Why do people bend their knees when they land from jumping?
Topics Covered: momentum, impulse, impulse-momentum relationship
Let’s try something: stand at a height (whether it’s the chair you’re sitting on, table, bed, or whatever) and jump to the ground. Now freeze, and look at your knees. What do you notice? You should have noticed that you are bending your knees when you land from the height. For most of us, it feels “natural” to bend our knees when we jump to the ground; or at least, it feels more comfortable than trying to straighten our legs when we land (and I hope you don’t try this because it will hurt…) But why does it feel more comfortable? In order to answer this question, we’ll talk about momentum.
Momentum, also known as the moving inertia, is the product of mass and velocity. Because momentum depends on both mass and velocity, a lighter object can have the same momentum as the heavier object if it is moving at a faster speed.
When we talk about momentum, we usually refer to the change in momentum, so we use the symbol Δp to represent the change in momentum. Δp = mΔv (usually mass is constant).
Another concept we need to look at is impulse. Impulse is a force applied to an object for a certain amount of time. It is represented by J, and calculated as J = FΔt (force times change in time).
Now, here’s a very important concept: impulse causes change in momentum. In other words, if you apply a force to an object for a certain amount of time, it will change the momentum (usually velocity) of that object.
If we put that all together, we get this important equation: J = Δp = FΔt = mΔv!
So if you want to change momentum as much as possible, you should apply as much force as possible, for as long as possible!
Finally, let’s use this concept to answer our original question. When we jump from the height, our change in momentum stays the same (since our mass and change in velocity will be constant). So if we look at the equation Δp = FΔt, we can tell that F and Δt have an inverse relationship. That means if the time of impact increases, the force exerted will decrease to keep Δp constant. Now that means, when we bend our knees while jumping, our time of impact will increase (we’re hitting the ground at a slower time than when our legs are straight), and therefore the force exerted will reduce!
This is important because less force exerted prevents the chance of damage to our body! Now you know why you have to bend your knees when you land from jumping.
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