Unbalanced Forces
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Unbalanced Forces
Newton's 3rd Law: Forces come in pairs (An action force and a reaction force)
What action-reaction pairs do we have here?
Since action-reaction forces are equal why don't they cancel to zero?
We must isolate the system that we are considering.
We consider only the forces that are acting on that system.
We "free" or isolate the object from the environment.
We consider only the forces acting on the body in question.
It is generally customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction that the force is acting.
We only consider the force on the box (our isolated system) not the reaction forces the box exerts outside the system
A free-body diagram is:
- used to show the relative magnitude and direction of all forces acting upon an object in a given situation.-is labeled to indicate the exact type of force.-a special example of the vector diagrams we have been using.
The size of the arrow in a free-body diagram shows the magnitude of the force.
The direction of the arrow shows the direction that the force is acting.
Free-Body Diagrams
Newtons 1st Law
An object will keep moving (or sitting still) unless an unbalanced force acts on it.
When two forces are of equal magnitude and in opposite directions, they balance each other. They are at equilibrium.
Center of mass - the point in an object around which the mass of the object is balanced.
Applied Force
Fapp
Gravity Force
(also known as Weight)
Fgrav
Normal Force
Fnorm
Friction Force
Ffrict
Air Resistance Force
Fair
Tension Force
Ftens
Spring Force
Fspring
Center of mass - the point in an object around which the mass of the object is balanced.
Apple is falling in a Vacuum. Draw the forces acting on it.
Apple Falling to the ground. Draw the force vectors.
The coffee cup is at equilibrium. Draw the forces acting on it.
The box is at rest. Draw Vectors for the forces acting on the box.
The Space ship is moving with a steady velocity. What forces are acting on it?
Pulling a child on ice. Draw the force vectors.
The girl is pushing the box into the other room. Draw the force vectors.
Assume the bunny is not moving. What do we know about the net forces acting on it?
Home work:
.
Work on p. 25-27, and 30-31
(Read page 10-15)
http://www.physicsclassroom.com/Class/newtlaws/u2l2c.cfm#Questions
Some common free-body examples with answers:
http://www.physicsclassroom.com/Class/newtlaws/u2l2c.cfm#Questions
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