Download Unit 15 * Forces and Motion

Unit 15 – Forces and Motion
LESSON 1: FORCES
What are Forces?
 A force is a push or pull which causes a change in
motion.
 It can cause objects to stop moving, start moving,
move faster/slower, or change direction.
What are Forces?
 Forces are measured with spring
scales in units called newtons (N).
 When we use a scale, we are measuring
weight: the measure of the force of
gravity on an object.
Friction
 Friction: a force that opposes motion.
 It acts between two objects that are touching.
 If it is between air and a moving object, then it is
called air resistance.
Friction
 When two objects have friction between them, we
can feel heat coming off of them.
 This is because the energy of motion is being
changed into thermal energy.

When I say start, rub your hands together. Do not stop until I
say to. What happens?
Unit 15 – Forces and Motion
LESSON 1: FORCES
Magnetism
 Magnetism is the force of repulsion (pushing)
or attraction (pulling) between poles of magnets.
 Every magnet has two poles: a south pole and a
north pole.
Magnetism
 The north pole of one magnet is attracted to the
south pole of another magnet.
 The north pole of one magnet repels (pushes
away) from the north pole of another magnet.
Magnetic Fields
 A group of imaginary lines running from
the north pole to the south pole on a
magnet.
Gravitation
 Gravity is the force that pulls all objects
in the universe toward one another.
Gravitation
 The strength of gravitation depends on
the mass of the objects and the distance
between them.


More mass = more force
Less distance = more force
Gravitation
 Examples:
 The mass of 2 ping pong balls is much less than
the mass of a ping pong ball and the Earth.
 There is a much lower gravitational force with
the ping pong balls.
Gravitation
 Examples:
 Two ping pong balls on the same table will have
a stronger force than two ping pong balls across
the room because they are closer to one another.
Unit 15 – Forces and Motion
LESSON1: FORCES
Balanced Forces
 forces that are equal in size and opposite
in direction.
 Cancel each other out.
 Object’s do not move or change direction.
Unbalanced Forces
 When one force is
greater than its
opposite force.
 Object’s motion
changes – it can
stop, speed up, slow
down, or change
direction.
Forces Always Act in Pairs
 when one object acts on a second
object, the second object also acts on
the first object.
Net Force
 The net force is the combined forces
acting on an object at any time.
Net Force
 If two forces are acting on an object
from the same direction, you would
add the forces up.
 If they are acting in opposite
directions, you subtract the smaller
force from the larger one.
Example
 You and a friend are helping Ms.
Knapp move desks in the classroom.
You stand on the same side and both
push using a force of 15 N.
 What is the total force?
Example 2
 You and a friend are helping Ms.
Knapp move desks in the classroom.
You stand on one side, pushing with a
force of 25 N, and your friend wants
to go the other way. They push with a
force of 20 N.
 What is the total force?
Acceleration
 any change in an object’s motion.
 Speed up, slow down, stop, change
direction, etc.
Unit 15 – Forces and Motion
LESSON 4: NEWTON’S LAWS OF MOTION
Newton’s First Law of Motion
 Objects at rest don’t move unless an outside force
makes them move; objects moving don’t stop unless
an outside force makes them stop.
 No acceleration can happen without an unbalanced
force.
Newton’s First Law of Motion
 Inertia: the tendency of an object to resist a change
in motion
Newton’s Second Law of Motion
 Acceleration depends on the amount of force applied
to the object and the object’s mass.
 It can be written as a formula:
 Force = mass times acceleration
 F = (m)(a)
Newton’s Second Law of Motion
 The greater the force, the greater the acceleration.
 If you gently kick a soccer ball, it will not go as fast or
as far as if you kick it with more force.
 You must use more force to move an object with
greater mass.

Think about an empty grocery cart compared to a full grocery
cart.
Newton’s Third Law of Motion
 Forces always act in pairs that are equal and opposite
to one another.
 The first force is called the action force and the
second force is called the reaction force.
Newton’s Third Law of Motion
 This explains why a wall does not move when you
lean on it. You apply a force onto the wall, but the
wall applies the equal and opposite force back on
you.
 This also explains why a ball bounces back up, and
why a person rowing a boat moves forward when
they push back on the oars.
Motion in Space / Orbit
 Mass: the amount of matter in an object.
 Weight: the pull of gravity on an object.
 Gravity is what holds all things to the Earth, but it
also holds the Moon and other objects in an orbit.
Motion in Space / Orbit
 When an object travels in an orbit, it moves in a
circular path around another object.
 If the object were to be “let go” from its orbit, it
would fly off in a straight line.
Motion in Space / Orbit
 Astronauts on the ISS appear weightless in space due
to microgravity.
 This occurs because Earth’s gravity causes the space
station to fall toward Earth at a constant rate.
 ISS Tour