significant one to compute for moving objects?(1 vote)
- How fast would it go if it were traveling faster distance/velocity based on the rate of speed at which it was traveling?(1 vote)
- If we had a scenario where the car was only moving 19 m/s rather than 20 m/s like the 30,000 kg truck, would the momentum of the truck to our frame in the car be 30,000 kg m/s from (20-19 m/s) x 30,000 kg?(1 vote)
- Why does he call it an 18 wheeler when it has 14 wheels, 7 on each side?(0 votes)
- The rear two axels of both the truck and the trailer have four wheels each. In total that's four axels with four wheels (4x4=16). And then adding the front axle of the truck which only has two wheels gives us the 18 wheels.(2 votes)

- KE=1/2mv^2, p=mv, therefore p=2KE/v, so as v increases p decreases. does this make sense? Thank You!(0 votes)
- We have p = 2KE/v, but that doesn't necessarily mean that as v increases p decreases. Note that KE is dependent on v^2, so the factor of increase in KE outweights that of v itself.(1 vote)

- Just wondering...

How would you convert it into miles rather than km?(0 votes)- Formula

for an approximate result, divide the length value by 1.609(1 vote)

- Amazing video. However, when talking about velocity, why is "right" always the positive direction, why not left? Is there a specific reason?(0 votes)
- That is just how velocity is defined, and in a way it makes sense as "right" is always associated with increasing. Hope this helps!(0 votes)

## Video transcript

- [Instructor] In this video, we're going to talk a
little bit about momentum. And I encourage you to think about what does momentum mean
in everyday language. If I were to tell you that a
business has a lot of momentum, many people would imagine that means that it's doing really well, it's hard to stop that
business from succeeding. If I were to tell you that, and let's say a movie star
has a lot of momentum, it means that they keep making really, really, really good movies, it's hard to stop them. And in physics, momentum
essentially has that same notion. In fact, these everyday
language versions of momentum really came from the
physics version of it, and an informal definition
you could think of it is how hard to stop something. Stop something. So with that very informal definition, we'll get a little bit more
mathy in a few seconds. I have two pictures here. And let's say both of these vehicles, so we have a big 18-wheeler truck here, and here we have a Formula One car. Let's say that they both have a velocity of 20 meters per second. Positive 20 meters per second. We'll just think in one dimension. So let's say it's going to the right at 20 meters per second, so that's a magnitude and a direction. Okay, I could put a positive
here to make it clear that I'm giving a direction here. So they're both going in
the positive direction at 20 meters per second. But you can imagine their
masses are very different. This large truck would have a mass of let's just say 30,000 kilograms. That's roughly actually what
an 18-wheeler truck's mass is, I looked it up before this video. And let's say the mass of this Formula One car
is about 1,000 kilograms. Now pause this video and think about which one do you think
would have more momentum. All right, well, you're
probably imagining trying to stop either of these, and I guess either of these
would be difficult to stop, but a truck going at 20 meters
per second seems a lot harder than the Formula One car. This thing seems like
it would just be able to drive through anything. So if you picked the truck
having a higher momentum when it's going at the same
velocity as a Formula One car, you'd be right. But an interesting question is well, how can I quantify that? And that's where we have
the mathematical definition of momentum, and momentum is defined as mass times velocity. And because velocity is a vector, it's not just a magnitude, we assigned a direction, so we said in the positive direction. If we were in one dimension, if we said in the negative direction, we'd be going to the left. That's maybe the convention we could use. And since it's a scalar, which is mass, times a vector, momentum is
also going to be a vector. You're going to have a momentum
in a certain direction. So pause this video and see if you can calculate the momentum for each of these vehicles. And see also if you can figure out what the units are going to be. All right, now let's
think about the momentum for this 18 wheeler. It is going to be its mass, which is 30,000 kilograms, times its velocity, which is positive 20 meters per second. And so when I multiply those two things, I am going to get, let's see, I'm going to get a six with one, two, three, four, five zeros. One, two, three, four, five zeros. 600,000, and then the units are kilogram times meter per second. Kilogram meter per second. That's the momentum of the truck. Now what about the Formula One car? Pause the video and try to
calculate that momentum. All right, well, same notion. I'll have to squeeze
it in right over here. The momentum here is the
mass, 1,000 kilograms. 1,000 kilograms times the velocity, positive 20 meters per second. I'm stressing the positive
because we're in one dimension, and I'm saying positive is one direction, negative is the other direction. So I am giving it a direction here. And so this momentum
is going to be equal to 20,000 kilogram meters per second. Kilogram meters per second. And so when you evaluate the momentums, it's clear mathematically that the truck has much more momentum than this Formula One car. It has 30 times the momentum. Now I know what some
of y'all are thinking. Well, this is just from
the reference frame if I'm standing still on the ground while this truck and car are moving by. But what if I were moving along with them? Would they have the same momentum? And if you were asking that question, it's a very good one, and the answer is no, the momentum depends on
the frame of reference. So let's say that you are able to travel very quickly in, well, let's say you're in a car here. And you're also going at
positive 20 meters per second, so you're going in the same
direction as the truck. Well, relative to you, the truck's velocity is now zero. So from your frame of reference, if you wanted to calculate momentum, you'd have the mass of the
truck, 30,000 kilograms. But from your frame of reference, the velocity would now be zero. So if you're traveling in
this car at the same velocity as that truck, the momentum
you would calculate is zero. Now, what would then have momentum? Well, in that world, let's say that there's a
big rock over here that, let's say this thing's 100,000 kilograms. Well, when you are standing on the ground, that thing would have no momentum, but now that you're moving
20 meters per second to the right, positive
20 meters per second, the rock in your frame of
reference would look like it's going at negative
20 meters per second. It would look like it's
going 20 meters per second to the left. And so this would now, if you're moving in the same frame of
reference as that truck, this rock would then look like it has a very negative momentum, and I encourage you to
calculate it if you like. But I'll leave you there. This is really just an
introduction to momentum. But once you get an intuition for it, all sorts of interesting
things in physics start to emerge from it.