Download Newton`s Laws of Motion and Vectors

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Investigation 2
Newton’s Laws of Motion and Vectors
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.
2.
A physics student holds a ball in her hand.
a.
Name the two forces that are acting on the ball, and indicate the directions of the forces.
b.
The two forces must have the same magnitude but must be in opposite directions. Which one
of Newton’s Laws tells us this?
The physics student releases the ball. 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 it acceleration?
b.
Which one of Newton’s Laws applies to this situation?
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?
d.
Name the force (or forces) that is (are) acting on the ball as it is falling, and indicate its (their)
direction(s).
3.
A physics student who weighs 800 Newtons (about 170 lbs) stands on a bathroom
scale in an elevator that is not moving.
a.
b.
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?
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 or
traveling at a constant velocity, 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.
elevator motion
not moving
begins to move
upward
moving upward with
constant velocity
slows as it reaches the
top floor
starts downward from
the top floor
slows as it reaches the
bottom floor
bathroom scale
reading
“net force” direction
acceleration direction
800 Newtons
no net force
no acceleration
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 the gravitational force pulls her downward and the
seat pushes her upward so 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
she travels on a straight road at 120 mph
she slows because she hears a CHP siren
6.
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
7.
When two vectors are added together, they produce a resultant that is found using the
parallelogram rule. In the diagrams below, a and b are examples of parallelogram rule. Use these as
a guide to find the resultant of the vectors in c and d.
a
Below is a top view of an airplane being blown off course by wind in various directions. Use the
parallelogram rule to show the resulting speed and direction of travel for each case.
c
b
a
Which airplane is traveling the fastest across the ground? __________
9.
d
R
R
8.
c
b
d
Slowest? __________
Use the parallelogram rule in reverse to find the components of the vector R along the dashed lines.
In the diagrams below, and b are examples. Use these as a guide to find the components of R in c
and d.
a
c
b
R
R
d