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• Please turn in your week 6 accountability
sheet and pick up week 7.
• If you did not bring your workbook, share with
a table partner.
REVIEW: Newton’s first law
• Sometimes referred to as the law of inertia
• “An object at rest and an object in motion
stays in motion with the same speed and in
the same direction unless acted upon by an
unbalanced force.”
Balanced and Unbalanced Forces
• What’s that mean?
• This is equilibrium!
equal magnitude,
opposite direction
• Now consider a book being pushed to your right…
• Unbalanced
forces cause
Accelerations
These pics are called Free Body Diagrams
Drawing Free-Body Diagrams
• “diagrams used to show the relative
magnitude and direction of all forces acting
upon an object in a given situation”
• Use vector arrows
• Example:
Our first free body diagram
• A book is at rest on a tabletop
• A gymnast is suspended motionless from the
ceiling by two ropes. Diagram the forces
acting on the gymnast.
• An egg is free-falling from a nest in a tree.
Neglect air resistance. Diagram the forces
acting on the egg as it is falling
• A flying squirrel is gliding (no wing flaps) from
a tree to the ground at constant velocity.
Consider air resistance. Diagram the forces
acting on the squirrel
• A rightward force is applied to a book in order
to move it across a desk with a rightward
acceleration. Consider frictional forces.
Neglect air resistance. Diagram the forces
acting on the book.
• A rightward force is applied to a book in order
to move it across a desk at constant velocity.
Consider frictional forces. Neglect air
resistance. Diagram the forces acting on the
book.
• A college student rests a backpack upon his
shoulder. The pack is suspended motionless by
one strap from one shoulder. Diagram the
vertical forces acting on the backpack.
• A skydiver is descending with a constant
velocity. Consider air resistance. Diagram the
forces acting upon the skydiver.
• A force is applied to the right to drag a sled
across loosely packed snow with a rightward
acceleration. Diagram the forces acting upon
the sled.
• A car is coasting to the right and slowing
down. Diagram the forces acting upon the car.
Determining the Net Force
• A.
B.
C.
• net force is the vector sum of all the forces that
act upon an object (“Where is the imbalance?”)
• Figuring out the net force: Ask yourself these
questions: In what direction and of what
magnitude is the imbalance?
Determine the net force acting upon
the object
The net force is known for each situation. However, the
magnitudes of a few of the individual forces are not
known. Analyze each situation individually and
determine the magnitude of the unknown forces.