Transcript Forces - Leon County Schools

Week 4
Chapter 8: Forces & Motion
Forces
April 12 & 13
Today’s Objectives
Students will be able to:
• Identify different types of forces.
• Explain how mass and distance affect gravity.
• Differentiate mass and weight.
• Distinguish between contact forces and noncontact forces.
Bellringer #3
Which friend do you
agree with most?
Explain your thinking.
Describe what you think
a force is.
Bellringer #4
Think back to activity you participated in during the previous period.
• What caused the book and sheet of paper to move?
• Did anything make contact with the items to make them move?
• Did anything push or pull the objects without making contact with them?
Forces
• A force is a push or a pull on an object.
• Force has both size and direction.
• You can use arrows to show the size
and direction of a force.
• The unit for force is the newton (N).
How do forces relate to energy?
Forces
• A contact force is a push or a pull on one
object by another object that is touching it.
 What types of contact forces exist?
• A force that one object can apply to another
object without touching it is a noncontact
force.
 What type of noncontact forces exist?
Gravity
• Gravity is an attractive force that exists between all objects that have mass.
• Although gravitational forces always exist between objects, they only become
observable when the masses are as large as those of planets, moons, or stars.
• The size of a gravitational force depends on the masses of the objects and the
distance between them.
• If the mass of an object increases, the gravitational force increases between it
and another object.
Gravity
• If the mass of an object increases, the
gravitational force increases between it and
another object.
• This fact may lead you to believe that falling
objects accelerate at different rates.
• By applying the formula F = ma, you can see
that if the force due to gravity increases as
mass increases, the acceleration will remain
constant.
• Near the Earth’s surface, the acceleration due
to gravity is 9.8 m/s^2
Gravity
 How do we measure the force of gravity?
• Weight is a measure of the gravitational force acting on an
object’s mass.
 What is the difference between mass and weight?
• Weight=mass x g (w= Newtons, m=kg, g= 9.8 m/s/s)
• The weights of objects on the Moon are smaller than objects
on Earth because the mass of the Moon is smaller.
Gravity
• The size of a gravitational force also depends on the distance between two objects.
• Gravitational force between an apple and Earth is about 2N on Earth’s surface.
• It would be about .001 N if the apple was 380,000 km away.
 If the size of the force of gravity did not affect the object’s acceleration, what
caused the objects to fall differently?
Friction & Air Resistance
• Objects can have their motion opposed by friction with air.
• Air resistance is the frictional force between air and objects moving through it.
• Friction is a contact force that resists the sliding motion of two surfaces that are touching.
• The force of friction acts in the opposite direction of an object’s motion.
• Air resistance opposes the downward motion of falling objects.
• As objects fall air resistance gradually becomes equal to the pull of gravity.
• When this happens, there is no acceleration.
 What does this imply about the object’s motion?
• The object continues to fall, but at a constant velocity.
Wrap Up
 How do contact and noncontact forces affect objects’ motion?
 How did our activity demonstrate this?
Acceleration
• Acceleration is a measure of how quickly the velocity of an object changes.
• When the velocity of an object changes, it accelerates.
• The action of slowing down is called negative acceleration.
• When an object’s acceleration is in the same direction as its motion, this is called positive
acceleration.
• When the roller-coaster car
increases speed, decreases speed,
or changes direction, it accelerates.
• Acceleration takes place whenever
velocity changes.
Combining Forces
April 15
Bellringer # 5
• What happens when two forces act on an object at once?
• Think of a real-life example.
• Remember, a force is a push or pull on an object.
Today’s Objectives
Students will be able to:
• Explain what happens when forces combine.
• Relate balanced and unbalanced forces to motion.
Combining Forces
• When more than one force acts on an object, the forces combine and act as
one force.
• The sum of all the forces acting on an object is called the net force.
• When two forces act on the same object in opposite directions, you must
include the direction of the forces when you add them to calculate net force.
How can you include direction when calculating net force?
• If the net force on an object is 0 N, the forces acting on the object are called
balanced forces.
• When the net force on an object is not 0 N, the forces acting on the object
are unbalanced.
Unbalanced Forces
• Forces that are unbalanced are unequal in size and act in the
same or opposite in direction.
• When unbalanced forces act on an object, the object’s
velocity changes.
Recall: What is velocity?
• Unbalanced forces can change either the speed or the
direction of motion.
• The train is able to pull away from the station because the
force of the engine is greater than the force of friction.
How does a curve in the track change the train’s velocity?
Does the train accelerate as it goes around the curve? How
do you know?
Balanced Forces
How are objects at rest and objects moving
at constant motion similar?
• Forces that are balanced are equal in size,
but opposite in direction
• When balanced forces act on an object, the
motion is constant.
• The object is either at rest or moving at a
constant velocity.
What forces keep the train resting on the
track?
Wrap Up
How do balanced and unbalanced forces affect motion?
Apply the concept of combining forces to Tuesday’s activity with the book
and sheet of paper.
Exit Ticket: Concept Mapping
With the person next to you, use ten or more
of the terms below to create a concept map.
Forces
Time
Acceleration
Balanced
Position
Friction
Unbalanced
Motion
Air resistance
Contact
Speed
Gravity
Noncontact
Velocity