Gravity is a powerful force that has a fundamental impact on the way we live our lives. Even walking, which we take for granted, is not possible without gravity. Gravity provides the necessary downward force on our bodies which creates friction between our feet and the ground, allowing us to walk (push our body weight forward with one leg and then the other).
When astronauts tried to walk on the moon, they found it extremely difficult, as the gravity on the moon is approximately one sixth of what it is here on earth. When we jump into the air, even though it is only for a second or two, we can be said to be momentarily overcoming the force of gravity.
Engineers have designed many ways to overcome the effects of gravity. For instance, in a Dodge Truck commercial, a truck is dropped to the ground from a height of perhaps three feet. The truck should be damaged by this fall, but the truck is equipped with shock absorbers and springs. The shock absorbers and springs of the truck dissipate the kinetic energy of the truck falling, compressing them almost to the point where the bottom of the truck hits the ground. The truck, because of the shocks and springs, finally returns to its designed position, with the bottom of the truck a foot or so off the ground.
When other forces are combined with gravity, such as motion (the movement of an object), inertia (the tendency of an object to resist change with regard to movement based on its mass), or power (the ability to exert energy over time), it may be impossible to prevent an impact which will cause damage.
For instance, if you roll an egg along the ground downhill at considerable velocity towards a wall, you can reasonably expect the egg to break. Your arm provided the force (power) to accelerate the egg to a certain velocity (motion). That motion is being increased due to the acceleration of the egg down the hill (gravity). The egg will not drastically vary its direction and avoid the wall (inertia tends to keep it moving in a straight line). The combination of power, gravity, motion and inertia will probably be sufficient to result in an impact between the egg and the wall that breaks the egg. This impact is called the primary impact.
There is a further impact which takes place when the egg hits the wall; this is when the mass inside the egg impacts against the inside of the wall of the egg. The egg white and egg yolk are usually in liquid form, and though liquid has considerable mass, the liquid inside the egg will rarely be the cause of the egg shell breaking. If you put a steel ball bearing into a plastic egg, and then shake the egg, you can hear the impact of the ball bearing hitting the inside of the egg, and it is easy to imagine the egg cracking because of the steel ball bearing. The impact resulting from the ball bearing striking the inside of the plastic egg due to the motion or change in motion of the egg is called the secondary impact.
Scientists and engineers have been working for many years to reduce the effect of impacts, primarily in the automobile industry. Efforts to reduce the primary impact (energy absorbing bumpers, crumple zones, modified chassis construction) and efforts to reduce the secondary impact (airbags, padded dashboards, collapsing steering wheels, and seat belts) are commonplace.
STAGE 1 - PLANNING/ DESIGNING
Lesson Background and Concepts:
The egg drop project involves several physics concepts that we have studied in class and other concepts that you will have to research. The main concepts are:
1. Momentum
Momentum is a measure of an object’s tendency to move at constant speed along a straight path. Momentum depends on speed and mass. Within a closed system of interacting objects, the total momentum of that system does not change value. This allows one to calculate and predict the outcomes when objects bounce into one another.
When an object is moving, it has a non-zero momentum. If an object is standing still, then its momentum is zero. To calculate the momentum of a moving object multiply the mass of the object times its velocity. Momentum is a vector, which means that momentum is a quantity that has a magnitude, or size, and a direction.
2. Pressure
Pressure is the force per unit area applied to an object in a direction perpendicular to the surface. Pressure is calculated by taking the total force and dividing it by the area over which the force acts. Force and pressure are related but different concepts. A very small pressure, if applied to a large area, can produce a large total force.
3. Air Resistance
A feather and coin will fall with equal accelerations in a vacuum, but unequally in the presence of air. This is because the air molecules cause a frictional force that opposes the motion of the falling objects. This air resistance diminishes the net forces for each. This will be a tiny bit for the coin and very much for the feather. The downward acceleration for the feather is very brief, for the air resistance very quickly builds up to counteract its tiny weight and surface area. The feather does not have to fall very long or very fast for this to happen. When the air resistance of the feather equals the weight of the feather, the net force is zero and no further acceleration occurs.
4. Angular Momentum
Angular momentum measures an object’s tendency to continue to spin. It can be obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. A spinning body also possesses spin angular momentum.
5. Gravity
Gravity is a powerful force that has a fundamental impact on the way we live our lives. Even walking, which we take for granted, is not possible without gravity. Gravity provides the necessary downward force on our bodies which creates friction between our feet and the ground, allowing us to walk (push our body weight forward with one leg and then the other).
When other forces are combined with gravity, such as motion (the
movement of an object), inertia (the tendency of an object to resist change with regard to movement based on its mass), or power (the ability to exert energy over time), it may be impossible to prevent an impact which will cause damage.
Did your vehicle work? What would you change or improve next time?
