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An Introduction to Forces:
More Practice
Draw a free-body
diagram for each of the
following objects:
(a) A car engine being
lifted from a car by a
rope attached to a pulley
An Introduction to Forces:
More Practice
Draw a free-body
diagram for each of
the following objects:
(b) an car moving with
constant velocity on a
level road
An Introduction to Forces:
More Practice
Draw a free-body
diagram for each of
the following objects:
(c) an apple hanging
from a tree branch
An Introduction to Forces:
More Practice
Draw a free-body
diagram for each of
the following objects:
(d) a skydiver being
slowed by a parachute
An Introduction to Forces:
More Practice
Selma is pulling across level snow a sled on
which is sitting her daughter Ivy. Tied to
the back of Ivy's sled is another tiny sled
on which Ivy's baby doll is sitting.
Draw the free-body diagrams for (a) Selma
An Introduction to Forces:
More Practice
Selma is pulling across level snow a sled on
which Iggy is riding. Tied to the back of
Iggy's sled is another tiny sled on which
his dog Quark is sitting.
Draw the free-body diagrams for (b) Iggy
An Introduction to Forces:
More Practice
Selma is pulling across level snow a sled on
which Iggy is riding. Tied to the back of
Iggy's sled is another tiny sled on which
his dog Quark is sitting.
Draw the free-body diagrams for (d) Iggy’s
sled
An Introduction to Forces:
More Practice
Selma is pulling across level snow a sled on
which Iggy is riding. Tied to the back of
Iggy's sled is another tiny sled on which
his dog Quark is sitting.
Draw the free-body diagrams for
(d) Quark's sled
Newton’s 1st Law of Motion:
Learning Goal
The student will be able to state Newton’s 1st
Law and apply it in qualitative and
quantitative terms to explain the effect of
forces acting on objects. (B3.4)
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
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
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.
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?
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:
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