Forces
&
Motion
Motion
• A change in the position of an object
• Caused by force (a push or pull)
Force
•
•
•
•
Anything that is pushing or pulling on an object
Measured in units called Newtons (N)
Measured with a spring scale
Forces act in pairs
Types of Force:
– Gravity- pulls objects towards each other and pulls
toward the center of Earth
– Friction- when two surfaces rub against each other
 opposes motion
 Increasing friction will slow an object down
 Decreasing friction allows an object to move more
freely
– Magnetism – iron and steel are attracted to magnets
 opposite poles of a magnet attract and like poles repel
– Electricity – these forces are produced when an
object gains or looses electrons.
 Charged objects can exert a push or pull force on each other.
Study Jams - Force Video
Inertia
Inertia is the tendency of an object to resist change
to its state of motion.
Newton’s 1st Law of Motion
• An object at rest will remain at rest until acted
upon by another force.
• An object in motion will remain in motion until
acted upon by another force.
Balanced Forces
• When all the forces acting on an object are equal
(net force is zero)
• Balanced forces do not cause a change in motion.
How Can Balanced Forces
Affect Objects?
• Cause an object at rest to stay at rest or an object
in motion to stay in motion (inertia)
• Cause an object moving at a constant speed to
continue at a constant speed
• In your notes, describe an example of a balanced
force affecting an object.
Study Jams - 1st Law of Motion Video
Newton’s Second Law of Motion
• The greater the force on an object, the
greater the acceleration on an object. The
smaller the mass of an object, the greater
the acceleration will be in comparison to
an object with a larger mass.
• Force= mass x acceleration
Unbalanced Forces
• When all the forces acting on an object are not
equal
• The forces can be in the same direction or in
opposite directions.
• Unbalanced forces cause a change in motion.
How Can Unbalanced Forces
Affect Objects?
• Acceleration is caused by unbalanced forces:
–
–
–
–
–
slow down
speed up
stop
start
change direction
• In your notes, describe an example of an unbalanced
force affecting an object.
Study Jams 2nd Law of Motion Video
Newton’s 3rd Law of Motion
• For every action or force, there is an equal
and opposite reaction or force.
Study Jams 3rd Law of Motion Video
Mass and Weight
• Mass is the amount of stuff (matter) in an object.
– The mass of an object will remain the same anywhere in the
universe.
– Mass is measured in grams or kilograms.
• Weight is the mass (kg) of an object multiplied by the
acceleration of gravity (9.8 m/s2).
– The weight of an object depends on the gravitational pull of the
location in the universe.
– The gravitational pull on the Moon is only 1/6 of Earth’s
gravitational pull.
– Weight is measured in newtons (N).
Net Force
• The total of all forces acting on an object:
– Forces in the same direction are added.
• The object will move in the direction of the force.
– Forces in opposite directions are subtracted.
• An object will move in the direction of the greater
force.
Calculating Net Force
10 N
30 N
http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg
A child was playing with a jack in the box. The lid pushed
down with 10 N of force, while the spring pushed up with 30
N of force. What is the net force applied by the spring?
In your notebook, calculate the net force and show all of your
work.
Check Your Answer
10 N
30 N
http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg
A child was playing with a jack in the box. The lid pushed
down with 10 N of force, while the spring pushed up with 30
N of force. What is the net force applied by the spring?
30 N up - 10 N down = 20 N up
Calculating Net Force
30 N
Tug of War
Game
50 N
Two competitors are playing tug of war.
What is the net force? Which direction will the rope move?
In your notebook, calculate the net force and show all of your
work.
Check Your Answer
Tug of War
Game
30 N
50 N
Two competitors are playing tug of war.
What is the net force? Which direction will the rope move?
50 N to the right – 30 N to the left = 20 N to the right