LAWS OF MOTION
Chapter 11
Newton’s First Law
 an object at rest remains at rest and
 an object in motion maintains its velocity (stays in
motion) unless it experiences an unbalanced
force (outside force).
 Aka Law of Inertia
INERTIA
 tendency of an object to resist being moved or
 if object is moving, resist a change in speed or
direction until an outside force acts on object.
 All objects resist changes in motion
 Less mass, less inertia
 More mass, more inertia
Chapter 11
INERTIA EXAMPLE
 Seat belts and car seats provide protection.
• Because of inertia, you slide toward the side of a car
when the driver makes a sharp turn.
• When the car you are riding in comes to a stop, your
seat belt and the friction between you and the seat stop
your forward motion.
WHY WEAR A SEATBELT?
SEATBELT IN THE BACKSEAT?
Newton’s Second Law
 unbalanced force acting on an object equals object’s mass
times its acceleration.
 Force = mass  acceleration
SI Unit:
 newtons (N)
 1 N = 1 kg  1 m/s2
F
m a
Zookeepers lift a stretcher that holds a sedated lion. The total
mass of the lion and stretcher is 175 kg, and the lion’s upward
acceleration is 0.657 m/s2. What is the unbalanced force
necessary to produce this acceleration of the lion and the
stretcher?
GIVEN:
WORK:
F=?
m= 175 kg
a = 0.657 m/s2
F = ma
F
ma
F = (175 kg) (0.657 m/s2)
F = 115 N
What is the net force necessary for a 1.6 x 103 kg automobile to
accelerate forward at 2.0 m/s2?
GIVEN:
WORK:
F=?
m= 1.6 x 103 kg
a = 2.0 m/s2
F = ma
F
ma
F = (1.6 x 103 kg) (2.0 m/s2)
F = 3.2 x 103 N
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:
WORK:
F = 1.4 N
m= ?
a = 9.8 m/s2
m = F/a
F
ma
m = 1.4 N / 9.8 m/s2
m = 0.14 kg
A sailboat and its crew have a combined mass of 655 kg. Ignoring
frictional forces, if the sailboat experiences a net force of 895 N
pushing it forward, what is the sailboat’s acceleration?
GIVEN:
WORK:
F = 895 N
m= 655 kg
a=?
a = F/m
F
ma
a = 895 N / 655 kg
a = 1.37 m/s2