(STF) Newton's Third Law Lesson
Newton's Third Law
A force is exerted on an object when that object interacts with another object in its environment. Newton's third law of motion states that for every action there is an equal and opposite reaction. This means that there are always opposing forces that control the movement of any object. We can demonstrate this with many toys that we are familiar with.
Think of a toy rocket. The thrust provided by the rocket engine pushes down and in return the rocket moves in the upward direction. The amount of force that the rocket engine provides is greater than the force of gravity that pulls the rocket toward the ground. This allows the rocket to move in the horizontal direction. Other toys use this principle, as well.
A Newton's cradle device suspends steel balls on strings that are arranged in order under a wire rack. As one ball is allowed to hit the system, energy is transferred through the other balls to the last one in the line. If one ball is pulled and released, one ball at the other end will be propelled in the opposite direction at the same magnitude. If two balls are pulled and released on one end, then two balls will be propelled in the opposite direction at the same magnitude. This is a perfect example of Newton's third law, for every action there is an equal and opposite reaction.
We also need to look at other types of force interactions that involve Newton's third law. As any object moves on Earth, it encounters friction. Friction is the resistance to movement caused by interactions between two substances. Friction occurs due to the attractive force between two objects.
Surface structure plays a big role in frictional force. When substances have many ridges and valleys, the frictional force is greater. If a surface is smooth, the force of friction is much less. Many toys utilize frictional forces to operate. Air hockey is a great example. Air flowing through small openings on the table provides a cushion of air that reduces the friction between the puck and the table. This allows the puck to move with great speed and allows the player to control their shots better. Other toys, such as friction cars, utilize friction to wind a spring and store energy. The user pulls the car in reverse and as the wheels encounter the friction of the ground, a spring is wound around the axle. This energy is released as the spring uncoils. The car accelerates as the spring releases its potential energy.
Friction
There are several types of friction that occur when objects are moving past one another. Friction occurs when one object is interacting with another object.
We can use our toy car again as an example; as the car rolls along a flat surface its inertia should continue its motion indefinitely according to Newton's first law of motion. If the car's components were frictionless, and the car was rolling along a totally friction free surface in a vacuum, that would be the case. In a real world application, friction is an opposing force that slows the car down.
In this case, only a small amount of the wheel interacts with the surface and friction is very low. This component is called rolling friction. The other forms of friction acting upon the car are sliding friction as the axle rotates inside a metal tube and wind resistance as the car moves through the atmosphere (also called drag). All of these forces act upon the car, causing it to slow down over time. The amount of friction changes depending on the two surfaces involved.
Think about it this way. On a snowy day, a sled sliding down a hill will go fast. It will slide on the snow because the surface is very slick and sliding friction is very low. If the same sled were sliding along the bare ground, friction would be very high and the sled might not move at all. As you can see, friction plays an important role in how objects move. When friction is low, objects move quickly. When friction is high objects move slow. This is all due to the resistant force in the opposite direction of the movement.
Weight
Some forces we experience are so familiar to us we never really think about them. Weight is a great example of this. We know how much we weigh, but do we know how it is calculated? We simply step on a scale and rely on the reading to tell us our weight. Sometimes we see weight limit on toys, such as a swing set or a bicycle. Do we really know what it means? If we look at this mathematically, we can better tell what weight really means. Weight is a result of two components: mass and gravity. On Earth we can use a constant value to represent gravity. The gravitational pull on the Earth is fairly constant and we give it a standard value of 9.81 m/s2. If you move above or below sea level, this number will vary slightly. Unless you are calculating intricate scientific measurements, you can estimate the acceleration due to gravity. Let's look at how weight is calculated:
W = m x g
W = Weight (N)
m = mass (kg)
g = acceleration due to gravity on Earth (m/s2)
Gravity varies in the universe based on the mass of the object that is creating the gravity. Our moon is an independent body with a gravitational force equal to 1/6th of the Earth. Let's compare weight on Earth and on the moon.
Example 1: In the year 2050, a boy who has a mass of 45 kg is sent to the moon on a spaceship. When he gets there he wants to play with some of his toys. He reads on the side of his toy moon rover that on Earth the maximum weight limit is 100 N. Can he still ride his favorite toy while he is on the moon?
On Earth
W = m x v
W = (45kg)(9.81 m/s2)
W = 441.45 N
On the moon gravity is 1/6th that of the Earth. Instead of gravity having an acceleration of 9.81 m/s2 gravity will now be 1.64 m/s2. His weight would be calculated as follows:
W = m x v
W = (45kg)(1.64 m/s2)
W = 73.8 N
While on the moon he could still ride his toy but once he returned to Earth he would have to give the toy to his younger brother as he would be too large to ride the toy moon rover.
Consider this:
When an object falls through a vacuum (space that is empty of matter) , such as outer space, it is subjected to only one external force, gravity. Therefore, regardless of the mass of the item, it will fall at the same rate. Don't believe this is true? Check out the following video recorded on the moon during the Apollo 15 space mission. The astronaut will drop a hammer and a feather at the same time.
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