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# Momentum: Ice skater throws a ball | Impacts and linear momentum | Physics | Khan Academy

Welcome back. I’ll now do a couple of more

momentum problems. So this first problem, I have

this ice skater and she’s on an ice skating rink. And what she’s doing is

she’s holding a ball. And this ball– let me draw

the ball– this is a 0.15 kilogram ball. And she throws it. Let’s just say she throws it

directly straight forward in front of her, although

she’s staring at us. She’s actually forward

for her body. So she throws it exactly

straight forward. And I understand it is hard to

throw something straight forward, but let’s assume

that she can. So she throws it exactly

straight forward with a speed– or since we’re going to

give the direction as well, it’s a velocity, right, cause

speed is just a magnitude while a velocity is a magnitude

and a direction– so she throws the ball at 35 meters

per second, and this ball is 0.15 kilograms. Now, what the problem says is

that their combined mass, her plus the ball, is 50 kilograms.

So they’re both stationary before she does

anything, and then she throws this ball, and the question is,

after throwing this ball, what is her recoil velocity? Or essentially, well how much,

by throwing the ball, does she push herself backwards? So what is her velocity in

the backward direction? And if you’re not familiar with

the term recoil, it’s often applied to when someone,

I guess, not that we want to think about violent things, but

if you shoot a gun, your shoulder recoils back,

because once again momentum is conserved. So there’s a certain amount of

momentum going into that bullet, which is very light

and fast going forward. But since momentum is conserved,

your shoulder has velocity backwards. But we’ll do another

problem with that. So let’s get back

to this problem. So like I just said, momentum

is conserved. So what’s the momentum at the

start of the problem, the initial momentum? Let me do a different color. So this is the initial

momentum. Initially, the mass is 50

kilograms, right, cause her and the ball combined are 50

kilograms, times the velocity. Well the velocity is 0. So initially, there is 0

velocity in the system. So the momentum is 0. The P initial is equal to 0. And since we start with a net 0

momentum, we have to finish with a net 0 momentum. So what’s momentum later? Well we have a ball moving at

35 meters per second and the ball has a mass of 0.15

kilograms. I’ll ignore the units for now just

to save space. Times the velocity

of the ball. Times 35 meters per second. So this is the momentum of the

ball plus the new momentum of the figure skater. So what’s her mass? Well her mass is going

to be 50 minus this. It actually won’t matter a ton,

but let’s say it’s 49– what is that– 49.85 kilograms, times her new velocity. Times velocity. Let’s call that the velocity

of the skater. So let me get my trusty

calculator out. OK, so let’s see. 0.15 times 35 is

equal to 5.25. So that equals 5.25. plus 49.85 times the skater’s

velocity, the final velocity. And of course, this equals

0 because the initial velocity was 0. So let’s, I don’t know, subtract

5.25 from both sides and then the equation becomes

minus 5.25 is equal to 49.85 times the velocity

of the skater. So we’re essentially saying that

the momentum of just the ball is 5.25. And since the combined system

has to have 0 net momentum, we’re saying that the momentum

of the skater has to be 5.25 in the other direction, going

backwards, or has a momentum of minus 5.25. And to figure out the velocity,

we just divide her momentum by her mass. And so divide both sides by

49.85 and you get the velocity of the skater. So let’s see. Let’s make this a negative

number divided by 49.85 equals minus 0.105. So minus 0.105 meters

per second. So that’s interesting. When she throws this ball out at

35 meters per second, which is pretty fast, she will

recoil back at about 10 centimeters, yeah, roughly 10

centimeters per second. So she will recoil a lot

slower, although she will move back. And if you think about it, this

is a form of propulsion. This is how rockets work. They eject something that maybe

has less mass, but super fast. And that, since we have a

conservation of momentum, it makes the rocket move in

the other direction. Well anyway, let’s see if we

could fit another problem in. Actually, it’s probably better

to leave this problem done and then I’ll have more time for the

next problem, which will be slightly more difficult. See you soon.

63 comments

Thanks

THEN DELETE IT

hello im from brazil

i`ll watch all your videos in these hollydays

ur helping me a lot

thank you

Unless they're self-aware robot-guns. Then we're all doomed.

DANG SHES HOT!

If it's defense against roboguns, I totally agree with you. Also zombies. Without guns they'd be roaming this planet freely.

You have good taste. 🙂

the whole time i just wondered what iceskating had to do with this problem… i get it now though!! thanksssss!!!!!!!! 🙂

35 m/s?

quiet an arm she got there for an ice skater

XD

our science teacher showed us this video in class. btw me and my friends named the ice scater sally 🙂

@pregnantlameboy lol-=)

shouldn't you have also taken to account the mass of the ball in the initial momentum calculation (50+.15kg) even though it will end up being multiplied by 0 anyways?

This helped me a lot! Keep up the good work.

@RE90 the ball AND the skater have a mass of 50kg

is that Justin bieber?

@frozenlicks She is a girl! and he is ___!!

@BeeezyFBaby23 No, it's the other way around. The equation is this:

-5.25 = 49.85 x Vs

We want to find Vs, right? To isolate it, we need to get the coefficient over to the other side, so we divide -5.25 by 49.85.

Think about it like this…

4 = 2x

How do we find "X"?

We divide both sides by 2, so we get 2 = X, which makes sense because 2 * 2 = 4.

Who is Justin bieber?

0:12 "lets just say she throw it directly in front of her although she's staring at us" hahahaha funny

Even though it does not matter, P_i should be 50.15 kg* 0 m/s and not 50 kg…Even though it doesn't really matter either way, it might give some people the impression that the mass of the snowball doesnt have to be accounted for.

this video was great

Your The Best , I just feel comfortable when i hear your voice

Can you do more momentum problems? Please.

I always like to learn on my own, but the physics book is worse than japanese brail to read. The real problem though is senioritis right now.

She's Iceskating in space. Explains the lack of friction friction and perfectly straight throw 🙂

@ one que what if i produce a accleration of 2m/s2 in a very heavy body lets say bout 1000 kg

so it have a force of 2000N now if this body struck a very light mass body like a ball of 1kg it will produce a huge accelaration in isn't it but does it not get the reaction force of 2000N back according to newton's third law of motion

pls reply

yeah since shes stationary before she throws the ball

Shouldn't her mass be her + the ball? 50 + .15?

no because he said that the ball+her weigh 50

ow ok.

its good to know youre concentrating in this video lol

i have made a contrifbution to your foundation

it is small but remember im a student, when i graduate ill hook you up more

Well eventually she'll come to a rest and yes it will equal zero. But, when she throws the ball she is moving backwards at .10 m/s.

Hey thanks for posting. I'm a huge nerd and I like this kind of stuff.

THANKS FOR THE SUBTITLES!

Sir, you explained in a matter of 20 minutes or so what my teacher could teach in 5 80 minute classes! thank you! im sure you get this a lot but you are helpful beyond belief!

She's an ice skater, can throw a ball straight ahead without looking, and she conserves momentum!! Looks like I found a wife!!

There are no calculators on the MCAT. It would be useful to show approximations, not exact calculations.

you can hear him fart around 1:18

helpful video

it would be useful to learn how to do them yourself.

Why do the last 5. something became negative?

Violent things… Since when are guns violent things?

Since when was just shooting a gun violent?

Why does the momentum has to stay zero?

LOL "although she is staring at us, she is forward for her body…"

LMFAO so a 110 pound girl in ice skates can throw a baseball (same as 0.15kg) 78 mph in a perfect horizontal direction! she must be the most freakishly athletic female in the history of sports!

why is shooting a gun violent? just because some people use gun to commit violent crimes doesnt mean the object/ tool should be scary…..

Thank you for your sharing and teachings

h

Heyyy! You have to convert the kilograms to grams first

give the bloody question

why add the skater's momentum as final momentum?

1:18

Only in America a highly educated public figure will use terms like recoil and examples of shooting a gun. Proud to be an American! MAGA!

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It is not possible to throw ball without applying inclined pressure on earth….

video is 240p, but its "quality" is over 1080p.

Thank you so much. This really really helps! You are amazing.

"the minus is too low, ignore it" LOLZ

That is one creepy figure skater…

Can we say that the recoil is a feedback of a certain force?

Omg. I finally understand this :-: thank you so much khan academy