Transcript Slide 1

Forces and Motion
Chapter 12
Chap 12 Pretest
• What is relative motion?
• Relative motion is movement in relation to
a frame of reference.
Chap 12 Pretest
• What is the difference between distance
and displacement?
• Distance is the length of a path between
two points.
• Displacement is the direction from the
starting point and the length of the straight
line from the starting point to the ending
point.
Chap 12 Pretest
• How is average speed calculated?
• Total distance is divided by total time.
• v=d/t
Chap 12 Pretest
• On a distance-time graph, what does the
slope represent?
• The slope represents the speed.
Chap 12 Pretest
• What is velocity?
• Velocity is speed with direction.
Chap 12 Pretest
• A backpack falls out of an open window.
The backpack starts from rest and hits the
ground 1.0 second later with a velocity of
9.8 m/s. What is the average acceleration
of the backpack?
• A. 9.8 m/s
• B. 9.8 m
• C. 9.8 m/s2
• D. All of the above
Chap 12 Pretest
• How is acceleration related to velocity?
• Acceleration is the change in velocity, that
is, any change in speed, direction, or both.
Chap 12 Pretest
• How are mass and weight different?
• Mass is the measure of inertia; weight is
the measure of the force of gravity acting
on an object.
Objects in Free Fall
• What factors affect a falling object? Perform the
following simple activity to begin learning about the
forces that act on falling objects.
• Stand beside your desk. Hold a sheet of notebook paper
level at eye level. Release the sheet of paper and watch
it fall. Describe the motion of the paper.
• Hold a sheet of notebook paper that has been crumpled
into a tight ball at eye level. Release the crumpled paper
and watch it fall. Describe the motion of the paper.
• How do the motions of the flat sheet of paper and
crumpled ball of paper compare? What forces do you
think are acting on each sheet of paper?
12.1- Forces
• A force is a push or a pull that acts on an
object.
• A force can cause a resting object to
move, or it can accelerate a moving object
by changing the object’s speed or
direction.
12.1 - Forces
• Force is measured in newtons.
– Newtons are abbreviated as N.
• One newton is the force that causes a 1kg
mass to accelerate at a rate of 1 meter per
second each second.
– 1 N = 1 kg•m/s2
• The newton is named after Sir Isaac
Newton.
12.1 – Representing & Combining
Forces
• Forces can be represented by arrows
showing the direction and strength of the
force.
• The net force is the overall force acting on
an object after all of the forces are
combined.
– Force arrows are combined. Forces in the
same direction add together, forces in
opposite directions subtract from one another.
12.1 – Balanced Forces
• Balanced forces are forces that combine
to produce a net force of zero.
• When the forces on an object are
balanced, the net force is zero and there is
no change in the object’s motion.
• Examples: tug of war, arm wrestling
12.1 – Unbalanced Forces
• An unbalanced force is a force that results when
the net force acting on an object is not equal to
zero.
• When an unbalanced force acts on an object,
the object accelerates.
• Examples: pushing a resting book on table,
winners/losers of arm wrestling or tug of war
matches
12.1 - Friction
• All moving objects are subject to friction, a force
that opposes the motion of objects that touch as
they move past each other.
• Friction acts at the surface where objects are in
contact.
• There are four main types of friction: static
friction, sliding friction, rolling friction, and fluid
friction.
12.1 – Friction Types
• Static Friction – the friction force that acts
on objects that are not moving.
– Always acts in the direction opposite of the
applied force.
• Sliding Friction – a force that opposes the
direction of motion of an object as it slides
over a surface.
– Sliding friction is less than static friction, so
less force is needed to keep on object moving
that to start it moving.
12.1 – Friction Types
• Rolling Friction – the friction force that acts on
rolling objects.
– Rolling friction is about 100 to 1000 times less than
the force of static or sliding friction.
– Examples – moving dollies, ball bearings
• Fluid Friction – the force that opposes the
motion of an object through a fluid.
– Fluid friction acting on an object moving through air is
known as air resistance.
– Examples – stirring cake batter
12.1 - Gravity
• Gravity is a force that acts between any
two masses. Gravity is an attractive force,
pulling objects together.
• Earth’s gravity acts downward toward the
center of Earth.
12.1 – Falling Objects
• Both gravity and air resistance affect the motion
of a falling object.
• Gravity causes objects to accelerate downward,
whereas air resistance acts in the direction
opposite to the motion and reduces acceleration.
• Terminal velocity is the constant velocity of a
falling object when the force of air resistance
equals the force of gravity.
12.2 – Newton’s First Law of Motion
• According to Newton’s first law of motion,
the state of motion of an object does not
change as long as the net force acting on
the object is zero.
– Unless an unbalanced force acts, an object at
rest remains at rest, and an object in motion
remains in motion with the same speed and
direction
12.2 – Newton’s First Law of Motion
• Newton’s first law of motion is sometimes
called the law of inertia.
• Inertia is a tendency of an object to resist
change in its motion.
12.2 – Newton’s Second Law of Motion
• According to Newton’s second law of motion, the
acceleration of an object is equal to the net force
acting on it divided by the object’s mass.
• Mass is the measure of inertia of an object and
depends on the amount of matter the object
contains.
• Acceleration = net force / mass, or a = F/m
12.2 – Practice Problems
• A boy pushes forward a cart of groceries with a
total mass of 40.0 kg. What is the acceleration
of the cart if the net force on the cart is 60.0 N?
• a = F/m = 60 N / 40kg=1.5 m/s2
• What is the upward acceleration of a helicopter
with a mass of 5,000 kg if a force of 10,000 N
acts on it in an upward direction?
• a= F/m = 10,000 N / 5,000 kg = 2 m/s2
12.2 – Practice Problems
• An automobile with a mass of 1,200 kg
accelerates at a rate of 3.0 m/s2 in the forward
direction. What is the net force acting on the
automobile?
• a = F/m, or F = ma = 1,200kg*3.0 m/s2 =3,600 N
• A 25 N force accelerates a boy in a wheelchair
at 0.5 m/s2. What is the mass of the boy and the
wheelchair?
• a= F/m, m = F/a = 25 N / 0.5 m/s2 = 50 kg
12.2 – Weight and Mass
• Weight and mass are not the same thing.
– Weight is the force of gravity acting on an
object.
– Mass is the measure of inertia of an object
and depends on the amount of matter the
object contains.
• Weight = Mass x Acceleration due to gravity
– W = mg
12.2 – Weight and Mass
• Do you weigh more on Earth or the moon?
• Hints:
– Your mass is the same in both locations.
– The acceleration due to gravity on Earth is 9.8
m/s2, and the moon’s acceleration due to
gravity is 1/6th that on Earth.
• Answer:
– You weigh only 1/6th as much on the moon as
on Earth!
12.3 – Newton’s Third Law of Motion
• According to Newton’s third law of motion,
whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first
object.
• These two forces are called action and
reaction forces.
12.3 – Action and Reaction Forces
• Action-Reaction forces are equal in size and opposite in
direction.
• Action-Reaction forces may or may not result in motion.
• Action-Reaction forces do not act on the same object so
they do not cancel or have a net force of zero.
– Only when equal and opposite forces act on the same object do
they result in a net force of zero.
• Examples:
– Bumper cars (motion occurs)
– Pushing against a wall (no motion occurs)
12.3 - Momentum
• Momentum is the product of an object’s mass and its
velocity.
– Momentum = Mass x Velocity
• Momentum is measured in units of kilogram-meters per
second.
• An object has a large momentum if the product of its
mass and velocity is large.
– An object with large momentum is hard to stop.
• The momentum for any object at rest is zero.
12.3 - Momentum
• Which has more momentum, a 0.046kilogram golf ball with a speed of 60.0
meters per second, or a 7.0-kilogram
bowling ball with a speed of 6.0 meters per
second?
• Momentum golf ball = 0.046 kg x 60.0 m/s =2.8kg*m/s
• Momentum bowling ball = 7.0 kg x 6.0 m/s = 42 kg*m/s
12.3 – Conservation of Momentum
• According to the law of conservation of
momentum, if no net force acts on a
system, then the total momentum of the
system does not change.
• In a closed system, the loss of momentum
of one object equals the gain in
momentum of another object – momentum
is conserved.
Momentum vs Time graph
• At what time did the ball have
zero momentum?
– At t = 0 s; the ball has zero
momentum before it is
released.
• At what time did the ball have
the greatest momentum?
– At t = 2.5s; about 6.5kg*m/s
• What is the ball’s speed after
1.25 seconds?
–
–
–
–
–
–
Momentum = mass x velocity
Momentum = 3.25 kg*m/s
Mass = 0.25 kg
Velocity = momentum/mass
Velocity = 3.25 kg*m/s / 0.25 kg
Speed = 13 m/s