Physics Internet Activity:
Newton’s Laws of Motion – Part I
Lesson 1:
Newton’s First Law
1) State Newton’s 1st Law of Motion.
An object at rest tends to stay at rest and an object in motion tends to stay
in motion with the same speed and in the same direction unless acted upon
by an unbalanced force.
2) Click on the “Animation” button. Explain how the law of inertia can
be observed when the car slams against the brick wall throwing the
passenger over.
The passenger and car are traveling at the same velocity. When the car
hits the brick wall, it is forced to stop, however the passenger follows the
law of inertia by trying to stay in motion with the same velocity causing
him to be thrown out of the car.
3) Try to provide explanations for the following applications of
Newton’s 1st Law of Motion.
a. Headrests are placed in cars to prevent whiplash injuries during
rear-end collisions.
When you are sitting in a car at rest and another cars hits you
from behind, the car will be forced to move forward, however,
your head would want to remain in its original position when it
was at rest causing your head to move backwards. If the headrest
is present, your head will move forward with your body during a
rear-end collision.
b. While riding a skateboard (or wagon or bicycle), you fly
forward off the board when hitting a curb, a rock or another
object which abruptly halts the motion of the skateboard.
You have the same motion as the skateboard before it hits a curb.
When you hit the curb, the skateboard is forced to stop. However,
your body keeps moving in the same direction with the same
speed causing you to fly forward.
Inertia and Mass
4) You roll a ball down an incline that is 2 meters high on to a flat
surface. According to Galileo, when will the ball stop? (Assume
friction does not exist.)
The ball will stop when it acquires a height of 2 meters or until an
outside force (like a wall or person) stops the ball.
5) How is inertia and mass related?
Mass is determines the amount of inertia an object has. The larger the
mass, the greater the resistance to change.
State of Motion
6) Inertia is the tendency of an object to resist acceleration or change in
velocity.(2 ways to write the answer)
Balanced and Unbalanced Forces
7) When are forces balanced? When are they unbalanced?
When all the forces acting upon an object balance each other, the object
will be at equilibrium; it will not accelerate. When an object experiences
forces that do not balance each other, the object will accelerate.
Lesson 2:
The Meaning of Force
8) What is force? Give the two types of forces.
A force is a push or pull upon an object resulting from the object's
interaction with another object. The two types of forces are contact forces
and forces resulting from action-at-a-distance.
9) What is a Newton (N)?
One Newton is the amount of force required to give a 1-kg mass an
acceleration of 1 m/s2.
Types of Forces
10)
Briefly describe the following forces and include their symbol.
a. Applied force
An applied force is a force which is applied to an object by another object
or by a person. Fapp
b. Gravity force
The force of gravity is the force with which the earth, moon, or other
massive body attracts an object towards itself. By definition, this is the
weight of the object. Fgrav
c. Normal force
The normal force is the support force exerted upon an object which is in
contact with another stable object. Fnorm
d. Friction force
The friction force is the force exerted by a surface as an object moves
across it or makes an effort to move across it. The friction force opposes
the motion of the object. Ffrict
e. Air resistance force
Air resistance is a special type of frictional force which acts upon objects
as they travel through the air. Fair
f. Tension force
Tension is the force which is transmitted through a string, rope, or wire
when it is pulled tight by forces acting at each end. Ftens
g. Spring force
The spring force is the force exerted by a compressed or stretched spring
upon any object which is attached to it. Fspring
Drawing Free-Body Diagrams
11)
What are free-body diagrams? How is it similar to vector
diagrams?
Free-body diagrams are diagrams used to show the relative magnitude and
direction of all forces acting upon an object in a given situation. The size
of the arrow in a free-body diagram is reflective of the magnitude of the
force. The direction of the arrow reveals the direction in which the force
acts just like vector diagrams.
12) Draw a free-body diagram for the following:
a) A vase sitting on a window sill.
b) A large box being pushed rightward by
two students.
Fnorm
Fnorm
Ffric
Fapp
Fgrav
Fgrav
c) A leaf falling from a tree. Air resistance
is a factor.
d) A book falling from a top shelf. Ignore
air resistance.
Fair
Fgrav
e) A pail being lowered in a well by a rope.
Ftens
Fgrav
f) A hockey puck sliding left on an ice rink
slowing down.
Fnorm
Ffric
Fgrav
Fgrav
Determining the Net Force
13)
Using the same rules for finding vector resultants, how would
you find the net force on an object? Give at least the two basic rules.
If two forces are acting in the same direction, simply add the forces and
follow the same direction. If two forces are acting in opposite directions,
simply subtract the smaller force from the larger force following the same
direction of the larger force.
14)
How is the net force related to the acceleration of an object?
(Hint: Compare the motion of an object with a net force to the motion
of an object with zero net force.)
If the net force is zero, the object is either at rest or moving in constant
velocity. If a net force is present, the object is accelerating which means
velocity is changing.
15)
Draw a free-body diagram for the following and determine the
net force.
a. A 2 N rock is falling. Ignore air resistance.
Fgrav = 2 N
Fnet = 2 N, downward
b. A boy is pushing a sled weighing 500 N rightward with a force
of 250 N across the snow. What would be the net force if the
friction force is 100 N?
Fnorm = 500 N
Fnet = 150 N, rightward
Ffric = 100 N
Fapp = 250 N
Fgrav = 500 N
c. A sheet of paper weighing 0.001 N is falling from a desk. If the
air resistance is 0.0002 N, what would be the net force on the
paper?
Fair = 0.0002 N
Fnet = 0.0008 N, downward
Fgrav = 0.001 N
d. A teacher is pushing a cabinet with a force of 450 N across the
floor. What would be the net force if the cabinet weighed 600 N
and the friction force is 450 N?
Fnorm = 600 N
Fapp = 450 N
Ffric = 450 N
Fgrav = 600 N
Fnet = 0 N