Name__________________________________________ Mods ____________ Date _________
Forces
Newton’s First law of Motion assumes that forces acting on the object are balanced. When a
book is at rest on a table, the force of gravity pushing down on the table is equal to the force of
the desk pushing up. The forces acting on the book are balanced, so the book stays at rest. The
same is true for objects in motion. If the forces acting on a moving object are balanced, and no
other outside forces interfere, the object would keep on moving forever.
Unbalanced forces cause a change in position or motion. The balanced forces cancel each other
out, causing a state of equilibrium where there is no motion or change. Unbalanced forces
always result in motion.
In the absence of a net force objects do not change their state of motion. The combination of
all forces acting on an object is called the net force. It is the net force that that changes an
objects state of motion.
When more than one force acts on an object, the net force is the sum of the forces. When
forces act in the same direction, the net force is the sum of the forces. When forces act in
opposite directions, the net force is the difference of the forces.
Force – any push or pull
Measured in Newtons (N)
Net force- the sum of all forces acting on an object
Objects in rest
The equilibrium rule: when net force is zero.
The sum of the upward vectors equals the sum of the downward vectors. ΣF = 0 and the
staging is in equilibrium.
Problem.
Consider the gymnast hanging from the rings.
If she hangs with her weight evenly divided between the two rings, how would scale readings in
both supporting ropes compare with her weight? Suppose she hangs with slightly more of her
weight supported by the left ring. How would a scale on the right read?
The reading on each scale will be half her weight. The sum of the readings on both scales
then equals her weight.
When more of her weight is supported by the left ring, the reading on the right is less
than half her weight. No matter how she hangs, the sum of the scale readings equals her
weight. For example, if one scale reads two-thirds her weight, the other scale will read
one-third her weight.
Support Force
Weight – force of gravity on an object
The upward force that supports the object is called the SUPPORT FORCE or NORMAL FORCE.
(normal in math means 90 degrees)
What force act on the book while it is motionless on a table? The table pushes up on the book
with a force equal to the downward weight of the book. It produces net force of zero. The other
force is the support force of the table. Support force is often called normal force. Normal to
means at right angles to. This force acts at right angles to the surface.
The spring pushes up on your hand with as much force as you exert to push down on the spring