# Why does a cup full of liquid not spill if it hold it up high when a car goes over a speed bump, but spills if I don’t?

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If I know a bump is coming while I’m driving and I hold my cup up high it doesn’t spill. But if I leave it in the cup holder or even hold it around my stomach area, it will spill. I know it’s a physics thing but I never took physics. What causes this?

In: 2

It’s not the height AFAIK. It’s likely you’re holding it too tightly when it’s closer to your body, which restricts the movement of the cup but not the liquid inside. Basically you want the cup to move with the movement of the vehicle, because that’s what the liquid is doing, but the movement of the liquid is slower than the speed of the vehicle

Edited for clarity

Your body acts like a shock absorber. When you go over the bump you can keep your hand in the same plane to compensate for the bump. When it’s in the cup holder it can not compensate for the bump.

You’re using your arm as a shock absorber, even if you don’t realize it.

As you go over the bump, you’re lowering then raising your hand so that the cup stays at close to the same height. Because the cup isn’t really changing height, its the same as if it never went over the bump to begin with. The lower you have the cup, the harder it is for you to do that.

For one: the lower the cup, the less room you have to lower your arm. But the higher you hold the cup, the more you can just let your elbow bend, which is something it can do naturally without you really needing to think about it. The lower you hold the cup, the more you need to rotate your shoulder to keep the cup at the same height, which is a much more complicated movement that requires a lot more thought to do.

There’s also the element of “knowing” where the horizon is so you can hold the cup horizontal thus minimizing the chance of spilling it’s contents. It’s easy to see the extremes of this learning. Give a bowl of water (filled to within an inch of the top) to a 5 year old and ask him/her to walk across the room with it. She/he will look at the bowl of water while walking in an attempt to keep it level. An adult will “look ahead” to avoid tripping over carpet wrinkles, bumping into furniture, etc. That’s because an adult has learned through experience to intuit horizontal. The kids will learn soon enough.

Same way I had to teach my kids to carry drinks without lids. Away from your body, your arm can keep it from spilling pretty easily. Tucked against your chest and it’s spilling and sloshing every step

These were all super helpful, thank you!