Download 3.3 Using Newton`s Laws • Which of Newton`s laws explains why a

October 13, 2016
3.3 Using Newton's Laws
• Which of Newton's laws explains why a skateboard
moves backward when you step off of it?
• Car crashes...wear your seatbelt!
> designed to keep you from bouncing around in the
car, and your seat to lengthen the impact time.
This results in a smaller force on the passenger
> Airbags do the same thing - lengthen the impact
time for the passenger
Newton's Second Law and gravitational acceleration
• Anything that is dropped experiences acceleration due
to the gravitational force. You can still use:
F = ma
• Air resistance - a friction-like force that opposes the
motion of the objects that move through the air.
> air resistance reacts in the direction opposite to the
motion of an object moving through air
> amount of air resistance depends on the size,
shape, and speed of the object, as well as the
properties of the air
> air resistance is the reason feathers fall more
slowly than pennies.
> The more spread out an object
will change air resistance
Glenco Physical Science 2012
October 13, 2016
Glenco Physical Science 2012
Speed and Terminal Velocity
• amount of air resistance increases as velocity increases
• gravity causes objects to accelerate as they fall
• terminal velocity - maximum speed an object will reach
when falling through a substance (like air)
> occurs when the upward air resistance force
becomes large enough to balance the downward
force of gravity and the net force on the object is
zero
> An object's terminal velocity depends on its size,
shape, and mass
October 13, 2016
Free fall - occurs when gravity is the only force acting
upon an object
• Satellites are in free fall around Earth
Weightlessness
• objects orbiting in a spacecraft are weightless and
seem to float because they are falling around the
Earth inside the spacecraft
show a zero weight
Glenco Physical Science 2012
October 13, 2016
Centripetal Force - a force exerted toward the center of a
curved path
• Anything moving in a circular path does so because
centripetal force is accelerating it toward the center
> Forces that can act as centripetal force
– gravity
– amusement park rides - the push of the wall on
riders
– friction between car tires and the road on a curve
Glenco Physical Science 2012
October 13, 2016
Glenco Physical Science 2012
Law of conservation of momentum - if no external
forces act on a group of objects, their total momentum
does not change
October 13, 2016
Rocket propulsion
The hot gases that are created by
burning rocket fuel exerts a force on
these gases and causes them to
escape out the back of the rocket
October 13, 2016
Homework:
P 92 # 22,23,25,27