49Unit 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

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 F
net
= 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/s
2
) + 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?

51
…………………………………………………………………………………………
………………………………………………………………………………
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?
…………………………………………………………………………………………

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 100
0
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/second
2

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.54
PRACTICE
Exercise 1: Write conditional sentences by combining one clause from A with a suitable one
from B.
A
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

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.