In some cases, an applied force is
balanced by an opposite force, and
there is no change in motion. In
other cases, an applied force is not
balanced by an opposite force, and
the result is acceleration in the
direction of the applied force. Look
at the following illustrations, and
identify the forces and motion in
each one.
A
C
B
D
1. In one drawing, no motion is likely to occur. Which drawing is it?
2. In which diagram are the forces clearly balanced? How does this
relate to your answer to #1? If more force is exerted by the person,
does the opposite force increase to match the new force, stay the
same or decrease?
3. Suppose there is enough friction in the wheels of the wagon in
diagram C to balance the force with which the wagon is pulled. How
will this affect the motion of the wagon?
Chapter 11
Forces
Section 1
Laws of Motion
Watch the video.
What happens to the people inside the car?
Why?
I. Newton’s 1st Law
A. An object at rest remains at rest and
an object in motion maintains its
velocity unless it experiences an
unbalanced force.
B. Inertia – tendency of an object to
either:
1. Keep moving
2. Remain at rest
C. Mass determines force:
1. Big mass = big force.
2. Small mass = small force.
D. Moving objects want to stay moving
while objects at rest want to stay at
rest.
1. Large objects (Large Mass) =
more inertia.
2. Small objects (Small Mass) = less
inertia.
II. Newton’s 2nd Law
A. Force equals mass times acceleration.
F = ma
The unbalanced force acting on an object
equals the object’s mass times its
acceleration.
B. Force is measured in Newtons (N).
One newton = 1kg x 1 m/s2
1 pound (lb) = 4.4 newtons (N).
Problem:
What is the net force necessary for a 1.6 x 103
kg automobile to accelerate forward at 2.0 m/s2
GIVEN:
m = 1.6 x 103 kg
a = 2.0 m/s2
F=?
WORK:
F = ma
F = (1.6 x 103 kg) (2.0 m/s2)
F = 3,200N
F
m
a
Problem:
A baseball accelerates downward at 9.8 m/s2. If the
gravitational force is the only force acting on the baseball and
is 1.4 N, what is the baseball’s mass?
GIVEN:
m=?
a = 9.8 m/s2
F = 1.4 N
WORK:
F = ma
m = F/a
m = 1.4 N/9.8 m/s2
m = 0.14 kg
F
m
a