English for students of Physics_Unit 9

Unit Nine WEIGHT AND MASS READING PASSAGE Weight and weightlessness Perhaps nothing is so ingrained in our senses as the perpetual pulling of the earth on our surroundings.. Earth’s g

Unit Nine

WEIGHT AND MASS

READING PASSAGE

Weight and weightlessness

Perhaps nothing is so ingrained in our senses as the perpetual pulling of the earth on our surroundings It’s always there, never changing It’s been hugging solids, liquids and gases to the earth’s surface for over 4 billion years Earth’s gravity is built into our descriptions of our world with words like up, down, and weight

Exactly what is weight? A weight is a force, nothing more Your weight is the pull of earth’s gravity on your body Likewise, the weight of your car is the force of the earth’s attraction for it The greater the mass is, the larger the attraction Two identical pickup trucks weigh exactly twice as much as one But mass and weight are not the same; they are measures

of two different things, inertia and force

For example, consider the rocks brought from the moon’s surface by astronauts Because

of the Earth’s stronger gravitational attraction, these rocks weigh more on Earth, about six times as much as they weighed on the moon But their mass, their resistance to a change in velocity, is still the same; they have the same quantity of matter on earth as they did on the moon

Even though weight and mass are not the same, most of us do not make a distinction between them, suppose someone hands you two books and asks which is the more massive Almost certainly you would “weigh” one in each hand choose the heavier book That’s okay, because the heavier one does have more mass But if the two books were on a smooth table, you could just push each book back and forth to see which has the larger inertia (Their weights don’t come into play, being balanced by upward pushes from the table) Even then, pointing to the one that’s harder to accelerate, you might from habit still say

“That one is heavier” The point here is “that one” is harder to accelerate only because it has greater mass An astronaut could pick up a large rock on the moon with much less force than required on earth But if the astronaut shoved the rock in a horizontal direction, it would take just as much of a push to accelerate it at, say, 5 feet/second2 as it would take on earth There is a difference between weight and mass

To measure your weight you can use a bathroom scale, which is a spring that stretches if

it is pulled (or compresses if it is pushed) As you step onto the scale, the spring’s pointer

register a larger and larger force until you are at rest, supported entirely by the scale The scale then shows you how much force (from the spring) balances gravity’s pull on your mass, and this force is equal to your weight If you step down and drink two cups of coffee and then step back on the scale, you’ll weigh about 1 pound more

But suppose some fellow strapped a small scale to his feet and jumped from the top of the stepladder You can imagine what would happen, although you should not actually try it While he was falling, the scale would fall with him- it wouldn’t support him, and he couldn’t press against it In this situation, the scale would show a reading of zero Gravity’s pull would still be there, of course, pulling on him as he fell He would still have weight, the pull of gravity on his body It’s just that nothing would stop that fall, there would be no supporting force opposing the gravitational pull, so he would feel weightless

To jump with a scale would be awkward (and dangerous) But if you strap on a small backpack stuffed with books and hop down from a chair, you can feel the pack’s weight vanish from the shoulder straps while you are falling Perhaps, you’ve jumped piggyback with

a friend into a swimming pool If your friend is on your back and you jump, your friend’s weight disappears from your back while the two of you are in midair Nevertheless, the weight of your friend doesn’t disappear; it causes your friend to accelerate right along with you, at the rate of g, towards the water This is why news reporters often say astronauts are

“weightless” when they are in the orbit But a better way to describe their condition is to say they are in free fall Since everything in a spaceship falls together around the earth, nothing inside supports anything else It’s true that the astronauts hover and float within their spacecraft as if they were weightless, but gravity still pulls on their bodies, so they do have weight The term weightlessness is a misnomer, but it gets the ideas across While in free fall, things seem to have no weight relative to each other

Provided there’s no air resistance, everything near the earth’s surface falls with acceleration g We can use this fact and the formula Fnet = ma to find the weight of an object

If something is falling freely (in vacuum), its weight is the only force acting, so its weight is

the net force The acceleration a is simply g, and substituting in the formula, we find weight =

mg (When anything is at rest, the acceleration is zero, of course, because the force from the ground balances the weight.) We measure weight in pounds or newtons, the usual units of force

As an example, we’ll find the weight of 1 kg mass on earth in both newtons and pounds: weight = mg = (1kg) (9.8m/s2) + 9.8N = 2.2lb

(Adapted from Physics, an introduction by Jay Bolemon, 1989)

READING COMPREHENSION

Exercise 1: Answer the following questions by referring to the reading text

1 What is the weight of a body?

………

2 What is the difference between the weight and the mass of the same body?

………

………

3 What makes the difference to your body on Earth and on the Moon? And what is the difference?

………

………

4 Is weight a scalar or vector quantity? Why?

………

………

5 In which situation can you be considered to be weightless? What really happens in this situation?

………

………

Exercise 2: Fill in the blanks with the words you have read from the reading text These

statements will make up the summary of the reading text

1 We describe _ with words like up, down, and weight

2 The weight of a body is the _ of earth’s gravity on it

3 Mass is to measure _ and weight is to measure force

4 The Earth’s is 6 times greater than that of the Moon

5 is the quantity of matter of a body

6 Common people normally do not _ between mass and weight

7 The feeling of weightlessness results from the fact that there’s no _ _ opposing the gravitational pull

8 Without air resistance Everything near the Earth’s surface falls with

9 Astronauts are weightless when in

10 When a body’s in free fall, its weight is the

Exercise 3: New version - Fill in the blank in the following text about weight

The weight W of a body is a (1)……… that pulls the body towards a nearby astronomical body; in everyday circumstances that (2)……… body is the Earth The force is primarily (3)………… to an attraction – called a gravitational attraction – between the two bodies Since (4) ……… is a force, its SI unit is the Newton It is not mass, and

its (5)……… at any given location depends on the value of g there A bowling ball might (6)………… 71 N on the Earth, but only 12 N on the Moon, where the (7)……… acceleration is different The ball’s mass, 7.2 kg, is the same in either place, because (8)……… is an intrinsic property of the ball alone (If you want to lose weight, climb a mountain Not only will the exercise reduce your mass, but the increased elevation means you are further from the center of the Earth, and that means the value of g is less So your weight will be less) We can weigh a body by (9) ………it on one of the pans of an equal-arm balance and then adding reference bodies (whose masses are known) on the other pan until we strike a balance The masses on the pans then match, and we know the mass m of the (10)………… If we know the value of g for the location of the balance, we can find the weight of the body with the following formula: W = mg

GRAMMAR IN USE

I) If-clauses

An if- clause is commonly called a conditional clause in complex sentences You have learnt all types of conditional sentences, but in a brief summary, we should recall all such types:

There are four types of conditional sentences:

Type 0:

1 If your friend is on your back and you jump, your friend’s weight disappears from your back while the two of you are in midair

2 If we heat iron, it expands

Type 1:

1 If you step down and drink two cups of coffee and then step back on the scale, you’ll weigh about 1 pound more

2 If we heat water up to 1000 C, it will evaporate

Type 2:

1 If the astronaut shoved the rock in a horizontal direction, it would take just as much

of a push to accelerate it at, say, 5 feet/second2 as it would take on earth

2 If we used a larger amount of matter in our experiment, we would conclude that mass really does not remain the same

Type 3:

1 If you had worked carefully, you would have found that all the changes in mass that you observed were within the experimental error of your equipment

In science writing, the last type is much less frequently used than the first three ones The reason for this lies in the function of each type that we recall as follows:

Type 0: If … + present … + present

This type is used to express one thing that always follows automatically from the other (or we can understand it in the way that this pattern is used to express a truth.)

Note: We can use when instead of if

For example: When/if we heat iron, it expands

Type 1: If … + present … + will (modal base)

This type is used to express an open condition It leaves an open question of whether the action will happen or not

Type 2: If … + past … + would (modal past form)

This type is used to express an imagined condition or a presumption for the action that happens to follow

Type 3: If … + past perfect … + would + perfect

This type is used to express something unreal or an imaginary past action, meaning it did not really happen

II) Special patterns of comparison

You have learnt all the basic patterns of comparison of adjectives and adverbs The following will present only two common special patterns that are used quite a lot in science writing:

Pattern 1: the … + comparative … the … + comparative

This pattern is used to express a parallel increase or to say that a change in one thing goes with a change in another

Example:

1 The greater the mass is, the larger the attraction gets

2 The more careful you are when conducting the experiment, the better the results

3 The more thoroughly you examine the phenomenon, the narrower the limitations of

your conclusion (will be)

Pattern 2: comparative and comparative

This pattern is used to express gradual and continuous decrease or increase

Example:

1 As you warm a piece of candle wax in your hand, it becomes softer and softer

2 As the Earth recedes into the distance, the potential increases more and more slowly

PRACTICE

Exercise 1: Write conditional sentences by combining one clause from A with a suitable one

from B

1 a straight stick is inserted

obliquely into water

2 we examine the works of a clock

3 one side of a block is rougher

than the other sides

4 the conductor is touched while

the charged body is still near it

5 someone claimed that he/she had

done an experiment in which as

much as one-millionth of the

mass disappeared or was created

6 a body is suspended on a scale

7 we were on the Moon

8 two different loads stretch a

spring identically at a pole

9 we dissolve some sugar in water

10 no matter is added to a body

and not a single particle is

separated from it

a we will find that separate trains

of wheels drive the hour hand and the minute hand

b it is impossible to change its

mass, regardless of what external actions we resort to

c it will appear to be bent at the

surface of the water

d the charge which has the same

sign as the inducing charge disappears

e we will be able to find the force

of its attraction by the Earth

f this identity is completely

preserved even at the equator

g friction is increased when the

block rests on that surface

h our weight would be different

i we should treat the result with

great suspicion

j the mass of the solution will be

precisely equal to the sum of the masses of the sugar and the water

Exercise 2: Decide whether two of the sentences in each pair are exactly the same in

meaning or not Write (S) for the same and (D) for the different

1 a The frictional force is greater when the contact force is greater

b The greater the contact force, the greater the frictional force

2 a When the mass of the attracting body is larger, the force of gravity changes more rapidly at a given distance

b The larger the mass, the larger its tidal force at any given distance

3 a If you climb a mountain, your potential energy increases as you go up

b The higher you are in the air, the greater your potential energy gets

4 a As the rocket goes up, the Earth’s pull on it gets gradually less

b The higher the rocket is up, the Earth’s pull on it gets smaller and smaller

5 a As we move further away from the Earth’s surface, the equipotential lines

become further and further apart

b The further we move away from the Earth, the further apart the equipotential lines get

6 a The atoms of a solid vibrate more and more as the temperature rises

b The higher the temperature, the stronger the atoms of a solid vibrate

7 a Since the force is the same at all points in a uniform field, it follows that the energy of the charge increases steadily as we push it from one plate to the other

b In a uniform field, as the force is unchanged at any point, the energy of a charge gets higher and higher when we push it from one plate to the other

8 a The potential energy of the test charge increases more and more rapidly the closer you get to the repelling charge

b The closer you get to the repelling charge, the more rapidly the potential energy of the test charge increase

9 a The strength of a magnetic field depends on how concentrated the flux is

b The stronger the strength of a magnetic field, the more concentrated the flux is

10 a Through a conductor length L in a magnetic field, a current I will feel a force

F; the stronger the field, the greater the force

b In a magnetic field, a current I through a conductor length L feels a force F

which is proportional to the strength of the field.

PROBLEM SOLVING

Describing process in chronological order

When we describe a process or procedure, say, an experiment, we often use the

present passive tense to give a general description (But when we report, we use the past

passive tense.)

Sequence, or order, is important in this type of description That’s why the sequence

markers e.g first, then…finally are often used These help not only to link the sentences

but to describe actions in a chronological order as well

The following are the commonly-used markers:

First(ly), … second(ly),… third(ly), ….etc …then/next/after that/afterward…finally/lastly

One, … two, … three, … etc … The next (following) step is/ then/ next/after that/afterward … finally/ lastly

And some others:

while (whilst) …, … at the same time, … … in the mean time, before – ing, … after – ing

Sometimes, in order to avoid repeating a subject, pronouns and relative clauses are used

Read the following examples:

1 First, a hole is made in the cap of a large plastic water can and the valve from an old bicycle tyre is glued to it Then, the cap is put back on the can and the can is weighed on a pair of balances After that, extra air is pumped into the can and

the can is weighed again It will be found that the can weighs more after the extra air is pumped into it than it did before

2 First, two pieces of platinum foil are connected to a battery with one piece to the positive terminal and the other to the negative They are then placed in blue copper sulphate solution contained in a beaker Next, a test tube is filled with the solution and fixed over the anode Finally, the current is switched on The

current passes from the anode to the cathode through the solution It will be seen that the blue solution of copper sulphate gradually becomes paler as the current

passes through it At the same time, gas is given off from the anode and is

collected in the test tube

Combine each set of the following statements into a paragraph, using suitable sequence markers, pronouns and relative clauses as well

1 Electrolysis using copper electrodes

Two copper plates are weighed

They are connected to a battery

They are placed in a vessel containing copper sulphate solution

The current is switched on

The current passes from one place to the other through the copper sulphate solution

After half an hour the current is switched off

The plates are removed from the copper sulphate solution

They are dried

They are weighed again

2 Oil refining

Crude petroleum is placed in a metal vessel, or still

Steam is passed over the petroleum

This provides enough heat to change the lightest oils into vapors

These vapors are carried to a number of pipes surrounded with water, or condensers

The vapors are cooled and become liquid in the condensers

The still is heated

Heavier oils are changed into vapors

The vapors are led to condensers

The vapors are liquefied

3 The making of alloys

The two metals which are the ingredients of the alloys are melted

The main gradient is melted

The other ingredient is melted

The other ingredient is added to it

The other ingredient dissolves

The mixture is poured into metal or sand moulds

It is allowed to solidify

4 Welding

The ends of two pieces of metal are carefully cleaned

They are heated

The ends become white hot

A flux is applied to the heated ends

The flux melts

The ends are pressed or hammered together

The joint is smoothed off

5 The preparation of oxygen

Potassium chlorate crystals are mixed with black manganese (IV) oxide powder The mixture is placed in a test tube

The test tube is fitted with a delivery tube

The delivery tube leads to a trough of water

A glass jar containing a column of water is placed upside down in the trough

The test tube is heated

The potassium chlorate decomposes

Oxygen is released

It passes through the delivery tube

It is collected in the glass jar

(Adapted from English in Physical Sciences, Student’s edition by J.P.B.Allen,

H.G.Widdowson, Oxpford University Press, 1997)

TRANSLATION

Task one: English-Vietnamese translation

1 Suppose a piece of gold balances a piece of wood, and the piece of wood balances

a piece of brass Then we say that the masses of all three are equal If something else balances the piece of brass, it also balances the wood and the gold and therefore has the same mass The equal- arm balance gives us a way of comparing masses of objects of any kind, regardless of their shape, form, color,

or what substance they are made of

2 Issac Newton found that any two particles with masses m 1 and m 2 pull on each other, directly towards each other, with forces that are equal and opposite He found that the force between two particles varies as the product of their masses, divided by the square of their distance, or F gravity varies as m 1 m 2 /d 2 where d is the distance between them The farther apart the particles are, the smaller the attraction they have for each other As d becomes larger, m 1 m 2 /d 2 becomes smaller – at an ever greater rate Nevertheless, the distance must become infinitely large before F gravity vanishes completely Here’s a force that acts over a distance as great as you can imagine, straight through anything that is in its way

3 Weight is the force with which a body is attracted by the Earth This force can

be measured with a spring balance The more the body weighs, the more the spring on which it is suspended will be stretched With the aid of a weight taken as the unit it is possible to calibrate the spring – make marks which will indicate how much the spring has been stretched by a weight of one, two, three, etc., kilograms If, after this, a body is suspended on such a scale, we shall be able to find the force (gravity) of its attraction by the Earth, by observing the stretching of the spring For measuring weights, one uses not only stretching but also contracting springs Using springs of various thickness, one can make scales for measuring very large and also very small weights Not only coarse commercial scales are constructed on the basis of this principle but also precise instruments used for physical measurements