Newton's Third Law of Motion
Action-reaction experiments can sometimes be mishandled. You may also misunderstand what happens if you put a fan on a sailing ship. Please watch.
Experiment 1: If you put a fan on a sailing ship, will you move forward?
you need : Light ships such as PS, small motors, propellers, batteries, sails
Attach the propeller to a ship made of PS etc. What happens to the ship when I move the propeller?
扇風機が質量mkgの空気を速さｖm/sで押し出したとします。船の質量をM㎏として考えます。
Suppose an electric fan pushes out air with a mass of mkg at a speed of vm / s. Consider the mass of a ship as M kg. Ship momentum of change
M · V-M · 0 = F · ⊿t next air momentum transfer
m · v-m · 0 = F · ⊿t next, from two equations
M · V = -m · v Then, V =-(m / M) v.
The magnitude of the force received is F = Mα = MdV / dt = --mdv / dt.
This reaction causes the ship to move to the right. This is almost the same as when a rocket flies in the sky. This is easy to understand. Imagine a fan on a sailing ship. The bottom line is that the ship can move forward, depending on the size of the sail. If you like, please watch the video from the column below.
As shown in the middle figure, the reaction that the fan receives (blue arrow) and the reaction that the sail receives from the wind (blue arrow) are the forces applied to the ship. Now consider the reaction that the sail receives. Suppose the air hits the sail and returns in the opposite direction. In reality, not everything goes back, and the direction of return is not the exact opposite, but it simplifies the calculation. As usual, the change in momentum
Change in ship momentum M ・ V－M ・ 0 ＝ F ・ ⊿t
Change in air momentum m ・v－(－m ・ v) ＝ F ・ ⊿t
From the two equations, MV = 2m ・ v And V = (2m / M) v.
The magnitude of the force received is F = Mα = MdV / dt = 2mdv / dt.
It is up to twice as large as the reaction of the electric fan. The force that subtracts the reaction that the electric fan will receive from now on will be the force applied to the ship. Actually, it depends on the size and angle of the sail. The ship can move forward to the left, stationary or to the right. Please watch the video from the link below to see how it moves forward.
In fact, jet planes are landing using this principle. It's called reverse injection. The photo on the right is in the opposite direction of the figure. Apply the thrust of the engine to the reflector and put it out at an angle of 45 degrees. This reaction shortens the landing distance. The plane can also be backed up using this retrorocket.
Points that are easy to make a mistake due to action and reaction
There is an apple on the desk as shown in the figure below. Illustrate the reaction of gravity acting on an apple.
If you have such a problem, you will want to write the normal force from the desk, but the correct answer is the attractive force acting from the apple to the earth. It is due to the force of balance and confusion.
Correct the mistake in the following sentence.
A and B wrestlers played against each other, and A pushed B out. However, according to the law of action and reaction, the force that A pushes B and the force that B pushes A should be equal to each other. Therefore, A and B should not move. This sentence is incorrect only in this part of the conclusion "Therefore, A and B should not work." Since the force of action and reaction is the force acting on different objects, it cannot be synthesized. This is important. The conclusion is "But B was pushed out by the force applied by A."