Transcript Newton’s 1st and 2nd Laws of Motion

st
1
Newton’s Law of
Motion
Net Force
The net force is the sum of all forces acting on an
object.
Example: A weightlifter holds a weight above his
head by exerting a force of 1.6 kN [up]. The
force of gravity acting on the weight is 1.6 kN
[down]. Draw a FBD of the weight. What is
the net force on the weight?
Net Force
FA=1.6 kN
Fg=1.6 kN
The net force is zero: 1.6 kN + (-1.6 kN) = 0
Newton’s First Law of Motion
If there is no net force acting on an object, the
object will remain at rest
Newton’s First Law of Motion
If there is no net force acting on an object, the
object will remain at rest or will keep moving at
the same constant velocity.
(Conversely, if an object is at rest or is moving at
constant velocity, there is no net force acting
upon it.)
Inertia
This is the principle of inertia, first articulated by
Galileo:
Inertia
“Inertia is a property
of matter.”
Inertia is the property of matter that causes it to
resist changes to its motion.
The greater the mass of an object, the greater its
inertia.
Newton’s First Law of Motion
Question: An object is being pushed along at
constant velocity by a force of 5 N [left]. What
is the force of friction acting on the object?
If the velocity is constant, there is no net force, so
the force of friction must be equal in magnitude
and opposite in direction to the applied force:
Ff = 5 N [right]
More Practice
Explaining Why with Newton’s 1st Law of Motion