Force and Inertia
Dynamics

Kinematics is the study of how things move.
• Position, velocity, and acceleration compared to time
• Trajectories in space

Dynamics is the study of why things move.
• What causes motion?
• How do we use the changes in position and velocity?
• Force and mass
Force
Force is
Force is not

A push or pull on an object.

Energy.

A vector with magnitude and
direction.

Power.

Momentum.

Velocity.
Natural Motion

A ball is thrown into the air without friction. Before it
reaches the top it is subject to
• A) a steadily decreasing upward force from the initial toss.
• B) an increasing downward force of gravity and a steadily
decreasing upward force.
• C) an almost constant downward force of gravity and a
steadily decreasing upward force.
• D) an almost constant downward force of gravity.
• E) a natural tendency to return to the Earth.
Identifying Forces

Find the pushes and pulls on
the ball in the air.

Isolate the object under
study.

Look at forces on the object
alone.
ball
hand
In flight,
only gravity
is pulling on
the ball.
No force
from the
hand acts on
the ball after
release.
Fundamental Forces




Gravity is a fundamental force.
It acts upon objects from a distance away from the
source (such as the Earth).
There are two other fundamental forces.
Electroweak force is
common in everyday life.
•
•
•
•
Electricity
Magnetism
Light
Radioactive decay

Nuclear force is uncommon
in everyday life.
• Nuclear fission (nuclear
power plants)
• Nuclear fusion (stars)
Contact Forces

Many forces are due to contact between objects.
•
•
•
•

Kick a ball
Push with a bulldozer
Tug from a rope
Friction due to the ground
The actual force is electricity, but the atoms are so
small we can treat the forces as coming from contact
by larger objects.
Newton’s Laws

Ancient scientists looked to the
natural properties of objects.

Motion was a result of the object’s
properties.

Newton defined motion based on
forces acting from outside an object.

Three laws were used to define the
behavior of forces on objects.
Law of Inertia
1
An object continues at rest, or in uniform motion in a
straight line, unless a force is imposed on it.

This describes constant velocity, so no acceleration.
No acceleration means no force, and vice versa.
Fnet = 0  a = 0 (Newton’s first law).


no force
rocket
constant
velocity
Inertial Reference Frames


No acceleration can mean an object at rest.
No acceleration can also mean a constant velocity.

Newton’s laws remain the same for observers at rest
or at a constant velocity.

An observer cannot use physics to distinguish
between inertial reference frames.
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