What are Forces?
What are forces?
• A force is a push or a pull.
• We learned that Newton’s 2nd Law states
that a Force is equal to the mass of a
moving object times its acceleration.
• We learned that Newton’s 3rd Law states
that for every force there is an equal and
opposite reaction force.
Newton’s Second Law of Motion
• Force = mass X acceleration
• F = ma
• Force and acceleration are directly
proportional. The greater the force, the
greater the acceleration.
• Mass and acceleration are inversely
related. The greater the mass of an object
the less the acceleration if the same force
is applied.
What is friction?
• Friction is a force which opposes motion.
• Friction is an important force in our lives.
We rely on friction in many ways. An
athlete usually wears shoes which provide
him or her with a greater friction between
the shoe and the surface. We rely on
friction as an important aspect of our
motion. In what other ways does friction
play a role in our everyday lives?
From Glencoe Physical Science
What causes friction?
• There are two factors which affect
friction between two surfaces:
– Kind of surfaces in contact (rough or
smooth)
– Amount of force pressing the
surfaces together.
The rougher the surface and the
stronger the force between the
surfaces, the greater the amount of
friction.
What is static friction?
• If you were trying to move a box like
pictured below and the box did not
move, this would be static friction.
• Static friction is friction between two
surfaces which are not moving past
each other.
From Glencoe Physical Science
What is sliding friction?
• Sliding friction occurs when a force is
great enough to overcome the static
friction.
What is rolling friction?
Glencoe Physical Science
•Rolling friction is the friction
which enables wheels to
turn and objects to move. If
there were no friction,
turning wheels would not
enable an object to move.
Air Resistance – another type of
friction??
• Air resistance is a force which
opposes a moving object. This can
be easily seen with falling objects.
When a piece of paper and a
crumple piece of the same type of
paper are dropped, they fall at the
same rate.
• A penny and a feather dropped in
air will not fall at the same rate, but
a penny and feather dropped in a
vacuum (no air) will.
What is terminal velocity?
• Forces cause objects to accelerate (2nd
Law).
• When the force of gravity on a falling
object equals the force of the air
resistance going against gravity, the forces
balance out and the object stops
accelerating.
• The object will travel at a constant velocity
– the terminal velocity.
What is Gravity?
• Gravity is a force of attraction between two
objects.
• Law of Gravitation – any two masses exert an
attractive force on each other. The amount of
attraction depends upon two things: the mass of
the objects and the distance between the
objects.
What is gravitational acceleration?
• When objects fall they accelerate toward
the ground.
• Using Newton’s 2nd Law we can see this
relationship: F = ma
• The force caused by gravity acts upon a
certain mass to cause it to accelerate
toward the ground at a constant rate.
• Acceleration due to gravity is 9.8 m/s2
Gravity causes weight
• The weight of an object is caused by the
force of attraction between the earth and
objects on the surface of the earth.
Gravitation force is equal to weight, therefore…
How much does a person with
a mass of 70.0 kg weigh on
the earth?
Weight = mass X 9.8 m/s2
Weight = 70.0 kg X 9.8 m/s2
Weight = 686 N
Remember that 1 N = (1 kg) (1m/s2)
What is weight?
• When an astronaut moves away
from the earth they “lose” weight.
Weight is caused by the force of
attraction between the earth and
objects on its surface. The greater
the distance an object has from
the earth, the less the force of
attraction which would exist.
What is the difference between
mass and weight?
• Mass is the amount of matter in an object. This
does not change in an object as it moves away
from the earth.
• Weight is caused by gravity. Therefore, the
greater the distance, the less the force of
attraction. The less the distance the greater the
force of attraction.
• Your weight is dependent upon the mass of the
earth. If you were able to go to the planet,
Jupiter, your weight would be 27 time greater,
because the mass of Jupiter is 27 times greater.
What causes “weightlessness”?
• Even in the space shuttle, there is a
force of attraction exerted by the
earth on the shuttle and its contents.
• When the space shuttle temporarily
“falls” toward the earth, the contents
of the space shuttle appear to be
weightless, but in fact they are falling
with the space shuttle. This is called
“freefall”.
What is projectile motion?
When an object is thrown into the air, two forces and
two motions affect the movement of the object.
The object may be thrown horizontally with a certain
force, but the vertical force of gravity combines to affect
the motion.
Slow projectile - shoot a
monkey
Fast projectile - shoot a
monkey
What is centripetal force?
• “centripetal” means to move toward the
center.
• Centripetal acceleration is the acceleration
of an object toward the center of a curve
or circular path.
• Centripetal force is a force toward the
center of a circle which holds the moving
object in its circular path.
What is Newton’s 3rd Law of Motion
• For every action there is an equal but
opposite reaction.
• When one object exerts a force on a
second object, the second one exerts a
force on the first that is equal in size and
opposite in direction.
• Action-Reaction Law
Two objects in contact with one another and
moving must use the action-reaction law.
Rocket propulsion involves the action-reaction
law. The explosion from one end of the rocket
causes a force in that direction. This causes an
unbalanced force in the opposite direction
causing the rocket to accelerate. Video Clip!
What is momentum?
• The momentum of an object is equal to
the product of its mass and its velocity.
The unit for momentum is kg m/s
What is the Law of
Conservation of Momentum?
• When object collide the momentum of one
object is transferred to the other object.
• View this video to see a demonstration of
conservation of momentum.
• Notice that the soccer ball has greater
momentum since it has a larger mass. Its
momentum is transferred to the smaller ball
which has a smaller mass and will therefore
have a larger velocity. Try this with a soccer
ball and a tennis ball!