Download The Laws of Motion - St. Thomas the Apostle School

The Laws of
Motion
Chapter 2
What causes these riders to move around?
What prevents them from falling out?
How do forces change the motion of the riders?
Types of forces
• Force- a push or pull on an object, all forces have strength and
direction.
• Contact force- a push or pull on an object by another that is touching
it.
Non-contact force- a force that one object can apply on another
without touching it.
Examples- magnetism,
Electricity and
Gravity.
GRAVITY
• Gravity- is an attractive force that exists between all objects that have
mass.
• Mass- The amount of matter in an object
The law of Universal gravitation
• Issac Newton, an English Scientist and Mathmetician, developed the
law of universal gravitation.
• This law states that all objects are attracted to each other by a
gravitational force.
• Gravitational force between objects decreases as the distance
between the objects increase.
• When the mass of an item increases the force between them alos
increases.
• All forces are measured in Newtons.
Weight- a gravitational force
• Weight is a gravitational force exerted on another object.
• Weight is proportional to mass. On Earth, the weight of an object in
Newtons is 10X the object’s mass in Kilograms.
• Near Earth’s surface , an objects weight is the gravitational force
exerted on the object by the Earth.
FRICTION- a force that resists the motion of
two surfaces that are touching.
• Static Friction- prevents surfaces from sliding past each other.
• Sliding Friction- opposes the motion of surfaces sliding past each
other.
• Fluid Friction- friction between a surface and a fluid
Describe a time when you have experienced
gravity, static friction, and sliding friction.
What causes friction?
Microscopic bumps and dips on surfaces that touch each
other and weak electrical charges on the surface.
What Reduces Friction?
Lubricants. They separate the surfaces that are in contact with
each other.
Newton’s First Law of Motion
• Net force- The combination of all the forces acting upon an object.
• To understand the motion of an object, you need to understand the
forces acting on it.
Besides flying, what are some of the actions that require the osprey to
exert force?
Why does spreading her wings increase the osprey’s air resistance?
Compare an ospreys wings and a parachute from a sky diver.
• A reference direction- a direction that you choose from a starting
point to describe an object’s position.
• When you combine forces in two opposite directions, one force is
positive and the other force is negative.
• When forces in the same direction combine, the net force is also in
the same direction. The sum of the net force is the sum of the forces.
What forces can act on an object at a given time?
Gravity, friction, and applied force
• Balanced forces- forces acting on an object that combine and form a
net force of zero.
• Unbalanced forces- forces acting on an object that combine and form
a net force that is NOT zero.
Newton’s first law of Motion- states that if the net force
of an object is zero, the motion of the object doesn’t change.
•When balanced forces act on an object,
the objects velocity does not change.
•If unbalanced forces act on an object at
rest, the object will start moving.
•If unbalanced forces act on a moving
object, the object will change its
velocity.
• Inertia-the tendency of an object to resist a change.
When a car stops short, a person continues moving forward because of
its inertia.
A book sitting on a table stays in place because of inertia.
How do forces change motion?
• Forces change an objects motion by changing its speed, its
direction, or both.
• Only unbalanced forces change the velocity of an object.
• Unbalanced forces are acting on an object that is at rest
when the object starts moving.
• Unbalanced forces change the velocity of a moving object.
• Another name for change in velocity over time is
acceleration.
• Unbalance forces can make an object accelerate by changing
the object’s speed, direction or both.
Newton’s second Law- states the acceleration of an
object is equal to the net force acting on the object
divided by the object’s mass.
• Forces change an object’s motion by changing its speed, its direction,
or both its speed and direction.
• When Unbalance forces act on an abject at rest, it moves in the
direction of the net force. Unbalanced forces cause an object to
speed up or slow down.
Make up a story problem that uses Newton’s
second Law of Motion.
Problems should require solving for force, mass or
acceleration using the equation a=f/m
If you throw a .5kg basketball with a force of
10N. What is the acceleration of the ball?
• Circular motion-is any motion in which an object is moving in a
curved path.
• Centripetal Force-In a circular motion, a force that acts perpendicular
to the direction of motion, toward the center of the curve
Newton’s Third Law
• When one object exerts force on a second object, the second object
exerts an equal force in the opposite direction on the first object.
• When one object exerts a force on another object, the second object
exerts a force on the first object in the opposite direction.
Using Newton’s Third Law of Motion
• When you push against an object, the force you apply is
called the action force.
• Newton’s third law establishes that the object you push on
applies an equal and opposite reaction force against you.
• According to Newton’s second Law, when the reaction force
results in an unbalanced force, there is a net force and the
object accelerates.
Momentum
• The measure of how hard it is to stop a moving object.
• Momentum is the product of an object’s mass and it’s velocity.
• What is the momentum of a 12 kg bicycle
moving at 5.5m/s?
The law of conservation of momentum
• States the total momentum of a group of objects stays the same
unless outside forces act on the objects.