Xiaoyu and Xiaoli run in the 100 meter race, and each time Xiaoyu reaches the finish line 10 meters ahead of Xiaoli. If Xiaoyu goes back 10 meters and competes with Xiaoli (both start at the same time, and their average speed is the same as before), the result is () A. Xiao Li gets to the end first B. get to the end together C. Xiao Yu gets to the end first D. can't be sure

Let Xiaoyu use time t, then the speed is V1 = 100MT; Xiaoli use speed V2 = 100m − 10MT = 90mt; in the second competition, S1 ′ = 100m + 10m = 110m, S2 ′ = 100m, because the speed is not changed, Xiaoyu use time T1 = S1 ′ V1 = 110m100mt = 1110t; Xiaoli use time T2 = S2 ′, V2 = 100MT, 90mt = 109t

Xiaoming bought a soft notebook for 12 yuan, and Xiaoli bought a hard notebook for 21 yuan. It is known that each hard notebook is 1.2 yuan more expensive than the soft notebook, Xiaoming

If the Soft Notebook costs X Yuan, the hard notebook costs (x + 1.2) yuan
12 21
------- = ----------
X X+1.2
By solving the equation, we get x = 1.6. After testing, x = 1.6 is the solution of the original equation
At this price,
12 / 1.6 = 7.5
21 / (1.6 + 1.2) = 7.5
They all bought 7.5 laptops, which is not in line with the actual meaning
A: Xiao Ming and Xiao Li can't buy the same number of notebooks

Xiaoming, Xiaoliang and Xiaoli want to know the age of the children in the community they live in. Xiaoming investigates the children who play in the yard that day. Xiaoliang investigates the children in the second unit he lives in. Xiaoli investigates the age of the children in the two households on the first floor of each unit. Who is the better way? Why? What is the better way

In contrast, Xiaoli's survey method is better, and her survey results are closer to the average among the three people
A better way of investigation is to conduct a more comprehensive investigation and go directly to the neighborhood committee

The volume of 1G air =? CM & # 179; the density is 1.29kg/m & # 179;

It's so complicated. Can you tell me more about it

The density is 1.29kg/m, and the volume of 1G air =____ cm³

Known: ρ = 1.29 kg / m3, M = 1 g = 0.001 kg
Ask: v
From the density formula ρ = m / V
V = m / ρ = 0.001 / 1.29 = 7.75 * 10 ^ (- 4) M3
That is, v = 7.75 * 10 ^ (- 4) * 10 ^ 6 cubic centimeter = 775 cubic centimeter

The alcohol density is 0.8 * 10 ^ 3kg / m ^ 3= G / cm ^ 3 air density 1.29kg/m ^ 3=__ g/cm^3

0.8
1.29*10^-3

(3 / 3) what is the sand content per cubic meter of river water (the total volume of pure water and sand is 1 cubic meter) in this section? (the density of sand is 2.5 × 10 cubic kg / cubic meter)

If the sand content of each cubic meter of river water is XKG, the volume of sand is x / (2.5 * 10 ^ 3) cubic meters, and the volume of water is 5 * 10 ^ (- 3) - X / (2.5 * 10 ^ 3) cubic meters
So there is: x + 1000 * [5 * 10 ^ (- 3) - X / (2.5 * 10 ^ 3)] = 5.3, and the solution is x = 0.5
So the sand content is 0.5 kg

If the density of an object is less than that of pure water, can the hydrostatic weighing method be used to measure the density of the object? If it is possible, the measuring method and experimental formula

Yes
First measure the scale V1 of water, then measure the scale V2 of water when the object floats freely, and then measure the scale V3 when the object is just submerged by external force
ρ = ρ water (v2-v1) / (v3-v1)

What is the density of pure water at 4 ℃

1000 / = 1kg / cubic decimeter, because the kilogram is defined according to this definition. The original kilogram mass unit was derived from the length unit meter adopted in France at the end of the 18th century. The mass of 1 cubic decimeter of pure water at the maximum density (the temperature is about 4 ℃) was determined as 1kg

The volume is 0.27 cubic cm and the density is 19.32 cubic cm. What's the weight?
How do you calculate that?

If the density is 19.32 g / cm ^ 3
Then G = ρ VG = 19.32 × 0.27 × 10 ^ (- 6) · g = 5.2164 × 10 ^ (- 6) · G
If G is 9.8 m / S ^ 2, the weight is 5.1121 × 10 ^ (- 5) Newton
If G is 10 m / S ^ 2, the weight is 5.2164 × 10 ^ (- 5) Newton