A few physics questions in the second year of junior high school will be rewarded with 20 points!!

float=mg=

Density = m v =

2.Alcohol density: another density = another volume: alcohol volume = 1 2:3 4 = 2:3

take g=10m s2

1. Buoyancy = =m v=

2. Buoyancy is equal to gravity, so it is 1:1

Let the total volume be v

1=g/(g3/4v)

2=g/(g1/2v)

1 2=g (g3 4v) g (g1 2v) This is not well written, you can calculate it yourself.

(1) The maximum force of a person standing on the ground is equal to his own weight: f=50g=50*10=500n

The weight of the object is g = 1600N, each movable pulley saves half of the force, and the self-weight of each movable pulley g1 = 50N, and n movable pulleys are required

nf≥1600+n*50

n≥1600/(f-50)=1600/(500-50)=1600/450≈

Take an integer greater than n=4, that is, at least four movable pulleys are required.

The pulley block comprises four movable pulleys and four fixed pulleys, and the four fixed pulleys are fixed on the upper support;

one end of the rope is fixed from the upper support, and the other end is passed through the first movable pulley below, and then through the first fixed pulley above;

then pass through the second movable pulley below, and then pass through the second fixed pulley above;

Then pass through the third movable pulley below, and then pass through the third fixed pulley above;

Then pass through the fourth movable pulley below, then through the fourth fixed pulley above, and finally hang the rope head down and the person standing on the ground pulls the rope end.

2) Regardless of the friction between the pulley and the shaft, lift the actual pulling force of the object f2

nf2=g+n*50

f2=(g+50n)/n=(1600+50*4)/4=1800/4=450n

3) If the person pulls the rope at a speed of 24cm s, what is the height of the object rising in 10s?

Each movable pulley saves half of the force, and the rope length of the rope is doubled, the pulley group of four movable pulleys, the rope movement distance is four times the distance of the heavy object, and the rope movement distance of 10 seconds: 24*10=240cm

Object raised height = 240 4 = 60cm

After the two resistors are connected in series, in order not to burn out the electrical appliance (the resistance in the question), the maximum current allowed to flow through the circuit should be amperes. So Maximum Voltage = Current Total Resistance = Volts.

Because it is not possible to burn out any resistance, the current in the series circuit is the one with the smallest, that is, ampere, so the total resistance is multiplied by the total current (the current in the series circuit is equal) (10 5).