Problems in physical mechanics in junior high school, problems in physical mechanics in junior high

Experiments should be done with control variables. Water is fluid, and it is better to do this experiment than to fill it with sand. Because the sand is almost immobile in the bottle, the bottle and the sand can be seen as one object.

And water has inertia to shake in the bottle and cannot be seen as an object. The empty glass vial rolls at almost the same distance as the glass vial filled with sand, indicating that the experimental design is inadequate. Because the rolling friction is very small and the friction surface is not rough enough, the experimental effect is not obvious.

If you want to test operability, you need to change the friction surface and change the rolling to sliding. Good luck with your experiment.

The air resistance is related to the size of the object, and the resistance of large objects is greater. If the center of gravity of an object moves horizontally, does it also have an effect if the center of gravity changes?

The magnitude of the rolling friction resistance moment is related to the gravity of the object, the initial velocity is the same, the kinetic energy is different, the resistance work is different, and the distance between the empty bottle and the full sand bottle is the same, so it just so happens. The density of sand is greater than that of water, the initial kinetic energy of the water is small, and the resistance work affects a part, so it is normal to not go far.

Guess: Does the sand bottle go x 1 times as far as the water bottle?

Question 2 should be "The coasting distance is related to the quality of the car", right?

Choose D, because as long as the fan is working, it will push the air down, and according to the force is reciprocal, the air will push the fan up. So choose D

c, gravity and tension are a pair of balancing forces.

Let the tensile force of the person be xn, which can make the wooden board stand still in the horizontal position, and let the ob length be l, and oa 2l, then the pressure of the human on the wooden board at this time f=60*10-x

The upward tensile force at the end of a is f=x

f*l=f*2l

600-x)*l=x*2l

x=200(n)

The pressure of the person on the plank = 600-x = 400 (n).

The rally force should be 300N

The pressure is 600N

The fixed pulley does not change the magnitude of the force

The plank is a lever around b 2 ob= ab, so the pulling force is half of the gravitational force, and the gravitational force is 600n for people, so the pulling force is 300n

200n；400n

Principle: Leverage balancing principle.

Let f be the tensile force of man, and the gravitational force of man be g, oa=ob=l, ab=2l;

From the principle of leverage equilibrium, it is obtained:

l*（g-f）=2l*f

Solution: f=200n

For human pressure on the board: n=g-f=400n

I haven't written a question like this in a few years. Ignorance. Hehehe.

300 300 Everyone's very profound, if you feel that I am right.