Forces and Motion PowerPoint

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Forces and Motion
Physical Science, Physics in the Workplace
Anne Rettig, Instructor
Part 1:
Forces
Forces affect Motion
 A force is a push or a
pull that changes an
object’s motion.
 This means that forces
make objects speed up,
slow down, or change
direction.
 Forces also transfer
energy from one object
to another object like
when you hit a ball with a
bat.
 The SI unit used to
measure force is the
newton. (N).
 One newton is the amount
of force needed to accelerate
a 1 kg mass by 1 m/sec2.
Gravity and Friction are Forces
 The force of gravity on an
 The force of friction acts to
object is called weight.
slow objects that are already
moving or make them stop.
 Weight is different than mass.
 Mass is the amount of matter  Friction is caused when
objects rub together.
in an object and weight
measures the force of gravity  There are three kinds of
on that mass.
friction: sliding, rolling, and
fluid friction.
 Mass does not change on
different planets, but weight
does, depending on how strong
the force of gravity is!
Types of Friction
 Sliding friction - when two
objects slide over one another;
determined by object’s weight
and types of surfaces
 Fluid friction - when an
object moves through a
fluid; the smallest force of
the three types of friction for
similar objects
 Rolling friction -when an
object rolls over a surface; a
smaller force than sliding friction
for similar objects
Balanced and Unbalanced Forces
 According to Newton’s 3rd
Law, forces always act in
pairs.
 Forces may be balanced
(equal in amount and
opposite in direction).
 Balanced forces cause no
change in an object’s
motion.
 Forces can also be
unbalanced (opposite in
direction but unequal in
amount).
 Unbalanced forces do
change an object’s motion
(The racquet’s force makes
the ball reverse direction!
Net Force
 Net force is the sum of all the forces acting on an object.
 If the forces are acting in the same direction, add them together to
find the net force. Which way will the boats be pulled? (Hint: Don’t
forget direction.)
N
W
5N
E
A
S
15 N
Net force on A = _____
balanced or
unbalanced?
 If the forces are acting in the opposite direction, subtract to find the
net force.
4N
15 N
B
10 N
16N
C
12 N
Net force on B = _____
balanced or unbalanced?
Net force on C = _____
balanced or unbalanced?
Universal Forces
 There are four forces that work together to control the
universe as we know it. Some of these forces work over
very large distances and some can only be detected at very
small distances. The four forces are:
1.
Gravitational Force - is the
3.
weakest of the universal forces.
It depends on the mass of the
objects and the distance between
them.
4.
2. Electromagnetic Force -may
attract or repel. These are much
stronger than gravity.
Strong Force - holds the
nucleus of an atom together by
binding the protons and the
neutrons.
Protons, neutrons, and electrons
are made of smaller particles
called “quarks.” Weak force glues the quarks together.
Quarks stuck
together to make a
proton
Part 2:
Measuring Motion
Speed, Velocity, and Acceleration
 Since forces cause an object’s motion to change, we
need to be able to measure those changes.
 There are three important measurements you must be
able to make to understand how forces affect motion.
These measurements are speed, velocity, and
acceleration.
Speed
 Speed measures how fast something moves.
 Speed is measured in units like meters/sec or
miles/hour (always a unit for distance over a unit for
time).
 Speed = distance / time
D
s
t
If you know any two of these
values, you can solve for the third
one:
D=st
s=D/t
t=D/s
Velocity
 Velocity measures both speed and direction.
 Velocity is also equal to distance / time, but you must tell the
direction of the movement too.
 These are correct ways to describe velocity: “The car
traveled at 70 miles/hour east.” “The rocket’s velocity was
34,000 m/s straight up.”
 Velocity has the same units as speed.
If you know any two of these values,
you can solve for the third one:
D=vt
v=D/t
t=D/v
D
v
t
Compare these formulas to the ones
on the previous slide! What do you
see?
Acceleration
 Acceleration measures how fast you change velocity.
 Acceleration happens whenever you speed up, slow down,
or change direction.
 When you speed up, acceleration is a positive number.
 When you slow down, acceleration is a negative number,
and is sometimes called “deceleration.”
 Acceleration is measured by change in velocity / time. The
unit for acceleration is meters/sec2.
Δv
a
t
If you know any two of these values,
you can solve for the third one:
Δv=at
a = Δv / t
t = Δv / a
terminal velocity, the
Falling Objects  At
upward force of friction and the
 If no other force acts on an
object, the force of gravity
will make the object fall
towards the center of the
earth.
 As the object falls, it will
accelerate (speed up) by 9.8
m/sec2 every second it falls
until it hits something.
 If the object falls from a very
high place, like a tall building
or an airplane, it may reach
terminal velocity before it
hits the ground.
downward force of gravity are
equal and opposite (balanced),
so the object’s speed stays the
same until it hits.
Projectile Motion
 A projectile is anything that is thrown or shot in a roughly
horizontal direction, like a baseball or a bullet.
 Projectile motion is always a curved path followed by the
moving object that was thrown or shot.
 This curved path of projectile motion is caused by the
combination of two forces: the horizontal push from the gun or
the hand and the downward force of gravity.
Downward Pull
of gravity
Horizontal Force of cannon firing
Part 3: Newton’s
Laws of Motion
Newton’s Laws of Motion
 1st Law: An object at rest
 2nd Law: Force =
stays at rest, and an object
(mass)(acceleration)
in motion stays in motion,
F=ma
unless acted on by an
outside unbalanced force.  3rd Law: For every force or
action, there is an equal and
opposite force or reaction.
Newton’s First Law
 First Law: “An object at rest stays at rest, and an object in
motion stays in motion, unless acted on by an outside
unbalanced force.”
 This means that an object will stay in place until you exert a
force on it (push or pull it).
 If the object is already moving, then it will keep going in the
same direction and at the same speed until something
pushes or pulls on it.
This heavy statue
does not move
unless it is pushed
or pulled!
That rocket travels in a
straight line at constant
speed unless something
makes it speed up, slow
down, or turn.
Newton’s Second Law
 Second Law: “Force = mass x acceleration”
 This means that it takes more force to make a heavy object
move than a light one.
 It also means that it takes more force to make an object
change speed fast than to change speed slowly.
F
If you know any two of these values,
you can solve for the third one:
F=ma
m
a
m=F/a
a=F/m
Newton’s Third Law
 3rd Law: “For every force or action, there is an equal and
opposite force or reaction.”
 This means that forces always act in pairs.
If a
wrecking
ball pushes
on a wall,
the wall also
pushes
back on the
wrecking
ball with
equal and
opposite
force.
When you
apply force to
a tennis ball
with a
racquet, the
ball also
applies an
equal and
opposite
force to the
racquet.
Circular Motion
 When objects move along a
circular path, they are
accelerating, even if they
don’t change speed—Why?
(Review the definition of
acceleration.)
 What things move in a
circular path? Think of at
least three examples.
 If an object is accelerating,
then a force must be acting
on the object.
 Centripetal force is the
force needed to make an
object move in a circle.
 Centripetal force always
points toward the center of
the circle.
 An object moving in a circle
has a special kind of
acceleration called
centripetal acceleration.
 Newton’s Laws of Motion
explain circular motion just
like other kinds of motion.