Following the rules of weighing, measure the mass of several solids. Measuring body weight on lever scales. Installation of analytical balances

Goal of the work:

Devices and materials:

WEIGHING RULES

In what units is it measured (list all)?

_____________________________________________________________

Do the exercises:

8.4 t = ___________ kg 500 mg =____________ g

0.5 t = ___________ kg 120 mg = ____________ g

125 g= ___________ kg 60 mg = _____________ g

100 g+ 20 g + 1 g 500 mg + 200 mg = ___________________________ g

20 g+ 10 g +1 g + 200 mg + 100 mg = ___________________________ g

Which scale pan is placed on:

body being weighed?____________________

weights?___________________________

PROGRESS

№ experience

Body name

Kettlebells

Body weight, g

Conclusion:____________________________________________________________

Laboratory work No. 3 “Measuring body weight on lever scales.”

Goal of the work: learn to use lever scales and use them to determine the mass of bodies.

Devices and materials: scales, weights, several small bodies of different masses.

WEIGHING RULES

Before weighing, make sure that the scales are balanced. If necessary, strips of paper should be placed on a lighter cup to establish balance.

The body to be weighed is placed on the left pan of the scale, and the weights are placed on the right.

To avoid damage to the scales, the body being weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.

You cannot weigh bodies heavier than the maximum load indicated on the scale.

Do not place wet, dirty, or hot bodies on the scales, pour liquids, or pour powders without using a pad.

Small weights and weights should be picked up with tweezers.

Having placed the body to be weighed on the left cup, a weight having a mass close to the body weight (by eye) is placed on the right one.

If the weight pulls over the cup, then it is put back in the case; if not, it is left on the cup. Then weights of smaller mass are selected in the same way until equilibrium is achieved.

Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred to the case.

TRAINING TASKS AND QUESTIONS

What physical quantity is determined using lever scales? weight

In what units is it measured (list all)? In SI - kg, in l/r - g

Do the exercises:

8.4 t = 8400 kg 500 mg =0.5 g

0.5 t = 500 kg 120 mg = 0.12 g

125 g= 0.125 kg 60 mg = 0.06 g

100 g+ 20 g + 1 g 500 mg + 200 mg = 121.7 g

20 g+ 10 g +1 g + 200 mg + 100 mg = 31.3 g

Which scale pan is placed on:

body being weighed? left

weights? right

What needs to be done on a lever scale before weighing?

Before weighing, make sure that the scales are balanced. If necessary, strips of paper should be placed on a lighter cup to establish balance.

PROGRESS

Knowing the rules of weighing, measure the mass of several small bodies with an accuracy of 0.1 g.

Record the measurement results in the table:

№ experience

Body name

Kettlebells, with which the body was balanced

Body weight, g

Conclusion: the mass of the body is approximately equal to the sum of the masses of the weights balancing the scales.

Goal of the work: study methods of measuring mass and acquire the ability to work with analytical balances.

Devices and accessories: analytical balance, weight, weighed body with known density.

Theory of the method

Body mass m is a physical quantity that is a measure of the inertia of a body in translational motion. The ratio of the masses of two bodies is equal to the ratio of their weights. This is the basis for comparing the masses of bodies using lever scales.

Lever scales are an equal-armed or unequal-armed lever that swings freely on a support or suspension. The main relationship that allows you to find the unknown mass of a body is the condition that the moments of force acting on the lever in the equilibrium position are equal to zero.

If a load with an unknown mass is suspended on one arm of a lever m, and on the other - the standard load balancing it m 1 , then in the equilibrium position

Where R– body weight of unknown mass; R 1 – weight of weight; F A And - Archimedean forces acting respectively on the load and weight in the air;
- length of the rocker arms.

Substituting the expression for weight into formula (1), we obtain

. (2)

If the lever arms of the scales are equal
, then expression (2) is simplified:

. (3)

The body weight in this case will be equal to

. (4)

Since it is technically impossible to produce ideally equal-armed scales, unequal-armed scales always introduce some error into the result. Elimination of this error during accurate weighing is achieved by using special weighing methods.

As noted above, equilibrium of the scale lever occurs not when the body masses and weight are equal, but when the differences in weight and Archimedean force for the body and weight are equal. The magnitude of the correction, which eliminates the error in determining the mass caused by the loss of body weight in the air, can be easily found from the following calculations:

Where R– body weight in vacuum; R 1 – weight of weight in vacuum; V And V 1 – body volumes and weight; - air density.

For body volumes and weight we can write

V= ;V 1 = ;

Where m And - the mass and density of the body, respectively; m 1 and - mass and density of weight.

Substituting expressions for volumes of bodies into formula (5), we obtain

, (6)

. (7)

Considering that
And
, we get the final formula:

. (8)

Whence it follows that the correction for the action of the Archimedean force is equal to

. (9)

To weigh small bodies with high accuracy (up to tenths of a milligram), analytical balances are used (Fig. 1). The main part of the analytical balance is the equal-arm lever BB, called a rocker arm, the support of which is the edge of a hardened prism A, located in the middle of the rocker and resting on a polished agate plate fixed at the top of the column A. There are prisms at the ends of the rocker arm bb, used for hanging cups SS. If there are no weights on the cups, then the rocker arm should be installed horizontally. To determine the position of the core
the long arrow serves as a meaning S, attached to its middle.

End of the arrow S moves in front of the scale D located at the base of the column A. When the rocker is in a horizontal position, the arrow should point to the middle division of the scale.

When the scales are not in use, they must be locked. This is done by the action of a special device inside the balance column, with the help of which the rocker arm and cups are slightly raised upward, as a result of which their prisms are released from pressure. Locking is performed by rotating the head V in one direction or another.

When weighing, special weights are used, the mass of which is known and indicated on them. In order not to use weights less than 10 pieces, use the so-called rater R, which is a thin wire bent into a hook (with an eye). The reuter is placed on one of the arms of the rocker, usually divided into 10 equal parts. Placing and removing the rater is done with a special device. It consists of a rod T, passing through the right side wall of the scale box and moving parallel to the rocker arm. The rod can rotate around its axis; at the outer end it is equipped with a head M, and on the inside – with a side lever R and a protruding pin; this latter is inserted into the ear of the rater and picks it up. Placing the rater first, second, third, etc. division of the rocker arm, counting from the middle, is equivalent to the action of a load placed on the cup 1, 2, 3, etc. PC.

When handling analytical balances, the following rules must be observed:

1. While the scales are not locked, you cannot place a load on the cups or remove them from them (you should not even touch the cups), and you cannot stop the rater on the balance beam.

2. Weights should be placed on the scale pan so that the overall center of gravity of the weights passes through the middle of the pan.

3. Take weights and place them on the scales only with tweezers.

4. When removing weights from the scales, you should definitely put them in each box in its designated place.

5. The rocker arm should not be completely released while the cups of the scales are still slightly balanced; it is released only enough so that one can judge which of the cups is lighter, noticing at the same time where the arrow deviates; after this, you should immediately lock the yoke and add or decrease the weight. When there is a small difference between the weights of the body being weighed and the weights of the weights, the rocker begins to swing pendulum-like.

6. The rocker arm should always be released and locked slowly and smoothly; if the scales swing, then the arrest must be done very carefully, while the arrows pass through the equilibrium position, otherwise the rocker will get a push.

7. If the cups swing pendulum-like, then you should first calm them down by touching their edge with a piece of paper and only then completely release the rocker arm.

8. When observing the swing of the scales, their doors must be closed.

9. If, after releasing the rocker arm, it turns out that the amplitude of oscillations is too small (less than 3-4 divisions in one direction or another), then, by closing the door slightly, you can wave your hand in front of the scales, then a stream of air usually imparts sufficient amplitude to the rocker arm.

10. You should not leave the load on the cups for a long time, especially when the scales are not locked; When weighing is completed, the scales must be locked, the loads removed and the doors closed.

In order to weigh on an analytical balance, you must:

determine the zero point of the scale;

determine the sensitivity of the scales;

weigh;

adjust for weight loss in the air;

introduce a correction for the inequality of the rocker arms.

Determination of the zero point of the scale. The zero point (equilibrium point) of the scale is the division at which, in the absence of friction, the arrow pointer stops when the rocker stops oscillating.

N
The zero point of the scale is determined using the swing method. After carefully releasing the rocker arm and passing several vibrations, note the position of the end of the arrow at the moments of the greatest deviations on the scale D (Fig. 2).

Due to friction, the vibrations gradually die out. Typically, the zero point of an unloaded scale is determined from five oscillations: three readings are taken in one direction and the other. If A 1 , A 2 ,......A 5 – successive counts to the left, and A 1 , A 2 ,......A 5 – successive samples to the right, then the zero point N 0 found from the formula

. (10)

Determining the sensitivity of the scales. Ratio of number of scale divisions n, by which the arrow of the scale is shifted, towards the weight, the overload that caused its displacement is called the sensitivity of the scale.

In order to determine the sensitivity of the scales, place the rater on the first division of the rocker scale, which corresponds to a load of 1 mg and find, as for unloaded scales, a new equilibrium position

Then the sensitivity of the scales is found as the difference between And N 0

n=-N 0 . (11)

Weighing. When weighing, the body is placed on the left pan of the scale, and on the right - a weight from the weight. First you need to try to balance the body with gram weights; If body weight is not expressed in whole grams, then it is necessary to continue balancing using decigrams and centigrams. If you cannot balance the body even with the help of centimeter weights, then you need to use a rater, loading the scales with milligrams. By moving the rater along the scale on the rocker, you can find two such positions, in one of which the weight of the weight and the rater will be greater than the body weight, and in the other - less.

Let the arrow deflection value N 1 corresponds to underload of the right scale pan. Therefore, the value N 1 lies to the right of the previously found value N 0 (zero point), and N 2 (the value of the needle deflection when the right scale pan is overloaded) will lie to the left N 0 .

Let the value N 1 corresponds to weight R mg. Then the value N 2 will match the weight (P+1) mg. Thus, to bring the scales to the zero weight point R mg is not enough, but (P+1) mg a lot. Needs to be added to weight R mg additional load
mg to balance the scales.

Since a weight difference of 1 mg corresponds to a deviation (N 1 -N 2 ), then the weight X will cause deviation: N 1 -N 0 . From the proportion we find

. (12)

Thus, the desired body weight R 1 will consist of the weight of the weights
on the right pan of the scale, the rater's weight for value N 1 and additional load X, calculated by formula (12):

R 1 =(
++
) mg. (13)

Adjustment for apparent weight loss in air is determined from formula (9). But, given that the difference between the apparent and true weight does not exceed 0.2%, in practice they are often limited to determining only the apparent weight.

The inaccuracy in determining the mass caused by the inequality of the rocker arms can be eliminated by the double weighing method (Gauss method). The essence of the method is that two weighings are performed: during the first weighing, the body under test is placed on the left pan of the scale, and during the second, on the right.

At first weigh-in for body weight R and weights of weights R 1 by the theorem of moments the equality is true

, (14)

Where - length of the left shoulder; - length of the right shoulder. For the second weighing -

. (15)

From (14) and (15) we find that

, (16)

, (17)

those. body weight is equal to the geometric mean of the product of weights of weights at the first and second weighing.

The purpose of the work is to learn how to use lever scales and determine the mass of bodies with their help.

Equipment and materials: scales, weights, several small bodies of different weights, a jar, shot or dry clean sand.

Directions for use

1. Read the appendix to the work “Weighing Rules”.
2. Following the rules of weighing, measure the mass of several solids with an accuracy of 0.1 g.

Additional task

There is a special weighing method called the taring method. When weighing using this method, an object whose mass they want to determine is placed on the left pan of the scale. A jar is placed on the right cup, in which pour dry sand or fine shot until until the scales come into balance. Then the object is removed from the left pan of the scale and weights are placed in its place and with their help the scales are brought into balance. The mass of these weights will be equal to the mass of the object. The taring method can be used to measure body weight quite accurately even on slightly out-of-tune scales.

Test this out. Place a pellet or a wad of paper on the left pan of the scale; this will upset the balance of the scale - the scale will go out of balance. Measure masses of the bodies you have using the taring method and compare them with the result obtained when weighing on balanced scales.

Think and explain why the taring method can be used to measure body weight quite accurately even on slightly out-of-tune scales.

Weighing Rules

1. Before weighing, you must ensure that the scale is properly balanced. If necessary, to establish balance, you need to place strips of paper, cardboard, etc. on a lighter cup.
2. The body to be weighed is placed on the left pan of the scales, and the weights are placed on the right.
3. To avoid damage to the scales The body to be weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.
4. You cannot weigh bodies heavier than the maximum load indicated on the scale.
5. Do not place wet, dirty, or hot bodies on the scales, pour powders without using a liner, or pour liquids.
6. Small weights should only be picked up with tweezers (Fig. 310).

Having placed the body to be weighed on the left pan, a weight having a mass slightly greater than the mass of the weighed body is placed on the right one ( selected by eye followed by inspection). If this rule is not followed, it often happens that there are not enough small weights and you have to start weighing all over again.

If the weight overtightens the cup, then it is put back in the case, but if it doesn’t overtighten, it is left on the cup. Then the same is done with the next most important weight and so on until there is equilibrium has been achieved.

Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred from the scale pan to the case.

Laboratory work No. 4

Subject: Measuring body weight on lever scales.

Goal of the work: learn to use lever scales and use them to determine the mass of bodies..

Equipment:

• scales with weights;
• several small bodies of different masses.

Directions for use

1. Following the rules of weighing, measure the mass of several solid bodies to within 0.1 g

2. Record the measurement results in a table

3. Draw a conclusion about the work done.

Application

Weighing Rules

1. Before weighing, make sure that the scale is balanced. If necessary, to establish balance, you need to place strips of paper, cardboard, etc. on a lighter cup.

2. The body to be weighed is placed on the left pan of the scales, and the weights are placed on the right. To avoid damage to the scales, the body being weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.

3. You cannot weigh bodies heavier than the maximum load indicated on the scales.

4. Do not place wet, dirty, hot bodies on the scales, pour powders without using a liner, or pour liquids.

5. Small weights should only be picked up with tweezers (see figure). Having placed the body being weighed on the left cup, a weight having a mass slightly greater than the mass of the body being weighed is placed on the right one (selected by eye and then checked). If this rule is not followed, it often happens that there are not enough small weights and you have to start weighing all over again.

6. If the weight overtightens the cup, then it is put back in the case, but if it doesn’t overtighten, it is left on the cup. Then the same is done with the next weight of smaller mass, etc., until equilibrium is achieved.

7. Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred from the scale pan to the case.

8. Check that all the weights are placed in the case and that each of them is in its intended place.

Laboratory work No. 3 page 161

Goal of the work: learn to use lever scales and use them to determine the mass of bodies.

Devices and materials: scales with weights, several small bodies of different masses.

Weighing – a method of measuring mass using scales.

Other units of mass:

1 t = 1000 kg

1 c = 100 kg

1 g = 0.001 kg

1 mg = 0.000 001 kg

Safety regulations.

1.Be careful with scales. Follow the weighing rules.

2. There should be no foreign objects on the table.

3. Place the scale in the middle of the table.

4. Do not lose weights and weights, especially do not put them in your mouth!!!

I have read the rules. I undertake to fulfill . ______________________

/Student's signature/

Weighing rules.

• Before weighing, make sure that the scales are balanced. If necessary, to establish balance, you need to place strips of paper, cardboard, etc. on the lighter pan of the scale.
• The body to be weighed is placed on the left pan of the scales, and the weights are placed on the right

Weighing rules.

3. To avoid damage to the scales, the body being weighed and the weights must be lowered onto the cups carefully, without dropping them even from a small height.

4. You cannot weigh bodies heavier than the maximum load indicated on the scales. (200g.)

Weighing rules.

5. Do not place wet, dirty, hot bodies on the scales, pour powders without using a liner, or pour liquids.

6. Small weights should only be picked up with tweezers.

Weighing rules.

7. Having placed the body to be weighed on the left pan, a weight having a mass slightly greater than the mass of the body being weighed is placed on the right pan (selected by eye and then checked). If this rule is not followed, it often happens that there are not enough small weights and you have to start weighing all over again. If the weight pulls over the cup, then it is put back into the case, but if it doesn’t, it is left on the cup. Then the same is done with the next weight of smaller mass, etc., until equilibrium is achieved.

Having balanced the body, calculate the total mass of the weights lying on the scale. Then the weights are transferred from the scale pan to the case.

Check that all the weights are placed in the case and that each of them is in its intended place.

Practice tasks and questions

• What physical quantity is determined using lever scales? ____________________

2. In what units is it measured (name all)?

________________________________

3.Do the exercises:

8.4 t = _______ kg

0.5 t =________ kg

125 t =________ kg

500 mg = ________ g

120 mg =_________ g

60 mg = _________ g

4.100 g +20 g + 2 g + 1 g +500 mg + 200 mg =___g

20 g + 10 g + 1 g +200 mg + 100 mg =_________g

5.Which cup is it placed on?

body being weighed? on ____________

weights? on ________________

6.What needs to be done on a lever scale before weighing?_____________

Progress.

1. Following the rules of weighing, measure the mass of several solids to the nearest 0.1 g.

Progress.

2. Record the measurement results in the table.

№ experience

Body name

Body mass

m , G

Cube

Body mass

m , kg

Body weight on electric scales m , G

Conclusion:

I learned to use lever scales and with their help measure the mass of various bodies with an accuracy of .......

Additional task.

• Which weights from the school set must be placed on a cup of educational scales in order to balance a piece of sugar weighing 10.50 g lying on another cup? (set of weights: 10g, 5g, 5g, 20mg, 20mg, 10mg).
• Express the mass of bodies in kilograms: 3.5t; 0.25t; 150g; 15
• How many grams are in 7.5 kg?
• The mass is designated by the letter ……….
• 100g + 20g + 2g + 1g + 500mg + 200mg =…..

Homework

§19, 20

Exercise No. 6 (1, 2, 3)