Transcript Forces & Newton`s Laws

Forces & Newton’s Laws
Ch. 4
Forces
• What is a force?
– Push or pull one body exerts on another
– Units = Newton (N)
– Examples:
List all of the forces that you can
encounter/exert in a day
Forces
• Types of forces:
1. Friction- force that opposes motion
between two surfaces that are touching.
– Air Resistance – Special type of frictional
force.
• Occurs when the object is traveling through the air
Forces
2. Applied Force – A force that is applied to
an object by another object.
Example: You, wind, water, etc
3. Gravitational Force – The force of gravity
on an object.
Forces
• Balanced forces: forces exerted on an
object of equal magnitude (size) but in
opposite directions.
Unbalanced Forces: unequal pulling of
forces on an object.
Forces
• Example:
In a tug of war, the net force causes an
acceleration in the same direction.
Are the forces balanced or
unbalanced in the picture?
Forces
If forces are balanced
(all forces cancel)
Acceleration = 0 m/s2
Fnet on object = 0 N
Object is doing
1 of 2 things…
Forces
Object is doing
1 of 2 things
At rest
v = 0 m/s
stays
at
rest
In motion
“constant” velocity
stays
in
motion
Forces
• Inertia
– The tendency of an object to resist any
change in motion.
– The velocity of an object will remain the same
unless a force acts on it.
Example:
Forces
• Inertia is dependent on mass.
• Would you expect a bowling ball and a
tennis ball to have the same inertia?
**Think about resistance to change motion**
• More mass greater inertia!!
Forces
• Newton’s 1st Law  Law of Inertia
An object at rest will stay at rest.
An object in motion will stay in motion.
Unless acted on by an outside force.
Forces
• Examples:
• A rolling basketball would
continue to roll forever if
an outside force did not
act on it.
• A rock will never move
unless an outside force
moves it.
Newton’s 2nd Law
If forces are unbalanced
(they do not cancel)
The mass accelerates
in the direction of
the larger force.
Newton’s 2nd Law
The relationship between
F and a
Fnet = ma
N
kg
2
m/s
“net” means total
Newton’s 2nd Law
• Newton’s 2nd Law  Law of Force
• If a larger force exists, then there must be
an acceleration in the direction of that
force.
Newton’s 2nd Law
• Helpful Hints:
N
F
m a
kg
m
s2
Newton’s 2nd Law
Example:
Newton’s 3rd Law
Equal and Opposite
• For every action there
is an equal and
opposite reaction
• Example:
– Imagine a rocket is being
launched from the earth.
Hot gases are pushed out
from the bottom of the
rocket as the rocket is
thrust upward. The force of
the gases pushing against
the surface of the earth is
equal and opposite to the
force with which the rocket
moves upward
Momentum
• A property a moving object has because of
its mass and velocity
• Momentum depends on both mass and
velocity
Momentum
• Momentum can be calculated using the
following equation
–p= mv
• p= momentum
• m= mass (kg)
v= velocity(m/s)
• Unit for momentum is kg·m/s
Momentum
• Objects that are not moving (at rest) have
ZERO momentum
• Example:
– A bicycle and a car are traveling at the same
velocity. Which has a greater momentum?
Which has a greater
momentum?
• WHY?
• The car would be have a greater
momentum, because the car has a greater
mass
Law of Conservation of
Momentum
• The amount of momentum a group of
objects does not change unless and
outside force acts on them.
• Example: