Download I. Force, Mass, and Acceleration

PSCI II – Chapter 3– Forces (pages 66 – 97)
Section 1 – Newton’s Second Law
I.
Force, Mass, and Acceleration
º
º
Remember Newton’s First Law.
Force and motion are connected.
Force and Acceleration
º Difference between a toss and a throw? – more force
º For any object the greater the force the greater the acceleration.
Force and Mass
º Difference between a softball and a baseball thrown? – mass
º Acceleration depends on mass.
º More mass means less acceleration – less mass means more acceleration.
II.
Newton’s Second Law
º States: the net force acting on an object causes the object to accelerate in
the direction of the net force.
º According to this equation acceleration = net force/mass.
º a = Fnet/m
º SI units would end up being kgm/s2 so scientists created a new unit called the
Newton (N) for force.
º Example: you are pushing a friend on a sled. You push with a force of 40N.
your friend and the sled together have a mass of 80kg. Ignoring friction,
what is the acceleration.
a
Fnet
Fnet
m
 40 N
m  80kg
a
40 N
 0.5 m s 2
80kg
PSCI II – Chapter 3 – Page 1 of 6
Using Newton’s Second Law
º Can be used to calculate force by Fnet = ma.
º Example: How much force would a tennis racket have to exert to give a ball
an acceleration of 5500m/s2 if the mass is 0.006kg?
Fnet  m  a
m  0.006kg
a  5500 m s 2
Fnet  (0.006)(5500)  33 kgm
III.
s2
 33N
Friction
º What happens when you push a skateboard?
º Newton’s law says it should go forever.
º Since it doesn’t there must be a force acting on the board.
º Defn. – force that opposes motion between two surfaces.
What Causes Friction
º Even the smoothest surfaces have microscopic bumps.
º These are called microwelds between the surfaces.
Sticking Together
º The stronger the forces pushing objects together the stronger the
microwelds will be.
º To break the microwelds force must be applied.
Static Friction
º Imagine you try to push a heavy box and it doesn’t move.
º According to Newton’s second law the net force must be zero.
º Another force is acting on the box – friction.
º Static friction – friction between two surfaces that are not moving past each
other.
º Push is not enough to move the box.
Sliding Friction
º Same box but you and a friend push it and it moves but not easily and stops
after you quit pushing.
º Sliding friction – opposes the motion of two surfaces sliding past each other.
º Caused by breaking and reforming of microwelds.
PSCI II – Chapter 3 – Page 2 of 6
Rolling Friction
º Think of a car stuck in the mud or on ice – what do you usually do to help
them? – Gravel gets put down so friction is increased.
º Rolling friction – friction between a rolling object and the surface its rolling
on.
º Microwelds break and reform as wheel rolls.
º Usually less than sliding or static friction.
IV.
Air resistance
º Acts on objects that fall through the air.
º Think of crumples and flat paper.
º Flat paper takes longer to fall so there’s a force acting on it.
º Air resistance acting up opposing gravity acting down.
º Acts opposite an objects motion.
º Depends on speed, size, shape, of the object.
º Everything falls at the same rate when no air is present (vacuum).
Terminal Velocity
º As objects fall they accelerate and the force of air resistance increases until
it cancels out gravity.
º At this point the forces cancel and are balanced and it falls at a constant
rate/speed (terminal velocity).
Section 2 – Gravity
I.
The Law of Gravitation
º We’re very attractive people (gravity attracts us to everything).
º Law of gravitation says that any two masses exert an attractive force on
each other.
º Depends on two things – the masses and the distance between them.
º So as mass increases gravity increases.
º As distances increases gravity decreases.
º Why do we not get pulled off the Earth then and into the sun? – To far away
Gravity – a Basic Force
º Four basic forces (electromagnetic, strong nuclear, weak nuclear and
gravity).
The Range of Gravity
º Even though something is very far away gravity never disappears.
º Gravity is a long-range force.
PSCI II – Chapter 3 – Page 3 of 6
II.
Gravitational Acceleration
º Near Earth’s surface all objects have an acceleration of 9.8 m/s2 (g).
º Remember that F = ma so if a = 9.8 m/s2 then F = m x 9.8 m/s2 .
º This is then the force of gravity and depends only on the mass of an object.
º Force of gravity is always down.
Inertia and Gravity
º If you dropped a bowling ball and a tennis ball off a bridge at the same time,
which would hit first? Neither – same time.
º True gravity on the bowling ball is greater but they hit at the same time.
º This is because more force is needed to change the velocity of the bowling
ball and that cancels out gravity.
III.
Weight
º No matter whether you’re standing, jumping or falling Earth exerts a force
on you.
º Weight – the gravitational force exerted on an object.
º Gravitational force = mass x acceleration due to gravity or W = mg.
º Example: a person has a mass of 50kg would have a weight of
º W = (50kg)( 9.8 m/s2) = 490 N
Losing Weight
º What happens to weight far from Earth?
º It gets weaker.
Weight and Mass
º Not the same.
º Weight is a force and mass is the amount of matter.
º More mass equals greater gravity.
º Different places can have different weight – planets.
IV.
Weightlessness and Free Fall
º Experienced in space very far from Earth’s pull.
º About 400km above earth gravity is about 90%
Floating in Space
º Picture yourself on an elevator standing on a scale.
º While you’re standing on the scale all forces are balanced.
º Now picture that the elevator is going down – this is kind of free fall and the
scale would read less than true value (maybe even zero).
PSCI II – Chapter 3 – Page 4 of 6
V.
Projectile Motion
º Thrown objects tend to curve downward.
º Anything thrown or shot through the air is a projectile.
º Projectiles curve because of inertia and gravity.
º They have a horizontal (the throw) and vertical (gravity) velocities.
Horizontal and Vertical Motion
º When you throw a ball you give it horizontal velocity.
º When you let go of the ball gravity acts on the ball too.
º Gravity is the unbalanced force that causes a change in motion.
º If you throw a ball and drop a ball from the same height what would
happen? – both would hit the ground at the same time.
Centripetal Force
º When a ball enters a curve remember it’s accelerating because direction is
changing.
º When a ball enters a curve the change in velocity is toward the center of the
curve.
º Centripetal acceleration – acceleration toward the center of a curved or
circular path.
º Centripetal means move toward the center.
º Centripetal force is acting as the unbalanced force to cause motion change.
º Defn. – a force directed toward the center of the circle for an object moving
in circular motion.
The Moon is Falling
º Talks about why the moon stays in orbit.
º Read the section.
Section 3 – The Third Law of Motion
I.
Newton’s Third Law
º States: when one object exerts a force on a second object the second
objects exerts a force on the first that is equal in size and opposite in
direction.
Action and Reaction
º How many action – reaction pairs can you come up with?
PSCI II – Chapter 3 – Page 5 of 6
How You Move
º If we have only equal action-reaction pairs how do we move?
º The pairs are acting on different objects.
º Example is swimming – a person pushes on the water and the water
pushes the person forward.
Rocket Propulsion
º Main principle behind rockets.
º Fuel is heated and produces gases.
º Engine then exerts a force on the gases to push them out.
º Newton’s third law gases exert a force as they’re pushed out forcing the
rocket forward.
II.
Finding Planets With Newton’s
º How we found Neptune.
º Read the section.
III.
Momentum
º Defn. – property that a moving object has because of its mass and velocity.
º Momentum (p) = mass (m) x velocity (v) or p = mv
º Units are kgm/s
º Has direction because velocity has direction.
Force and Changing Momentum
º We can make some substitutions for velocity to calculate for changing
momentum.
F
mv
f
 mvi 
t
mvi  initialmom entum
mv f  finalmomentum
Law of Conservation of Momentum
º momentum doesn’t change unless mass or velocity change.
º Can be transferred from one object to another.
º States: if a group of objects exerts forces only on each other their total
momentum doesn’t change.
º No momentum is created or destroyed.
When Objects Collide
º Both moving in same direction – second puck will continue but more quickly.
º Opposite directions – pucks will reverse direction and go with same speed
PSCI II – Chapter 3 – Page 6 of 6