Rocket League ball physics – Rocket Science #4
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Rocket League ball physics – Rocket Science #4

December 13, 2019


Hey guys, welcome back to Rocket Science. We’re going back to physics in this episode. You guys voted on twitter and “Ball physics”
won the poll so lets talk this topic through. First we need to talk about units. The engine uses a custom unit to measure distance. The Unreal Unit. You already know this unit from your camera
settings. 100 height means that the camera is 100 unreal
units above the ground. If you don’t want to get spoiled about the
Rocket League lore you might want to stop watching now because I’m about to reveal the
size conversion that the game uses. Alright? 1uu is equal to 1cm. I will be using the unreal units in this video
because the values make the most sense but I’ll put the conversions on screen whenever
possible. There are some boundary values in the Rocket
League physics. The ball and car both have a maximum speed
at which they can go. For the car this is 2300 uu/s and for the
ball it’s 6000 uu/s. The maximum spin that the ball can have is
also limited to 60RPM or 1 full turn a second. Another important number is the gravitational
acceleration. It is 650 uu/s² which is 2/3
of the gravity on earth. This means, whilefalling, the ball will increase it’s speed
downwards by 650 uu/s every second it’s falling. So how does the ball behave once it’s in the
air? Air resistance does not work like it does in real life. Spin doesn’t affect the
ball while it’s in the air. The ball does slow down however. It will lose 3% of it’s
speed every second or in other words it will half it’s speed every 22 seconds. Obviously
it will never be in the air that long so we can conclude that waiting for the ball to
slow down significantly when it’s in the air doesn’t work. On the ground there is some sliding friction
on top of this. It slows the ball at a rate of 230uu/s². Because the ball can’t spin
faster than 1 round per second it’s not fast enough to roll without sliding unless the
ball moves at a speed of less than 1 time it’s perimeter per second which is around
565uu. So once it gets that slow, there will be no sliding friction and the airdrag gets
counteracted by the rotational energy. The ball only slows down about 2.2% per second.
If the ball has sidespin it will also have a much bigger impact at those low speeds.
When the ball moves at a speed of less than 40uu/s for 2.5 seconds and has a spin of less
than 10 RPM it will just stop. If you can get it to spin perpendicular at a higher rate
than 10RPM it will never stop. The horizontal speed should never reach 0 either but at some
point the ball will keep the same position because of rounding errors in floating point
arithmetics. Before the ball goes sliding or rolling on
the floor it will have to bounce first. I need to clear up a common misconception right
here. Many think that the angle of incidence has to be the angle of reflection on a bounce
without spin but that is unfortunately not the truth. This happens with light but with
a normal ball you have friction and the strength of reflection to consider.
You can split a bounce that is coming in at angle into two different components. One is
parallel and the other one perpendicular to the wall. The physics of the latter are really
easy to explain. In Rocket League you will always get back 60% of the perpendicular momentum
in a bounce. This means if the ball is coming in with a perpendicular momentum of 1000uu/s
it will have 600uu/s in the opposite direction after the bounce.
Sadly you can’t calculate the parallel compenent this easily. Friction depends on the perpendicular
force, and the spin of the ball can affect the bounce to some degree.
What will hopefully help you alot more than a formula is explaining the general idea.
Since more perpendicular force will increase friction, a shallower angle also means less
friction. So if a ball is moving down the field and just barely touches the wall the
parallel speed will decrease less than it would if it bounces at a 45° angle. Now let’s get to arguably the most important
part. The car-ball interactions. The ball doesn’t actually interact with the model of
the car. All the cars have a seperate hitbox in the shape of a rectangular cuboid. The
slope of the octane is just for the looks and doesn’t do anything to the ball. That’s
not all though. Rocket League has another anomaly that you wouldn’t expect. In real
physics when the ball bounces on a surface like the cars roof there is something called
the impact normal which is perpendicular to the surface. The ball would behave the same
way it would with a wall bounce given the angle to the impact normal. In Rocket League
the impact normal isn’t perpendicular to the surface. Instead it points from the center
of mass of the car to the point of impact. This was done in order to make hits more consistent.
The real world impact normal changes drastically on an edge which would make those touches
almost unpredictable. What does that mean for the ball though? A
real-life object where every impact normal points from the center of mass to the surface
is a sphere. Given an impact point on the car we can extrapolate the position on a virtual
sphere and predict the bounce based on that. Essentially it becomes a game of pool with
the only differences being that that not all the balls are the same size and the car is
10 times the weight of a ball. An implication of this is that it’s impossible to have a
ball resting completely still on a car. That might seem bad at first but it’s actually
what gives you the control needed to balance the ball on your roof. Increasing speed, decreasing
speed and turning would usually not be possible with a flat shape. If you’re new to dribbling
try to picture your car as a sphere. The balancing act is pretty much the same as 2 spheres stacked
on top of each other. Okay, direction is one thing but how do we
hit it with the most power possible. You’ve probably heard somewhere that you should hit
the ball with the front of your car. Why is that? The front of your car is not inherently
more powerful. You can actually hit the ball with the top of the car for the exact same
power. The power is determined by combining 3 things. The speed of the part of the car
that hits the ball, the impact normal and the center of mass of the ball. In order to
get the most power you want them all to line up. The speed of the part of the car that
hits the ball going in the same direction as the impact normal and both pointing straight
through the middle of the ball. Knowing this, it’s pretty easy to explain why you usually
want to hit the ball with the front. Say the car is traveling in the direction of it’s
roof. If you don’t get the touch on the ball perfectly above the cars center of mass then
nothing is going to line up. Because of the shape of the hitbox the front is much more
forgiving. Add to that the fact, that your boost is at the end of your car and that there
is no flip that goes towards your roof and you get the reason as to why you almost never
see an insanely powerful roof hit. There is another part of your car though.
The wheels. They have a seperate hitbox that is connected to the rest of the car. If you
look at the cars model you can see that the wheels are not in a fixed position. The same
seems to be true for their hitbox. When you hit the ball with them they work like shock
absorbers which is why wheel hits are usually very weak. Alright that’s going to be it for this video.
I’ve decided that I want to cover redirects and pinches in an extra video. If you have
any further things you would like me to test regarding ball physics please tell me so I
can add them to my list. If you want to stay up to date please go follow my twitter.

Only registered users can comment.

  1. There's a could elements to power that you didn't mention, like orientation velocity and how hitting it with the corner of the front hitbox's right/left bumper can add more power.

  2. ah, and i thought hitting the corner front of your car will generate the most power. Great analysis by the way! 😀

  3. The center of mass you showed in the breakout isn't in the middle of the hitbox, so it is based on the actual shape of the car's model? If so, is there like extra logic to it like the motor having more weight or is it just the "middle" of the shape?

  4. Wait, wait wait wait wait! Hold on there for a second.
    I just can't get my head around the fact that there is 0% effect of the spin while the ball is in the air!
    Is the Ball camera that confusing to our eyes? I could swear i saw the Ball curving into the goal just so often…
    Please, i need an answer to this!
    It's just so important to calculate an aerial hit as well!

  5. Great video! Even though some of this is just trivia I found it very interesting. Especially because I had always been wondering how exactly consistent hits were possible with that hitbox. The amount of effort you seem to put into these videos is amazing.

  6. I remember reading somewhere that the top speed of your car gets clamped, and if you accelerate in a different direction, you will be slowed down in the direction you're moving to keep your car going at the same top speed even if there is no other force pushing in that direction. What I wonder is if this would also apply to the wheels of the car when they move up/down, and if this would affect the rest of your car in any way. I have no idea how this would be tested or if it would be significant at all but it's just a thought I had.

  7. lul how the ef do you have <2,000 subscribers? you deserve atleast >2,000,000. Your videos make Rocket League a science which makes it way easier to understand.

  8. I've just finished watching this video, and I'm literally speechless since I have been wondering how the RL physics work for a long time. What a great work !!

  9. Hey guys,
    unfortunately centre of mass car -> impact location is not the correct vector that the game uses. It's centre of mass car -> centre of ball. In a lot of cases that is almost the same direction and many of the things I said in the video still apply. For the full explanation please watch Episode 6!

  10. "Dribbling is pretty much the same as 2 spheres stacked on top of each other"
    Thank you so much for that quote. I've been visualizing it as a box this whole time and haven't been able to properly wrap my head around how the dribble actually works, but this quote actually makes it all make sense

  11. I can't play this game because it fucks with your intuition.
    The whole speed at which a ball falls down is so tentative you keep doing it wrong etc…

  12. Would you ever consider doing a video on dodge physics? I think a lot of people who frequent the subreddit has a general grasp of how it works, but I want to make sure I know with confidence that the my understanding is correct

  13. Does rotating your car give speed to the part of your car that you hit the ball with? like if I flat hit a ball vs air-rolling/flipping into a ball, does the rotation give extra speed? I don't think the physics works that way but Im not sure

  14. I think the part about the suspensions working as shock absorbers in rocket league is wrong? I remember Jared Cone talking about the suspensions in his GDC 2018 talk "It IS Rocket Science! The Physics of 'Rocket League' Detailed" and mentioning that the suspensions are actually just an effect in the graphics engine and not handled by the physics engine at all. Maybe the bottom is just composed of another physics material acting like a dampener when the ball hits it?

  15. I have a question guys,: If my car in rocket league is driving forward at a speed after i used 50 boost and i hit the ball, and some guy at a standstill just taps it, shouldn't i win the 50/50 due to the force? And wouldn't in reality the force i hit the ball with my car send the other guys car backward because of the force my car exerts on the ball? Thanks for the reply.

  16. Can you tell, how long the maps are? I think this is important, when we know the speed, lenght of the car etc.

    And thanks for the great work! Your videos are awsome!

  17. That’s why I can’t read bounces. I’m champion 2 but I still can’t read some bounces and that’s because it’s unrealistic. I play football a lot so I know my bounces very well but in rocket league I don’t and it screws me up alot

  18. Is there a plugin or some setting where the ball height/position is measured in real time? I'm doing a study of it for school.

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