Download Net Force

Net Force
Unbalanced Forces

Newton’s Second Law requires a net force.
• One or more forces act on an object
• Forces are vectors that can be added

Add all vector forces acting on a object.
• If the sum is zero – equilibrium.
• If the sum is not zero – net force.
Force Vectors

Force is a vector.

A block sliding on an inclined plane has forces acting
on it.

We know there is a force of gravity and normal force.
q
Force Diagram

Draw the forces acting on
the block.

The force of gravity points
down with magnitude Fg=mg.

The normal force points
away from the surface of the
inclined plane.
FN
q
Fg  mg
Force Diagram

Draw components of the
forces so the vectors can be
added.

The coordinates should point
along the surface.
FN
x
mg sin q
• The normal force is
unknown
• The motion will be along the
surface
q
mg cos q
Fg  mg

The components of Fg are
compared to the surface.
Net Force

• No motion or acceleration away from
the surface
• No force in that component
FN
mg sin q

Solve for the normal force:
Fy  FN  Fgy  FN  mg cos q  0
q
mg cos q
The block stays along the plane.
FN  mg cos q
Fg  mg

Solve for the net force:
Fnet  Fgx  mg sin q
Motion from Force



The net force causes the block to accelerate.
The amount of acceleration is force divided by mass.
This is a constant acceleration moving the block.
a
q
F
 g sin q
m
Inclined Plane

Pushing a block up an
incline is easier than
lifting.
Fapp  mg sin q

To get constant velocity
a force equal to the
downward acceleration
must be applied.
FN
mg sin q
q
mg cos q
Fg  mg
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