Transcript Lecture06 - Purdue Physics

Reading Quiz
Review
Choose the correct response. The inertial mass of
an object is
• The proportionality factor between the total applied
force and the resulting acceleration.
• The property discovered by Aristotle that causes
forces.
• The essential property of matter that Galileo used to
prove that Aristotle was wrong.
• All the other choices are correct.
PHYS 214-First Exam
A. First Exam is on Wednesday, February 4. It will cover
Chapters 1,2,3, & 4.
B. Previous exam questions are on the Class Web Site.
Question 1:
A bullet is shot horizontally with a speed of 200 m/s. The target
is 100 meters away. How far does the bullet fall before
hitting the target?
A. Zero - it doesn’t fall.
B. about one millimeter
C. about one centimeter
D. about 10 centimeters
E. more than one meter
Newton’s First Law
• An Object remains at rest, or in uniform
motion in a straight line unless acted on by
an external force.
(memorize)
Figure 4.4
Newton’s first law: In the absence of a force, an object
remains at rest or moves with a constant velocity.
Newton’s Second Law
• The acceleration of an object is directly
proportional to the the imposed force. The
proportionally constant is the mass of the
object.
(memorize)
Newton’s Second Law:
The acceleration of an object
• is directly proportional to the magnitude of the total
force on it, and
a F
• is inversely proportional to the mass (or inertia) of the
object.
a  1m
Hence,
F  ma
Question 2:
A bowling ball is rolling down the lane of a bowling alley, heading
for a strike that will knock all the pins down.(Choose the
CORRECT answer.)
A. The force that keeps the ball moving forward is left over from
the person who threw the ball.
B. There are no forces pushing the ball forward.
C. The force the keeps the ball moving comes from the air, which
rushes to fill the space behind it.
D. The ball would not continue moving forward if there were no
friction with the floor.
E. Because the ball is not accelerating, there are no forces at all
acting on the ball.
Question 3:
Consider a heavy, 16 pound bowling ball at rest with a string tied to
it, and the string is pulled with a constant force for one second,
causing the ball to move across a nearly frictionless floor.
Repeat this experiment with an 8 pound bowling ball, using the
same force also for one second. After one second,
A. both balls have the same acceleration.
B. both have the same velocity.
C. the lighter ball has a larger velocity and larger acceleration.
D. the heavier ball has a larger velocity and a larger acceleration.
E. (None of the above).
Figure 4.6
A block being pulled across a table.
Two horizontal forces are involved.
What is a Newton, our unit of force?
Kilograms/meter-second-second
Question 4:
The Moon is in a circular orbit about the Earth, with a period of
about 28 days.(Choose the CORRECT answer.)
A. The Moon is continuously accelerating towards the Earth.
B. The Moon never falls and crashes into the Earth because no
force is pushing or pulling it towards the Earth.
C. The Moon does not fall to Earth because the forces acting on
the Earth add up to zero. (They cancel out.)
One Newton is the force required to make
a 1 kg mass accelerate at 1 meter/sec/sec.
How much does a 1 kg mass weigh?
Weight=gravitational force= mass times g
= (1.0 kg)(9.8m/s2)
= 9.8 N
= 2.2 pounds
Thus, we say a kilogram is 2.2 pounds.
Note: kilogram is a unit of MASS,
pound is a unit of FORCE (=ma).
Note: We can also say that 1 pound = 4.45 N.
Note: Weight depends on value of gravity.
Mass is independent of gravity.