Egg on a Sheet
|Physics, Engineering:||Impulse, Force Transfers|
|Grade Range:||Middle School, High School|
This does require throwing eggs, so try to do this only in a large open space, and ideally outdoors.
- Fitted Bedsheet
- Eggs (Hard Boiled or Raw)
Please read the Flying Particles section of the Demonstration Safety page before performing this demonstration.
This demonstration requires: Safety Goggles
- Have two volunteers or co-presenters hold up the sheet. They should each hold a corner of the sheet, standing at the sides of the sheet, so that the elastic at the bottom of the sheet makes a pocket and is off the ground.
- Show the audience that you are holding an egg. Ask the audience if they think you can break the egg by throwing it at the sheet.
- Take aim, and throw the egg into the sheet. The egg doesn't break! take the egg out of the sheet to show that it is still intact. You can repeatedly throw the egg into the sheet, and it won't break!
- You can show the audience that it is a real egg after throwing it by cracking it open over a trash can or bucket. If it is a raw egg, be sure to wash your hands afterward.
Why This Works
This demonstration shows how Force, Momentum and Impulse are all connected. Force is mass times acceleration, like when a car is speeding up. Momentum is mass times velocity, like a car going the speed limit on a road. Impulse is when something has a change in momentum in a short period of time, like a car quickly slowing down. These three are all happening in this demo. When the egg is thrown, it is being accelerated forward by the throwers hand, and until it is released it is having a force applied to it. Once released, the egg travels through the air due to the momentum it has, since it is no longer accelerating forward and now travels at a certain velocity. Once it hits the sheet, it experiences the impulse, or sudden change in momentum. Usually, we see an impulse as a near-instant effect, like if the egg splattered against a wall. The sheet, however, absorbs the impact and allows the impulse to take longer. This means the momentum has more time to change, and so the egg doesn't break!
Force, Momentum and Impulse are all related to each other by the following equations:
|Momentum||p = m * v||d/dt p = m * v/t = m * a = F|
|Force||F = m * a||∫ F dt = F * tf - F * ti = F * ∆t = I|
|Impulse||I = F * ∆t||I = m * a * ∆t = (m * vf) - (m * vi) = ∆p|
Momentum is when a mass has a velocity to it. Force is when a mass has an acceleration to it. The derivative of momentum is force, since the derivative of a velocity is equal to acceleration. The integral of force is called impulse. Impulse is a change in momentum over time, and can be either positive (indicating an increase in momentum) or negative (indicating a decrease in momentum). If there is no change in momentum over time, then the impulse is equal to zero, and you would use the basic momentum equation. By connecting these three equations, we can accurately map out the situation that happens with the egg.
|Part of Movement||Egg Wind up||Egg Release||Egg Traveling Through Air||Egg Hits the Sheet||Egg Stopped by Sheet|
|Movement Type||Force (Positive)||Impulse (Positive)||Momentum||Force (Negative)||Impulse (Negative)|
|Equation||F = m *a(+)||I = (m * vf(+)) - (m * vi(0)) = (m * vf) - 0||I = 0 since vf = vi, so p = m * v||-F = m *a(-)||-I = (-F * tf(+)) - (-F * ti(0)) = (-F * tf) - 0|
When the egg strikes the sheet at the end, it does not break because of the time it takes for the egg to fully stop. We normally are used to seeing impulses acting on things instantly, like if the egg were to be dropped on the floor. This is because of the hard surface not allowing any recoil to happen, and therefore the eggshell having to try and absorb all of the damage from the strike. With a hard surface, the deceleration is too fast for the egg to handle. The sheet allows give, and with that the egg can take a slightly longer time to stop. This time difference, even though it seems to be only a little bit longer, makes a big difference in how well the egg can handle the force of impact. With a longer stop time, the egg experiences less force per time interval, which allows the egg to avoid breaking!