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### Course: Mechanics (Essentials) - Class 11th > Unit 9

Lesson 1: Why do trains stay on track?# Linear velocity comparison from radius and angular velocity: Worked example

Linear velocity from the radius and angular velocity for two pumpkin catapults with different arm lengths.

## Want to join the conversation?

- Wait, so here, You are saying that the MAGNITUDE OF LINEAR VELOCITY is equal to SPEED. SPEED EQUALS DISTANCE over TIME, or in this case, the arc length, aka DISTANCE, over Time. My question is that isn't the LINEAR VELOCITY equal to DISPLACEMENT divided by the TIME, just disregarding the direction?(3 votes)
- I'm confused as to why we can calculate linear velocity at all if velocity is dependent on displacement (so it should be measured from the direct distance between the starting point and end point). Can someone explain?(2 votes)
- Sal is actually using the magnitude(without direction) of velocity which is speed(because speed has not direction),hence displacement is used.(1 vote)

- This is unrelated to the video, but I just got to 50,000 energy points, but it isn't letting me move up in the Primosaur ranks.(1 vote)
- Click on your badges on the profile home page. Below the badge Icons is a "Check for new badges and avatars" Button click it and see if that gets you the ranks.(1 vote)

- I understand this. But what if you have, for example, angular acceleration in the small catapult (In the clockwise direction of course), while the largest catapult doesn't have angular acceleration? Wouldn't the speed of the small catapult eventually surpass the speed of the largest catapult?

Maybe my question is not related to this, because here we are talking about angular velocity, speed and radius, but that doubt crossed my mind(1 vote)- Is the velocity of trebuchets and catapults are either angular or linear? there is something unrecognize in my mind because I observed as the payloads travelled, definitely in the curved path.(1 vote)

- When it goes in a arc-like motion, how do the forces on the pumpkin differ from in a linear motion?

Just curious.(1 vote)- maybe because ones direction is changing constantly and the other isnt?(1 vote)

- last numerical answer is de reases by 2. shouldn't it be final frequency decreases by 1/2??(1 vote)
- Respected Sir, at 2.30 you said that angular velocity w is equal to 2pi radians. how can it be 2pi radians. the catapult has not covered a final angle of 2pi.(0 votes)
- I know I'm late but I'm going to answer just for my own sake (so you can ignore if you want because you probably already understand this). The angular velocity "W" is 2pi rad/sec, meaning it will cover 2pi radians-if given the chance- in 1 second. So, even though the catapult's arm has NOT covered the whole 2pi radians, it will do so in 1 second. Think of it as your car's speedometer, it shows you're travelliing at 40km/h even though you haven't actually travelled 40km, but you're just going to do so in 1 hour.(1 vote)

## Video transcript

- [Instructor] Let's say that
we have two pumpkin catapults, so let me just draw the ground here. And so the first pumpkin catapult, let me just draw it right
over here, that's its space. And then this is the part that actually catapult the pumpkin. So that's what it looks like. It holds the pumpkin right over here. That's the pumpkin that's
about to be shot away. And what it does is once it releases, I guess someone presses a
button or pulls a lever, the pumpkin catapult is going to release, and so this arm is going
to turn a certain amount and then just stop
immediately right over there. And then that pumpkin, that
pumpkin is going to be released with some type of initial velocity. So your pumpkin is just
gonna go like that. So this is our small pumpkin catapult. This is our small one. But let's say we also have
a large pumpkin catapult. Let me draw that. So our large pumpkin
catapult right over here, similar mechanism. But let's say its arm
is four times as long. So, that looks about four
times as long right over there. So this is the larger one. The large pumpkin catapult. Let me make sure to draw the pumpkin to remember what we are catapulting. And then it will go through
actually the exact same angle, you go to the exact same angle, and then let go of its pumpkin. And this is gonna be a very useful video because you will find yourself making many pumpkin catapults
in your life. (laughs) And then it will let it go, and you will have some linear velocity. Now we know a few things
about these pumpkin catapults. Let's say the small pumpkin catapult, the radius between the
center of the pumpkin, the radius between the
center of the pumpkin and the center of
rotation right over there, let's say that this is r. While for the large one, this one is, this distance right over here, is four r. We also know the angular velocity while this thing is moving. So we know that the angular velocity here, let's say the magnitude of
the angular velocity is omega. It would actually be negative if we were to write it as a vector 'cause we're going in
the clockwise direction because that's the convention. But this right over here is the magnitude of the angular velocity and just to make that tangible for you, let's say that this is
two pi radians per second. And let's say this thing,
while it's in motion, it also has the same magnitude
of its angular velocity, so this thing right over here is also. The magnitude of angular velocity is once again two pi radians per second. So my question to you is
how would the velocity, the magnitude of the
velocity of the pumpkin being released from the small catapult, so v sub small. If I put a little arrow on top of it, we'll be talking about velocity. Since I didn't put an arrow, we're talking about just
the magnitude of velocity. You can think about this as the speed. How does this compare to v sub large? We have the same angular velocity but we have different radii. Pause the video and see if
you can figure that out. Well, the key thing to realize here, and we've seen this in multiple
videos, is the relationship between the magnitude of angular velocity and the magnitude of
velocity, linear velocity. The magnitude of angular
velocity times your radius is going to give you the
magnitude of your linear velocity. So for the small right over here, we could write this as v small is going to be equal to omega. These are the same omega, that omega and that omega is the same. In fact, we don't even
have to know what this is. We could say v sub small is equal to omega times our radius, which is r. And what's v sub large going to be? Well, v sub large is going to be equal to that same omega. So I'm talking about this
particular omega right over here. So it's that same omega but now, our radius isn't r, it is four r, so we're talking about, so times four r or if we were to rewrite this, this would be equal to
four times omega times r. Four times omega times r. And what is this right over here? Omega times r, that is the
magnitude of the velocity of our smaller catapult or the pumpkin being released
from the smaller catapult. So just like that, you see by having the
same angular velocity, but if you increase your arm
length by a factor of four, your velocity is going to
increase by a factor of four. And so you have the magnitude
of velocity of the pumpkin being released from the large catapult is going to be equal to
four times the magnitude of the velocity of the
pumpkin being released from the smaller catapult.