Download Motion and forces (Ch 10 and 11)

Jones
Physical Science
MOTION AND FORCES
(CH 10 AND 11)
Friction
 What is friction?
 Friction: the unbalanced force that acts against
an object’s direction of motion
 Why does friction occur?
 The surface of any object is rough and sticks to
other surfaces that are also rough
 How does friction affect motion?
 It opposes any applied force
 How does a car keep moving?
Two types of Friction
 Static friction
 The friction between stationary (nonmoving)
surfaces
 Kinetic friction
 The friction between moving surfaces
 Can be either sliding friction or rolling friction
 Sliding friction: when two objects slide past each
other
 Rolling friction: when a round object rolls over a flat
surface
Friction
 Other than the friction caused by the tires
coming in contact with the ground, what
other friction is there to oppose the motion of
the car?
 Fluid friction
 Friction caused by an object moving through a
fluid, such as air
 What can car companies do to take care of
this potential problem?
Positive friction
 Without friction, the tires of a car would not
be able to push against the ground and move
the car forward.
 You are using friction right now to push ink or
lead on your paper so much to leave a mark.
 Without friction, you would slip and fall when
you tried to walk.
Positive friction
 How can positive
friction be
increased?
 Make surfaces
rougher
 Increase the force
pushing the surfaces
together
Negative friction
 How can negative
friction be
decreased?
 Use lubricants such as
motor oil, wax, and
grease
 Replace sliding
friction with rolling
friction
 Rolling friction is
usually less than
sliding
 Make the surfaces
smoother
Newton’s Laws of Motion
 Sir Isaac Newton established three laws that
describe the relationship between motion
and force.
Newton’s First Law
 An object at rest remains at rest and an
object in motion maintains its velocity unless
it experiences an unbalanced force.
 Inertia is the resistance in change of motion.
 Inertia is related to mass
 An object with a small mass has less inertia, and it
is easier to change its motion
 An object with a large mass has more inertia, and
it requires more force to change its motion
Newton’s First Law
 Explain the function of a car seat or a seat
belt in terms of mass and inertia.
 Is the person in motion if the car is in motion? YES
 If the car meets an unbalanced force, what will
happen to the person if there is no unbalanced
force acting on the person as well as the car?
Newton’s Second Law
 The unbalanced force acting on an object
equals the object’s mass times its
acceleration.
 Force = mass x acceleration (F=ma)
 Page 349 Fig 3
 Empty grocery car versus full grocery cart
 Imagine pushing both carts with the SAME force
 Which one will be moved farther? Why?
Newton’s Second Law
 Practice problems
 Page 351 #1-3 in your notes
Gravity
 All matter is
affected by gravity
 Gravitational force
increases as mass
increases
 Gravitation force
decreases as
distance increases
Free-fallin’
 When gravity is the only force acting on an
object, it is said to be in free-fall.
 The free-fall acceleration towards the center
of Earth of ANY object is 9.8 m/s2
What is the difference
between mass and weight?
 Mass – a measure of the amount of matter in
an object
 Weight – dependent on gravity; equal to an
object’s mass times free-fall acceleration
 Unit is Newtons (N)
 Weight = mass x 9.8 m/s2
Can weight influence the
shape of living things?
 YES!
 Large, land animals must have strong skeletons to
support their mass against the force of gravity
 Marine life does not require strong skeletons
because it can drift due to the downward force of
gravity being balanced by the upward force of the
water.
What happens to objects moving
through Earth’s atmosphere?
 Both air resistance and gravity act on the
object.
 Velocity becomes constant when the force of
air resistance is equal to the gravitational
force on the object.
 The object will stop accelerating and will
reach its maximum velocity, also called
terminal velocity.
Objects In Space…
 Objects in space are in free fall (there is no air
resistance)
 Weight is a gravitational force that depends
on the masses of objects and the distance
between them
 Therefore, it is impossible to be weightless
anywhere in the universe.
Objects In Space…
 Orbiting objects are also in free fall
 What causes orbiting?
 Orbiting: when an object is traveling in a circular
path around another object
 Two motions are responsible for creating an orbit:
 Path of the object (Moon’s path, direction of
spaceship)
 Earth’s gravitational pull
 When the forward motion combines with free fall,
the object follows the curve of the Earth’s surface.
Projectile Motion
 Projectile motion: the curved path an object
follows when thrown, launched, or otherwise
projected near the surface of the Earth
 Two independent components:
 Horizontal motion
 Vertical motion
 The two do not affect each other, but combine to
form a curved path
 P 358 Figure 13
Projectile Motion
 First component: Horizontal motion
 A force is exerted on an object to make it go horizontal
 Second component: Vertical motion
 Gravity pulls the object downward
 Putting these together result in a curved path
 Why do archers aim above the bull’s eye on a
target?
Newton’s Third Law
 For every action force, there is an equal and
opposite reaction force.
 Also called the “law of action and reaction”
Forces always occur in pairs
 Whenever a force is exerted, another force
occurs that is equal in size and opposite in
direction.
 Even when there is no motion
 Example:
 Your weight pushing down on the chair while
sitting: action force
 The chair pushing back up with a force equal to
your weight: reaction force
Force pairs do not act on
the same object!
 P 361 Fig 16
 Action force: the swimmer pushing the water
backward
 Reaction force: the water pushing the
swimmer forward
Momentum
 Momentum: a property of all moving
objects, which is equal to the product of the
mass and the velocity of the object.
 Momentum (p) = mass (m) x velocity (v)
 Practice problems Page 363 #1-2
Momentum is dependent on
mass
 Objects with less mass have less momentum
 Objects with more mass have more
momentum
 Momentum also has direction –
 An object’s momentum is in the same direction as
its velocity.
Changing momentum
 How can momentum of an object be
changed?
 Apply a force to the object
 The object collides with another object
 Transferring the momentum from one object to
another
Changing momentum
 By applying force
 Happens when you force an object to change its
motion
 Extending the time needed to change the
momentum lessens the force
 For example, moving in the direction of the
motion after landing a pole-vault, high-jump, or
gymnastics trick, extends the time of the
momentum change, which results in the impact
force decreasing.
Changing momentum
 When two objects collide, momentum is
conserved (total amount of momentum does
not increase or decrease).
 Some momentum may be transferred from one
object to another, such as in the game of pool.
 Fig 21 on Page 365
 The rocket pushes gases downward/backward
toward the surface of the Earth
 The gases push the rocket upward/forward toward
the sky in an equal amount.