Chapter 2 Physics in Action
Physics Talk
NEWTON’S SECOND LAW OF MOTION
Evidence for Newton’s Second Law of Motion
In the Investigate, you observed that it was difficult to push on an object
with a constant force because the object would move faster and faster.
This observation that a constant force produces an acceleration is very
important in physics.
You also found that if you pushed on a more massive object with the
same force, it did not accelerate as much. This observation that the
acceleration decreases with an increase in mass is also very important.
Physics Words
Newton’s second
law of motion:
the acceleration of
an object is directly
proportional to the
unbalanced force
acting on it and
inversely proportional
to the object’s mass.
The direction of the
acceleration is the
same as the direction
of the unbalanced
force.
Based on observations from investigations similar to yours, Isaac Newton
wrote his (Newton’s) second law of motion: The acceleration of an
object is directly proportional to the unbalanced force acting on it
and is inversely proportional to the object’s mass. The direction of the
acceleration is the same as the direction of the unbalanced force.
You saw the evidence for Newton’s second law in the Investigate.
When you pushed an object with a small force, the object had a small
acceleration. The speed of the object increased, but not very quickly.
When you pushed the object with a large force the object had a large
acceleration. Newton’s second law states this: “The acceleration of an
object is directly proportional to the unbalanced force acting on it.”
This is a mathematical way of saying that the larger force produces
a larger acceleration. As the force gets larger, the acceleration gets
larger — a direct proportion. In this Investigate, the force was a push.
You also found that the same force on a small mass produced a larger
acceleration than it did on a large mass. Newton’s second law states this,
“The acceleration of an object is… inversely proportional to the object’s
mass.” This is a mathematical way of saying that the larger the mass,
the smaller the acceleration. As the mass gets larger, the acceleration
gets smaller — an inverse proportion. To achieve a big acceleration,
you need to apply a large force to a small mass.
In one of the most important science books of all
time, Principia, Isaac Newton wrote his second law
of motion. It is interesting both historically and in
terms of understanding physics to read Newton’s
second law in his own words:
“The change in motion is proportional to the
motive force impressed; and is made in the
direction of the right line in which that force
is impressed.”
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When multiplying or dividing, the result should have no more
significant digits than the factor having the fewest number
of significant digits.
Multiplying and Dividing
When adding or subtracting, the final result should have the
same number of decimal places as the measurement with the
fewest decimal places.
Adding and Subtracting
There are also guidelines that you can use when making
your calculations.
Significant Figures in Calculations
• In a large number without a decimal point, the zeros are not
significant. In the measurement 2000 kg, the zeros are not
significant. The measurement has one significant figure.
However, if the zeros in 2000 were significant, it would be
written as 2000. or in exponential notation as 2.000 × 103.
• A zero at the beginning of a decimal number is not significant.
In the measurement 0.023 kg, the zeros are not significant. The
measurement has two significant figures.
• A zero at the end of a decimal number is considered significant.
In the measurement 1.50 N, the zero is significant. The
measurement has three significant figures.
• A zero between nonzero digits is a significant figure. In the
measurement 308 g, the zero is significant. The measurement
has three significant figures.
Zeros may or may not be significant, depending on their place
in a number.
All nonzero numbers are considered to be significant figures.
In the measurement 152.5 m, all the digits are significant.
The measurement has four significant figures.
There are guidelines that you can use to determine the number
of significant figures in a measurement.
Determining the Number of Significant Figures in a Measurement
Section 3 Newton’s Second L