🦋 Jumping into a pool of water Vs Jumping off a tall building.

Spring board diving is an Olympic sport. The divers do not hit the bottom of the pool and get injured.

Those who jump off tall buildings hit the ground and die or is seriously injured.

Why is the 2 so different?

We see children enjoying themselves jumping into a shallow pool without harm. Why is the depth of the pool or the height we jump off important?

In this post, we will look at various  forces and the concepts of terminal velocity and law of conservation of energy.

Whether we jump off the spring board or a tall building, there are 2 forces that affect us. For both cases the effect of the 2 forces are identical. 

Gravity makes us gain speed at the rate of 9.8 m per second per second. The greater the height we jump off, the more speed  and kinetic energy we will gain.

Another force that is acting on us all the time to slow us down is friction.

Air is not dense and so the amount of friction is not much. The faster we move the greater the amount of friction. Friction  varies with the square of our speed. Doubling our speed quadruples the friction.

Friction slows us down while gravity increase our speed. The rate at which we gain speed will decrease from the initial  9.8 m per second per second.

If our speed increase to the point where the force of friction is exactly  equal to the force of gravity we stop gaining speed and travel at a constant speed. The constant  speed is know as Terminal Velocity.

Because air is not dense, the terminal velocity for a human falling is very high. We will keep falling untill we hit the ground.

Water is much denser than air so friction of moving in water is much greater. The friction slows down our speed quickly. If the depth of the water is sufficient, we will reach a final velocity of zero. We stop falling. 

The pool used for spring board diving is deep enough for the diver to stop the descent of the diver.

In the case of jumping off a tall building, friction does not increase enough to overcome gravity. Although the rate of acceleration decrease as we fall, we keep accelerating untill we hit the ground.

The amount of Kinetic  energy  (KE) a body is based on the fomula

KE = 1/2 m vv 

(Sorry I can get my hanphone  to type v squared)

m is mass and v is velocity.

When we hit the ground our velocity become zero. Our KE become zero ( mass x zero x zero is zero)

Because of the law of conservation of energy, the KE is not destroyed but have been used for something. 

Part of the KE was converted to the thud sound as we hit the ground.

Most of the KE was used to cause injury to our body.

The taller the building, the more KE we had and the more serious  our injury.

Jumping from the edge of a swimming  pool is safe because the height of the edge of the pool to the surface of the water is so small.

When you take your children to a beach where there are big rocks, make sure that the water below the rocks is deep enough before you allow your children  to jump off the rocks.