Chapter 3
Forces
Section 1
Newton’s Second Law
Newton’s 3 Laws
1. Law of Inertia
2. Newton’s second law of motion connects
force, mass, and acceleration
3. For every action there is an equal and
opposite reaction
Newton’s second law
• Force and motion are connected
– an object will have greater acceleration if a greater
force is applied to it
• Ex. throwing a baseball
– the mass of an object and the
force applied to it affect
acceleration
• Ex. difference between
throwing a baseball and
a softball
Force = mass X acceleration
• Can also be written as….
• Force is calculated in Newtons
• Mass will be calculated in kilograms
• Acceleration will be in meters per seconds per seconds
or m/s2
Lets do an example…
• You push a friend on a sled. Your friend and
the sled together have a mass of 70 kg. If the
net force on the sled is 35 N, what is the sled’s
acceleration?
You can also calculated force
• F = ma
• A tennis player hits a ball. The acceleration is
5000 m/s2. The mass
of a tennis ball
is 0.06 kg. What
would be the net force
exerted on the ball?
Other forces exerted…
• Gravity
• Friction
• Air resistance
Friction
• Friction is a force that opposes motion
between two surfaces that are touching each
other
Amount of friction…
• Depends on two factors:
– the kinds of surfaces that are touching
– the force pressing the surfaces together
Microwelds
• Even apparently
smooth surfaces still
have microscopically
rough surfaces
• Microwelds are areas
where surface bumpers
stick together (these
are the sources of
friction)
Friction
• Static Friction – friction between two surfaces
that are not moving past each other
Friction
• Sliding Friction – force that opposes the
motion of two surfaces sliding past each other
Friction
• Friction between a rolling object and the
surface it rolls on is called rolling friction
Air Resistance
• Air resistance opposes the motion of objects
that move through the air
– The amount of air resistance depends on an
object’s shape, size, and speed
Terminal Velocity
• Gravity is a force and thus causes objects to
accelerate towards earth
• As something falls faster air resistance gets
stronger and thus is able to balance the
downward force of gravity.
• Terminal velocity depends on the size, shape,
and mass of a falling object.
Terminal Velocity
Section 2
Chapter 3
Gravity
• Law of gravitation – any two masses exert an
attractive force on each other
• Gravity – an attractive force between two
objects that depends on the masses of the
objects and the distance between them
Gravity depends on the
size of the planet…
Earth’s Gravitational Acceleration
• Every object on earth falls at the same rate af
acceleration due to gravity
• acceleration due to gravity = 9.8 m/s2
What is weight?
• Weight – is a gravitational force exerted on an
object
– Weight decreases as an object moves away from
Earth
– Weight results from a force
– W = mass (kg) X acceleration of gravity (m/s2)
– W = mg
Difference between Weight and Mass
• Weight is a force and mass is a measure of
how much matter an object contains.
• However… they are related.
• Weight increases as mass increases…
Projectile Motion
• When something is thrown it has a velocity in
the direction which it was thrown as well as a
velocity towards earth due to gravity.
• Therefore a projectile will have horizontal and
vertical velocities due to gravity, and follow a
curved path.
Centripetal Force
• Centripetal force is the force pulling toward
the center of a curved path.
• This is due to centripetal acceleration.
• Centripetal force is an unbalanced force.
Section 3
Chapter 3
Newton’s Third Law of Motion
• Law: To every action force there is
an equal and opposite reaction force.
Momentum
• A moving object has a property called
momentum that is related to how much force is
needed to change its motion.
• The momentum of an object doesn’t change
unless its mass, velocity, or both change.
• Momentum – related to how much force is
needed to change an object’s motion;
momentum equals mass times velocity
• momentum = mass (kg) X velocity (m/s)
• p = mv
Law of conservation of momentum
• Momentum can be transferred between
objects; momentum is not lost or gained in
the transfer.