If you throw 1 kg cotton and 1 kg watermelon at somone, why does the watermelon hit you harder despite it being the same weight?


If you throw 1 kg cotton and 1 kg watermelon at somone, why does the watermelon hit you harder despite it being the same weight?

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Kinetic energy.

The energy you use up accelerating the object is stored in it. It is released on impact. It takes much more energy to get the watermelon up to throwing speed than I does the cotton.

Also, the cotton will lose more energy to air friction than the watermelon due to much more interaction during flight.

Not a scientist involved in kinetic energy etc but here’s my take.

Assuming it’s thrown at the same speed it’s because of the density. A kilo of watermelon takes up one tenth of the volume of cotton wool. When it hits you, all the energy of the watermelon is transferred to your body (more or less) in one go. The cotton strikes you but it also has to squash (because it’s fluffy) so it takes longer to transfer the energy.

If you compress the cotton wool into a small block, it would hurt like hell.

The cotton deforms, absorbing the energy of the impact and spreading out the deceleration in time (longer time to stop = less force). The watermelon is rigid so things immediately proceed to “let’s see who breaks into pieces first”. If you compress the cotton *real* hard into a ball and shrinkwrap it in something that can hold it in that shape, it can definitely conk you at least as hard as a watermelon. This is the same with snowballs: if you make two identical snowballs that are loosely packed, then go ham and press the hell out of one until it is basically a chunk of ice, it will hurt a lot more on impact than the other, soft version (and still weigh the same, though in a smaller form).

The cotton would be less dense which gives it more air resistance, and the cotton can deform on impact so the force is spread out over more time. If the cotton were tightly compressed to the same size as the watermelon it would have a more similar impact.

Well of you compressed the cotton and somehow made it just as hard as the watermelon i would imagine the hit would be the same. Basically cotton is soft and melon is hard.

The real factor here is the area of damage. Watermelon is very dense, thus, it will hit in a smaller area causing higher pressure.

While cotton will hit you on a larger area, this pressure will spread and lose its power.

Pressure = Force/Area. Less area will make the pressure go up.

If the cotton was pressed and cubed in a 5inch by 5inch cube (arbitrarily and just for clarification) then the watermelon and cotton will do feel the same when they hit.

Think of it like water in the shower. When you increase the pressure by turning the shower head, what its doing is making water flow throw smaller holes which increases its pressure.

The watermelon is denser than cotton i.e There’s more material packed in it per unit volume than it is in cotton. That explains why the watermelon feels harder. Additionally when hit with a watermelon a smaller area of it actually contacts your body so the melon’s kinetic energy is dissipated over a smaller area. Also a 1kg ball of cotton thrown at you with the same initial velocity and from the same distance will slow down faster compared to the melon due to air resistance. It’s kinetic energy right before hitting you will be much smaller than that of the melon.

To add to other answers you could think of it like this:

1kg of water vs 1 kg of solid ice thrown at you.

The water is highly deformable and will flow around you spreading its energy over a large area and a longer duration of time. This means the overall force is reduced.

The ice does the opposite.