Physics 3.5.4a - Projectile Practice Problem 1

[@michaelkhalil7368]

I never write comments so when I do you know I mean this so sincerely. Physics has never ever made sense to me since middle and high school. Now in college I gotta get it so I don't fail these classes. This is the first time I've felt confident and actually understood even slightly what I was doing. So thank you Derek!!

[@MovieTrim]

Wow the quality and organization of his calculations as well as pace and verbal description of the problem beats even KHANA. Great quality content I'm subbed

[@superbaby0907]

love the fact that its colourful! helps to clearify things a lot!

[@abhisheksaini-lw8li]

seriously, i think people like u deserve a great prize... ive physics exam tommorow and i didnt know anything... thanks very much

[@felixtira5245]

Thank you Mr. Owen! I've been frustrated for hours LITERALLY. This is the best video out there! 10/10!

[@itsjasminagain]

Thank you! I've been having so much problems understanding this and I have my first exam tomorrow.. This helped so much!

[@micheleloga2099]

Absolutely AMAZING! I can't get my Physics teacher to explain any of this to me and my book is too confusing to help me with these kind of problems. Thank you so much for doing this video! I understand it so much more now.

[@gggg-gu2hl]

Been 9 years.. how did you do?

[@derekowens]

Yes, that's correct. That's because for a projectile, there is no acceleration horizontally, so the velocity is the same the entire time, and you can find it with the equation x/t.

[@lizrey63]

omg you are soooooo helpful!!!!!!! im so glad i found your videos! thanks for taking the time to make these. you dont understand how helpful these are. thank you!

[@derekowens]

I typically use v_0 for the initial velocity. That's v with a zero in the subscript position. It basically means "velocity at time zero" or "velocity when t=0". Then I use v by itself for velocity at some later time. Using u works also, though. Good notation definitely helps, but it's not as important as the concept.

[@princesspat5239]

Thank you so much Im doing my first year of biomed but i didnt learn much about physics in high school. I know theres projectile formulas but I've been trying to find videos where they use the kinematics one since that increases your understanding. Thank you so much for making these, it was very clear!

[@claymountain1300]

this video saved my life!! got a test tomorrow morning, but I finally understand now!

[@gggg-gu2hl]

Been 9 years how was the test? Did you graduate yet?

[@derekowens]

@Ell4Sh Yes, that approach would also work. There are typically multiple ways to set up a problem, all of which should give the same final answer. I set it up with 0 a the top because all of the motion and the acceleration are downward, in this problem, and setting it up this way makes all the displacement, velocity, and acceleration numbers positive. The other approach is fine, though.

[@ntokotom147]

11 years later, this is still helpful. Thank you sir

[@Vlb361]

Thank you so much for making these videos! They made studying for my exam so much easier. :)

[@SBchampsXLII]

Thank you so much for uploading these videos. You explain things so well, and your visuals are amazing.

[@derekowens]

@omar3211 Acceleration near the earth's surface, due to gravity, is 9.8 m/s^2 (or very close to that). That's basically a constant (if you're on earth).

[@derekowens]

The key here is to distinguish between the vertical and the horizontal. Once the bomb is released, it is a projectile, and there is no horizontal acceleration. There is vertical acceleration, though: gravity is pulling it down. It also gets more complicated when you include air resistance. Air resistance introduces some horizontal and vertical forces, which depend on the speed.

[@Denseworldproduction]

thanks WOW! your 8 min tutorial rlly helped me do a problem that wasnt even the same as this, so basically you improved my understanding :)

[@joshuabrucetaylor9897]

I'm brushing up on my physics for the MCAT and these videos have been great. Thanks for uploading Derek, I really appreciate it :)

[@RamanShrikant]

how was mcat!

[@gggg-gu2hl]

Been 9 years what did you do? How’s everything?

[@aaravmaheshwari3733]

This literally just saved my whole grade, thank you

[@EmmanuelGINI-vj8qf]

Bro your teachings are very smart and easy Thank you very much

[@derekowens]

@SohrabR93 It depends on how the problem is set up. Typically a problem can be set up with up being the positive direction (in which case gravity is -9.8), or with down being positive (in which case gravity is +9.8). It can be done either way, as long as you pick one way and stick with it consistently through the whole problem.

[@derekowens]

@Star123Euro In this problem, the object is a projectile, which means its motion influenced by gravity only. And gravity pulls straight down. The force of gravity does not have any horizontal component, so the horizontal acceleration is zero as long as it is in free flight.

[@derekowens]

One of the key points of projectile motion is that the horizontal motion is independent from the vertical motion. There is no acceleration horizontally, but there is acceleration (downward) vertically. A bullet fired is accelerated horizontally by the gunpowder exploding behind it, but once it leaves the barrel then it is just coasting, under the influence of gravity alone (which is downward). While it is coasting, it is considered to be a projectile.

[@supajustin4285]

This is very helpful for my AP class. Thanks so much

[@currentlyindisguise8322]

I m indian and from half an hour I was dealing with the concept that u made me understand in few mins thanks a lot

[@SuperMmmm33]

you are amazing!, ths tutorial helped me with a problem that i've been struggling with for days! :)

[@scraggsds]

Thanks for posting this. Helped me understand it easier and made my life a lot easier.

[@derekowens]

At the start of a problem, you need to choose which direction you will call positive. You can call up positive and down negative, or you can call down positive and up negative. Pick one, and just stick with it through the whole problem. If up is positive, then the acceleration due to gravity is negative. If down is the positive direction, then the acceleration due to gravity is positive.

[@derekowens]

@MsBiebaholic Yes, that is correct. The two equations you mention are actually the same, since horizontally there is no acceleration so the 1/2 a t^2 term reduces to zero. The larger equation simply reduces to the smaller in this case. I don't know an any easy way to memorize the equations of motion, but even if it's just by brute force or practicing, memorizing them is certainly a good idea.

[@derekowens]

I understand your reasoning. The problem is how theta is defined. In this context, theta is the angle measured relative to horizontal. The equations Vx = V cos Θ Vy = V sin Θ assume that theta is the angle measured relative to the horizontal. If you called theta the angle relative to vertical, you could still solve the problem, but the sine and cosine would get switched. Hope that helps! Derek Owens

[@derekowens]

@HairtUB It depends on how the problem is set up. If it is set up consistently, then y and a will both have the same sign in that equation, and there would be no negative square root.

[@FHO72]

holy shit, i finally UNDERSTAND. Jesus christ. I am jumping with joy right now. Thank you for making sense. Thank you. Seriously.

[@Purpleye815]

Thank you for posting this is helping me study for exams

[@derekowens]

@804YankeeFan Yes, that's basically correct. When I throw a ball into the air, the rotation of the earth does not cause the ball to be "left behind".

[@derekowens]

@lidyaFACE There are usually two (or more) ways to set up a problem. The acc. can be either positive or negative, depending on how it is set up. For a projectile, though, the horizontal and vertical motions are always independent, and the acceleration of gravity only applies to the vertical motion. For a projectile, the horizontal acceleration will be zero.

[@janithaamarasinghe4954]

These were difficult at first but after watching these videos I understood the main concepts. I became an expert in projectile motion questions. Thank you so much for doing this :)

[@stevetari3990]

perfect explanation, i am sure i've gained something from it....

[@Palmar3s]

@juschecknin because gravity doesn't affect it because it's going horizontal, that is why there is no horizontal acceleration, but if it's vertical then yes, because gravity pulls down, gravity doesn't work from side to side (horizontal) therefore there is no acceleration. As far I have seen, every time acceleration is mentioned I think of gravity working on the vertical axis, pulling down, never it acts sideways, unless there is another force of acceleration. I think it goes like that :/

[@lylass1232]

Good video. It help me to understand better in this chapter. Good job!

[@Applecheria]

I am at the verge of failing Physics class. Chemistry was a mess and I do not want to Jeopardize my GPA! Thank you so much for your help!

[@derekowens]

@alkhor999 I typically use x to indicate the horizontal position and y to indicate the vertical position.

[@ItsChenice]

You sir, are a life saver!

[@SohrabRouzbeh]

thanks man,grate explanation,but i don't know why in some cases other lecturers put gravity as -0.98 but it wasn't negative here!

[@redcamel7965]

thank you very much for this great video, it really helped me all .

[@OmarSN93]

thank you so much :) i solved it before you started solving it and my answer was the same as yours :D

[@iibornfighter]

WOW you are the best!!! Way better the over rated khanacademy

[@derekowens]

@anoorcc Not in this case. If it were a symmetrical parabola, going up and then back down, then we could double the t to find the whole time. In this case, though, we only have half of such a parabola.

[@TheCarlosfgm91]

Awesome! thanks for the help! please keep on making physics videos, they're a ton of help! 5stars!

[@rdbanks2823]

Derek Owens - Thank you so much! Resolving in the Y direction makes so much sense - then you can find the time it would take to drop. This idea links nicely to your last video!!

[@PhilberthBundo]

It's very helpful, thankyou for the video

[@avgrech]

10x a million ma 2 morro i got a test aout thease and i was sick all week no idea what to do but now i got a clue 10x m* ur da best vry good explenation btw

[@duanoor3061]

thank you very much for this great video, it really helped me allot

[@HienNguyen-jv1px]

thank you so much ! this was so easy to understand and it also makes sense!!!

[@thepunisher1951]

Thank you very much. It helps me a lot

[@derekowens]

@804YankeeFan The buoyant force is very tiny, and is generally considered small enough to ignore. One could, though, include both the buoyant force and the air resistance in the calculations. The calculations get beyond the scope of this course, though.

[@lonogugeadult]

@emmalainesmiles you'd use the equation y = yo + vo(t) + 1/2a(t^2) and plug in what you know for the vertical component to solve for t

[@riceisnice8643]

thanks for helping me I was on the verge of crying

[@utuntupamed6224]

Thanks man have finally made it in physics

[@isome9123]

Great vid as always! I have a question however (hope it's not too foolish): Why do we use the same t in the horizontal equation as the one we used in the vertical one? Maybe I interpret it wrong but doing so wouldn't it mean that it takes the same amount of time to move in a horizontal and vertical way independently? And if so, how do we know it does this? Hopefully I managed to explain my misunderstading well.

[@jioddon]

your d bosss...... u deserve a grammy mi bosss

[@Soccerstar7867]

thanks man helps alot. How smart can you get? and are you like a physics teacher, or like a prof? thanks again helps a lot.

[@mdewolfe33]

Got it perfectly right! Thank you

[@thejavellanas]

you explained very well. :)

[@AVerbene]

thanks to you, i am going to pass my physics midterm tomorrow

[@gggg-gu2hl]

Did you pass? It been 11 years!

[@arik2216]

Are you channel still active ? Where are your newest lesson teacher ? Could you provide the topics about Quantum physics, electronics component (semi conductor, mofset..) like how they work and made ?

[@TheOne-sp8vx]

Why is Y positive when it goes downwards? I always see people making Y positive upwards. Now this made me more confused😔

[@MohammedAl-Kobari]

Did you get the reason?

[@1234567890585]

Your tutoring skills is very good, but there's still a thing which I would like to clarify: when will you know that acceleration due to gravity is positive or negative? In what circumstances? because every in every Physics class, they will use positive and negative values of acceleration due to gravity. Please help. Thanks in advance.

[@mattmeyer8141]

Derek, Nice job explaining this problem!!!! What software/hardware do you use for your chaulboard?

[@LeonardATF]

Thank you I couldn't understand until I watch this

[@gggg-gu2hl]

Did you pass? It been 9 years

[@colorgreens]

your hand writing is beautiful! :)

[@jojosh234]

@jojosh234 the vertical distance should be at negative because it's going down to Y axis,

[@merlin88888]

im just gonna leave this here... cuz im not too sure if everything required here is in the vid. 1. A ball rolls off a table that is 1.5 m high and lands on the floor, 4.0 m away from the table. a. How long is the ball in the air? b. With what horizontal velocity did the ball roll off the table? c. What is the vertical velocity of the ball just before it hits the floor? d. What is the horizontal velocity of the ball just before it hits the floor?

[@RandomBoggoDude]

Nice vid, somewhere there is either a mistake in my books or with my interpretation of the formula. If I use the formula "delta x =[(Vf+Vo)/2*delta t]" i get exactly double your answer. That is if I use Vf as 0. Is using Vf as 0 wrong while working with projectiles or is the derivative formula I'm using wrong?

[@tinpot1978]

It's easier to use the equation Vo = range x square root of gravity divided by 2 x the height... i.e. 2 time the height is 200... 9.8 divided by 200 is 0.049... square root of 0.049 is 0.22135. times that by 95 (the range) and you get the answer 21.029. No need to find the time.

[@EpicJelly]

how is there no horizontal acceleration? the horizontal acceleration should be negative since it slows down as the object loses it's speed?

[@derekowens]

EpicJelly Yes, in reality it does lose some horizontal speed due to air resistance. If we neglect the air resistance, though, then the horizontal speed is constant, because gravity only acts in the downward direction, and does not speed the object up or slow it down horizontally. There are no horizontal forces, so there is no change in the horizontal speed. We could include the air resistance, but that changes the problems to a much more difficult problem. In this case we have made the simplifying assumption that air resistance can be neglected.

[@EpicJelly]

thanks a lot!

[@ryanjustinmarantan8146]

OMG TY FOR ASKING THIS QUESTION. AND THANK YOU FOR THE ANSWER, DEREK.

[@jcrlaptoptrading]

nice, thanks

[@harkendoich2]

If its motion is influenced by gravity only and gravity pulls straight down, why does the projectile travel 95m? I mean, if you dropped a bullet off the edge of the cliff it would fall straight down, but if it was accelerated out of a gun it would travel several hundred metres due to the acceleration. So in theory this projectile in the video must've had acceleration as opposed to just dropped off the edge?

[@ChaosGamerNick]

this video helped me so much thank you

[@RonaldTani]

Thank you job well done ✅

[@804YankeeFan]

@derekowens So, I have a last question I was wandering about. So are satellites orbiting the sun with the Earth while they are orbiting the Earth? Does this cause the Earth to be an inertial reference frame when calculating its orbit?

[@Nickmarvelouz]

here goes "THE ROCK" lol. thinking of wrestling while trying to study.

[@yadukrishnan]

😁

[@rockster16]

@RuthBuzzzzz It would be negative, but when you square anything its always the absolute value of that, so the negative or positive wouldn't matter

[@happyports2621]

THanks man!!!! It helps a lot dude

[@Heavenz94]

i think it was a lil bit too complicated. simply just find v initial first which is root 2*g*h. then use then formular v= u + at to get ur time. then substitute for speed = distance/time

[@FachrinabilPF]

how could you determine the way of which should you firstly use horizontal or vertical ?

[@softwareftw7080]

Thanks Derek I Solved it first made a mistake. Then saw you continue to work it and then reworked before you got your final anwser. Just one question in these questions should you always use the number to 2 decimal places.

[@derekowens]

There are actually some rules that tell us how we ought to round. The general idea is that the accuracy of our answer is limited by the accuracy of the given numbers. I discuss those rules (called "significant figures") in other places in the course, but don't always get into them when the main point of the discussion is some other important concept. In general, though, we don't need every digit that the calculator returns to be included in our answer.

[@T83707]

Derek Owens

[@rebeccacandranitasetiawan4721]

Isn't the horizontal is the x? and the vertical is the upward / the y? I'm confused.

[@coleader206]

Yup. You are correct.

[@diwakarsingh5466]

No you treat them as different the only thing common is the time which is to be calculated via downward component and horizontal component remains unchanged so use the calculated time in it I did it the other way used time outta accelerate motion and utilized it is horizontal

[@krayzie5968]

it may be stupid but i was following you until the very end when you went from X=Vsub0 t, To Vsub0=X/t. that maybe just automatic to some people but for me it confuses me.

[@rangiroa100]

Hi Derek. I got a question. An object fall to the earth at 9.8 meter per second. If the vertical distance is 100 meters, then 100 / 9.8 = 10.20 seconds. It would take 10.20 seconds for an object to fall 100 meters. Your calculation showed 4.52 seconds. Can you explain the discrepancy ?

[@JaredReabow]

Why did you not just use v^2-2aS=u^2 ? to find the horizontal

[@Stinglikeabee625]

i do not understand why the acceleration is positive in this problem! i thought because of gravity pushing down 9.8 would always be negative.

[@lidyaFACE]

1) how come there's no horizontal acceleration? 2) the way my teacher taught it, always use -9.8 for acceleration when an object is moving downwards because the object eventually slows down. hypothetically speaking, if you used -9.8, you would get the same number for the time but just negative. can I assume both ways are right but just make sure the number is positive because time can't be a negative number? lol

[@funforall6540]

respected sir you said that vertical initial velocity will be 0 but if we apply vectors initial velocity should be u sin(theta) and as it is a horizontal motion, so theta should be 90. now putting value of theta in u sin(theta) we get u sin 90 which as we know is equal to 1 as sin90=1. So please help me clear my doubt.

[@anuurcc35]

@ derekowens; When finding the initial horizontal velocity, should time be doubled @ 7:21 so that it account for the whole horizontal time. So instead of 4.52 should it be 9.04. Can anybody else see what I'm talking about as well?

[@ArrowedDragon]

THANK YOU! THE OTHER VIDEOS I'VE SEEN SUCK!

[@804YankeeFan]

@derekowens I was wandering If I drop a ball in the air, the air's bouyancy force is acting on me and accelerating me with the weight. Am I right? So why dont people consider the extra bouyancy force?

[@wasb911]

is it possible to find the speed if the vertical velocity wasn't 0 given the x and y distances only ?

[@kareemkourouma3976]

why is the intial velocity horizontally 0? wouldnt it need some velocity once it reached the edge of the cliff to keep moving horizontally.

[@omsnet63]

My only doubt is that if acceleration horizontally is zero the projectile should keep moving in the same direction (if its velocity is constant ) then why does it stop? ( i m sorry if its 2 obvious)