1. Solving mechanics and physics problems in junior high school

Option a, because it is a uniform decrease, so the balanced force, the object subject to the balanced force, the two forces are equal. The two forces experienced by the object are the upward air resistance and the downward gravity of the person and the umbrella, and these two forces are equilibrium and are therefore equal.

I choose AUniform motion and stationary objects are subject to the balance force, people and umbrellas as a whole, subject to downward gravity and upward resistance to do uniform motion, these two forces are the balance force, the balance force is equal in magnitude, the direction is opposite, acting in a straight line,

A When falling at a constant speed, the net force of the system is 0, so according to this theory, the two forces must be equal, and the force on the person and the umbrella has two kinds of gravity and resistance, so I choose A, I hope it will help you.

Select A, falling at a uniform speed, indicating that the person and the umbrella are affected by a pair of balanced forces, and the person and the umbrella are subjected to vertical downward gravity and vertical upward air resistance.

Option A: According to the law of inertia, the object remains in uniform linear motion or at rest when there is no force or the net force is zero.

Now the paratrooper falls at a constant speed and satisfies the uniform motion The resultant force is zero, and the paratrooper and the parachute are only balanced by gravity and air resistance, and the gravitational force on the person and the parachute is equal to the air resistance.

Passing by in the third year of high school... It seems that it should be a because it is falling at a uniform rate, so the drag should be equal to the gravitational force received.

a, as long as you see the word constant velocity, you can do it. If the speed is constant, the force should be balanced, so choose A.

A Newton's first law states that an object does a uniform linear motion or rest with zero resultant external force.

It's the teacher's fault, you don't have a problem!

Actually, it's not entirely the teacher's fault, it's just that he is trying to make it easier for you to understand.

It is true that two objects do not touch each other and there is indeed a forceful action, and this force acts through a field. For example, gravity, gravitational force, electric field force, magnetic force, and so on.

This question will naturally be understood by you in high school, but the teacher's second sentence is correct, two objects in contact may not have a forceful effect, they must be squeezed.

There are many kinds of forces, magnets are not without force, but the force between them you can't see, there is an interactive force in the middle, the teacher is not wrong, your understanding is one-sided.

The teacher is not wrong, your understanding is one-sided. It's not that the magnet doesn't have a force, but the force between them you can't see, and there is an interactive force in the middle. The teacher is just explaining that force can change the state of motion of an object.