Download Newton`s Laws of Motion

Newton’s Laws of Motion
Newton’s 1st Law
In an inertial reference frame, an object at rest
will remain at rest and an object in motion will
remain in motion with constant velocity unless
acted on by an external force.
Newton’s 1st Law
Inertial Reference Frame
- Non-accelerating
- Classroom (mostly...)
Expanded meaning of velocity
Train Example
Car on turn example
Newton’s 1st Law
Internal vs. External Forces
Multiple External Forces
Implications for problem solving
- a = 0 m/s2
- v0 = vf
- xf = x0 + v0t
Newton’s 2nd Law
The acceleration of an object is proportional to
the vector sum of the external forces acting on
the object and inversely proportional to its mass.
Newton’s 2nd Law
Newton’s 2nd Law
Units of force
Meaning of “vector sum”
Interaction with first law
Example 1
A 5 kg bowling ball is dropped out of a window
that is 4 meters above the ground.
a. What is the acceleration of the bowling ball?
b. What force must have been applied to the
bowling ball in order for it to have this
acceleration?
Weight vs. Mass
Mass is the amount of matter in an object,
measured in kilograms.
Weight is the gravitational force on an object,
measured in newtons.
Example 2
A 75 kilogram person stands still on a floor. What
force must the floor exert on the person?
Normal Force
Force of a surface on an object in contact with
the surface.
“Normal” = Perpendicular to surface
Often (but not always!) equal to weight.
Apparent Weight = Normal Force
Example 3
A 75 kilogram person stands in an elevator. The
elevator begins to move upward, with an
acceleration of 2.0 m/s2.
a. What must be the apparent weight of the
person?
b. When the elevator reaches its destination, it
slows down with the same acceleration. What
is the person’s apparent weight in this
situation?
Contact & Field Forces
Contact Forces require contact. Examples:
- Normal Force
- Tension
- Applied Forces
Field forces do not require contact. Examples:
- Gravity
- Electric Force
- Magnetic Force
Four Fundamental Forces
Image © Randall Munroe, used with permission. Link: https://xkcd.com/1489/
Normal Force is really electromagnetic force.
Newton’s 3rd Law
When an object exerts a force on a second
object, the second object exerts a force on the
first object with equal magnitude and opposite
direction.
Newton’s 3rd Law
Intuitive Examples
- Skating Rink
- Rocket Propulsion
Less Intuitive Examples
- Gravity
- Motorcycle vs. Truck
- Jumping / Normal Force
How do things change if it’s always
balanced?
Equal and Opposite Reactions always occur on
different objects.
Different masses yield different accelerations.
Example 4
You throw a 5 kg bowling ball out of a second
story physics classroom window. It accelerates
downward at 9.8 m/s2. What is the upward
acceleration of the Earth due to the bowling ball?
The mass of the Earth is 5.972e24 kg.