Download Newton`s Laws of Motion

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Investigation 2
Newton’s Laws of Motion
The motions we observe in our everyday life follow some simple rules. These rules are called Newton’s
Laws and can be expressed as follows:
First Law:
“Every object continues in a state of rest or of uniform speed in a straight line unless acted
upon by a nonzero net force.”
Second Law:
“The net force on a body equals the mass of that body times its acceleration, and the
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directions of both are the same.” ( F  ma )
Third Law:
“When one object exerts a force on a second, the second object exerts an equal and opposite
force on the first.;” or “To every action there is an equal and opposite reaction.”
Answer the following questions are clearly and concisely as you can.
1.
A physics student holds a ball in her hand at rest.
a.
Name the two forces that are acting on the ball, and indicate the directions of the forces.
Gravitational force of the earth, downward; hand holding ball, upward.
b.
The two forces must have the same magnitude but must be in opposite directions so that the
ball is at rest. Which one of Newton’s Laws tells us this?
First Law: both forces are acting on the same object (ball).
2.
The physics student releases the ball (lets the ball drop). Consider the ball as it falls toward the
ground without air resistance.
a.
Is the ball accelerating? If it is, then in what direction is its acceleration?
It is accelerating downward.
b.
Which one of Newton’s Laws applies to this situation?
Second Law: net force = mass x acceleration
c.
What does Newton’s Second Law say about the direction of the “net force” acting on the object
and the direction of the acceleration?
The direction of the net force and the acceleration are the same.
d.
Name the force (or forces) that is (are) acting on the ball as it is falling, and indicate its (their)
direction(s).
Only the gravitational force downward.
3.
A physics student who weighs 800 Newtons (about 170 lbs) stands on a bathroom
scale in an elevator that is not moving (at rest).
a.
What two forces are acting on him, i.e., what forces does he feel? How are the
strengths of the two forces related to each other? Which one of Newton’s laws
applies here?
forces:
gravitational force downward; scale pushing upward
strengths:
the forces have the same strength (equal to each other)
Newton’s Law:
b.
First Law: forces are on the same object (physics student)
On the diagram of the student in the elevator above, draw two vectors to represent the two
forces acting on the student. (Remember that vectors represent both the direction and the
magnitude of forces.)
In the following questions, you should think first about the type of motion – is the object stationary (at
rest) or traveling at a constant velocity (uniform speed in a straight line), or is the object undergoing an
acceleration? This will tell you whether Newton’s First Law or Newton’s Second Law applies.
4.
Refer to the situation in question 3. For the following motions of the elevator, indicate whether the
reading on the bathroom scale is the same as, greater than, or less than the reading when the elevator
was stationary, and indicate the directions of the “net force” on the student as up, down or no net
force. Also indicate the direction of the acceleration according to Newton’s Second Law as up, down,
or no acceleration.
bathroom scale
reading
“net force” direction*
acceleration direction*
same
no net force
no acceleration
>800 Newtons
upward
upward
moving upward with
constant velocity
same
no net force
no acceleration
slows as it reaches the
top floor
less than
downward
downward
starts downward from
the top floor
<800 Newtons
downward
downward
slows as it reaches the
bottom floor
greater than
upward
upward
elevator motion
not moving
begins to move
upward
*Remember that the direction of the net force and the acceleration are the same.
5.
A physics student sitting in a stationary Lamborghini (car), is holding onto the steering wheel, and is
strapped in with her seat belt. She knows that in the vertical direction, the gravitational force pulls
her downward and the seat pushes her upward and that the net vertical force is zero, thus resulting in no
upward or downward acceleration. In the horizontal direction, the back of the seat pushes her forward
and the steering wheel pushes her backward.
Compare the strengths of the two horizontal forces -- the back of the seat pushing her forward and
the steering wheel pushing her backward (which one is greater, or are they the same?) when
she accelerates away from a stop sign
6.
back of the seat pushing forward is greater
she travels on a straight road at 120 mph
they are the same
she slows because she hears a CHP siren
steering wheel pushing backward is greater
A physics textbook is at rest on a table. A calculator is
placed on top of the book. Name the three forces that are
acting on the book. Name the corresponding reaction forces.
calculator
book
table
action force
reaction force
table pushing upward on the book
book pushing downward on the table
calculator pushing downward on the book
book pushing upward on the calculator
earth pulling downward on the book (gravity)
book pulling upward on the earth