Transcript net force

Newton’s Laws of Motion
• What happens when one particle collides
with another particle (of the same mass)?
• What is an elastic collision?
• What is an inelastic collision?
• How would the situation change if one
particle was more massive than the other
particle?
http://www.picgifs.com/grap
hics/c/collision/graphicscollision-644627.gif
• How is the speed of an object defined?
• How does relative speed change the
outcome of a collision?
• What is the momentum of an
object/particle?
• Compare the momentum of two objects
of different masses moving with the
same speed. Which has a greater
momentum? Explain.
• What is a force?
• How does a force affect the motion of a
object/particle?
• Can one particle exert a force on another
particle without itself being affected?
• How does the force exerted on one particle
compare with the force on the other particle
during the collision?
• A collision (interaction) between two objects
of different masses occurs – say a bee and
a windshield of a car.
• How do the forces on each object compare
at impact? How do the objects’ resulting
accelerations compare at impact?
• Collisions between objects come
somewhere in between two extremes:
elastic and inelastic.
• An elastic collision is one in which the two
objects bounce off one another.
• An inelastic collision is one in which the two
objects stick together.
• Inertia is the resistance to change in motion
(AKA – the acceleration) of an object.
• The inertia (AKA – the mass) of an object is
measured in kilograms (sometimes in
grams).
• Speed is the time rate at which distance is
covered.
• Example 1: If a car has a speed of 70
miles/hour, what does this mean?
• How much time elapses every 70 miles?
• How many miles are traveled every hour?
• The speed of an object can be determined
by dividing the distance by the time
interval it took to cover that distance. In
symbols: S = d/t
• Example 2: if a marble travels a distance
of 30 centimeters in about 2 seconds, then
what is its speed?
• S = d/t = 30 cm/2 s = 15 cm/s
Observations with
colliding speeds:
1 (medium speed)
2 (faster speed)
3 (½medium speed)
Marble 1
(inertia: ________g
Marble 1
Marble 1
Marble 1
(inertia: _______g (inertia: _______g (inertia: _______g
• Follow the general directions on the lab
sheet and commence learning!
• At the completion of investigation 1 discuss
your data and observations within your
group, write a summary of your
observations, and finally debrief with your
instructor (as a group or class).
• What happens when a particle collides
with another particle of similar mass?
• In general, if the objects do not stick
together (an elastic collision), the initially
moving object will come to rest (stop
moving) and the initially stationary object
will move with about the same speed as the
once moving object.
• How does increasing (or decreasing) the
speed of the starting marble affect the
speeds of both marbles after the collision?
• In general, during an elastic collision
between two objects of the same mass,
whatever speed the first object has will be
transferred to the second object.
• What happens during the collision
between the moving marble and the
stationary marble? Or what causes the
stationary marble to begin moving, and
the moving marble to come to rest?
• During this interaction something called a
force causes both of these effects to occur.
• A force is a push or a pull on an object.
• During the collision, was the force on the
stationary object a push or a pull? Explain.
• A push.
• During the collision, which way was the
force on the stationary marble acting?
• In the direction that it began moving.
• During the collision, which way was the
force on the initially moving marble acting
and was it a push or a pull?
• It was a push in the opposite direction of
its initial motion. The force made it stop,
or nearly stop.
• Compare the size of the forces acting on
each marble during the collision. Is one
larger than the other? Explain.
• They are equal in size.
• How would you describe the total motion
of the two-marble system with similar
masses before and after the collision?
• The total quantity of motion before and
after a collision between objects of the
same mass is the same. Whenever a
quantity stays the same it is said to be
conserved.
Eureka! Law of Inertia video
• http://www.youtube.com/watch?v=by-7kkAu2Pg
Sir Isaac Newton – English physicist,
mathematician, astronomer,
1642 - 1727
Newton's First Law – The Law of Inertia
• The velocity of a body remains constant
unless the body is acted upon by an
external force.
• Meaning - If something is moving, it is
going to continue to move unaffected until
some force or other object is encountered.
• Net Force = 0 (upward force on the book cancels
downward force on the book; also known as balanced forces)
• Net Force = 0 (forces balance or cancel each other out)
• Net Force  0 (forces do not cancel each other out)
• Say a book is slid across the table to a
friend…what happens?
• The amount of inertia of an object is measured
by its mass.
• The units for mass in the metric system is grams.
• The standard for mass in the metric system is the
kilogram (1000 grams).
• If inertia is the resistance to a change in
motion, then what will happen if the
stationary marble is replaced with one of
a greater mass?
• How will the resulting speeds of each
marble compare after the collision?
• How will differing speeds of the smaller
marble affect the results?
• Follow the directions on the lab sheet for
Part II, and commence learning!
• At the completion of investigation Part II
discuss your data and observations within
your group, write a summary of your
observations, and finally debrief with your
instructor (as a group or class).
• What happens when a particle of smaller
mass collides with another particle of
larger mass?
• In general, during an elastic collision with an initially
moving smaller mass colliding with a stationary larger
mass, the result is that the larger mass will move in the
same direction as the smaller mass before the collision,
but with a smaller speed; and the smaller mass will move
in the opposite direction with a smaller speed.
• Using the relative inertia of each marble,
describe how the inertia of an object
affects its motion.
• During an interaction (collision), the object
with less inertia has a greater change of
motion than the object with more inertia.
• How did increasing or decreasing the
speed of the less massive marble affect
the resulting motion of each marble?
• The more massive marble had a greater
change in motion; it moved with a greater
speed. The less massive marble had a
greater change in motion as well in the
opposite direction.
• In Part I, speed or motion was “conserved.”
Was speed “conserved” in Part II? Explain.
• No. Before the interaction (collision) only the
smaller marble had a speed in one direction.
After the interaction, the smaller marble had a
speed in the opposite direction, and the larger
marble had a small speed in the same direction as
the smaller before the collision.
• Compare the size of the forces acting on
each marble during the collision. Is one
larger than the other? Before you answer,
remember that each marble has a different
amount of mass (inertia). Explain.
• They are equal in size. The smaller mass marble has less of a
resistance to changing its motion so we see a larger change in
its motion, and likewise the stationary more massive marble has
a smaller change in its motion. Therefore even though the forces
(the causes) are the same size on each marble, the resulting
changes in motion (effects) are different.
• Using what you learned from Parts I & II
predict what will happen in Part III.
• What will happen when a larger mass
marble collides with a stationary smaller
mass marble?
• Which marble will have a larger change
in motion?
• Follow the directions on the lab sheet for
Part III.
• At the completion of investigation Part III
discuss your data and observations within
your group, write a summary of your
observations, and finally debrief with your
instructor (as a group or class).
• What happened when a larger mass
marble collided with a stationary smaller
mass marble?
• The larger mass’s speed got smaller, and
the smaller mass speed increased.
• Which marble had a larger change in
motion?
• The smaller mass marble.
• 1st Law (the law of inertia): an object will
remain in motion at a constant speed in a
straight line, or at rest, until acted upon by
a net force. A net force on an object
causes the object to accelerate (change its
motion).
• A net force is the sum of all the forces
acting on an object.
• Two possibilities (flow chart – The Physics
Classroom.com)
• Net Force = 0 (forces balance or cancel each other out)
• Net Force  0 (forces do not cancel each other out)
• Say a book is slid across the table to a
friend…what happens?
• The amount of inertia of an object is measured
by its mass.
• The units for mass in the metric system is grams.
• The standard for mass in the metric system is the
kilogram (1000 grams).
• The Mechanical Universe : The Law of Inertia
• Practice with the law of inertia.
• If an object accelerates (changes its
motion) then a agent called a net force
acts on it.
• A force is a push or a pull.
• There are two categories of forces –
contact and non-contact forces.
• What might be an example of a contact force?
• Lifting a book with your hand, friction between
your shoe and the ground, pulling a door handle
• What might be an example of a non-contact
force?
• Dropping a pencil on the floor (the earth’s
gravitational attraction to it).
• Your hair attracted to a plastic comb (a static
electric force of attraction).
• A magnet picking up a paper clip (a magnetic
force of attraction).
• Newton’s 2nd Law of Motion: the
acceleration (change of motion) on an
object is directly proportional to the net
force acting on it.
• As the net force increases, the acceleration
increases; as the net force decreases, the
acceleration decreases.
• Newton’s 2nd Law - continued
• If the mass (inertia) changes, then the
resulting acceleration changes inversely.
• For a given constant forces if the mass
increases, the acceleration decreases; if
the mass decreases the acceleration
increases.
• Newton’s 2nd Law - continued
• These relationships are summarized in the
equation: Force = mass x acceleration, or
symbolically
F = ma
• Newton’s 2nd Law of Motion – Example
• Suppose a 800 kg car has an acceleration
of 8 m/s/s (it’s speed increases 8m/s
every second). What is the net force on
the car?
• Suppose a 800 kg car has an acceleration of 8
m/s/s. What is the net force on the car?
• From the 2nd law: F = ma = (800kg)(8
m/s/s) = 6400 N
• Newton’s 2nd Law Example 2
• Suppose a 1000 kg car smashes into a
telephone pole that exerts a force of 200,000
N on the car before breaking.
• What is the acceleration of the car?
• Suppose a 1000 kg car smashes into a telephone pole
that exerts a force of 200,000 N on the car before
breaking. What is the acceleration of the car?
• Again F = ma 
• -200,000 N = (1000kg)(a)
(note the negative sign indicates the
direction of the force)
• 200,000N/1000kg = -200 m/s/s
• Newton’s 3rd Law: forces always occur in
pairs, equal in size and opposite in
direction.
• In the example below: the ball exerts a
force on the catcher glove. What is the
reaction force? Symbolically: Fglove by ball
• Answer: The force on the ball by the glove.
Symbolically: Fball by glove
• What is the reaction force to Fpin by ball?
• Answer: The force on the ball by the pin.
Symbolically: Fball by pin
• Are you seeing the pattern?
• Newtons 3rd law Mythbusters
http://www.youtube.com/watch?v=yneECzfVkYk