Download Newtons 1st and 2nd Laws

Chapter 4
Newton’s 1st and 2nd Laws of
Motion
What do you think?
• If a net force acts on an object, what type of
motion will be observed?
– Why?
• How would this motion be affected by the
amount of force?
• Are there any other factors that might affect this
motion?
Newton’s First Law

An object moves with a velocity that is
constant in magnitude and direction,
unless acted on by a nonzero net force


“A body in motion tends to remain in
motion; a body at rest tends to remain at
rest.”
The net force is defined as the vector
sum of all the external forces exerted
on the object
External and Internal
Forces

External force


Any force that results from the
interaction between the object and its
environment
Internal forces


Forces that originate within the object
itself
They cannot change the object’s
velocity
Inertia

Is the tendency of an object
to continue in its
original state of motion

Either moving, or at rest
Mass
A measure of the
resistance of an object to
changes in its motion
due to a force
 Scalar quantity
 SI units are kg

Weight

The magnitude of the gravitational
force acting on an object of mass
m near the Earth’s surface is called
the weight w of the object

w = m g is a special case of Newton’s
Second Law


g is the acceleration due to gravity
g can also be found from the
Law of Universal Gravitation
Gravitational Force


Mutual force of attraction between
any two objects
Expressed by Newton’s Law of
Universal Gravitation:
m1 m2
Fg  G 2
r
Newton’s Second Law
The acceleration of an
object is directly
proportional to the
net force acting on it, and
inversely proportional to
the object’s mass
Newton’s Second Law


Fnet  m  a
• Increasing the force will increase the acceleration.
– Which produces a greater acceleration on a 3-kg model airplane,
a force of 5 N or a force of 7 N?
• Answer: the 7 N force
• Increasing the mass will decrease the acceleration.
– A force of 5 N is exerted on two model airplanes, one with a mass
of 3 kg and one with a mass of 4 kg. Which has a greater
acceleration?
• Answer: the 3 kg airplane
Newton’s Second Law
(Equation Form)

F  ma
• F represents the vector sum of all forces acting
on an object.
F = Fnet = m·a
Units for force: mass units (kg)  acceleration units
(m/s2) = kg·m/s2
The units kg•m/s2 are also called newtons (N).
Multiple Objects –
Example




When you have more than one
object, the problem-solving
strategy is applied to each object
Draw free body diagrams for each
object
Apply Newton’s Laws to each
object
Solve the equations
Classroom Practice Problem
• Space-shuttle astronauts experience
accelerations of about 35 m/s2 during
takeoff. What force does a 75 kg astronaut
experience during an acceleration of this
magnitude?
• Answer: 2600 kg•m/s2 or 2600 N