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Newton’s Laws
Weight
• The gravitational force (Fg) exerted on
an object by Earth is a vector quantity,
directed toward the center of Earth.
• The magnitude of this force (Fg) is a
scalar quantity called weight.
• Weight changes with the location of an
object in the universe.
Calculate the weight of a 53 kg object.
If an object weighs 98.1 N, what is the mass of that
object?
A 70-kg person travels to a planet where he weighs
1,750 N. What is the value of g on that planet?
Answer: 25 N/kg
Force
• A force is an action exerted on an object which
may change the object’s state of rest or motion.
• Forces can cause accelerations.
• The SI unit of force is the newton, N.
• Forces can act through contact or at a
distance.
Newton’s First Law
An object at rest will remain at rest and an object
in motion will continue in motion with a constant
velocity unless acted upon by a force.
Force Diagrams
• The effect of a force depends on both
magnitude and direction. Thus, force is a
vector quantity.
• Diagrams that show force vectors as arrows are
called force diagrams.
• Force diagrams that show only the forces acting
on a single object are called free-body
diagrams.
Equilibrium
• Equilibrium is the state in which the net force
on an object is zero.
• Objects that are either at rest or moving with
constant velocity are said to be in equilibrium.
• Newton’s first law describes objects in
equilibrium.
Tip: To determine whether a body is in equilibrium, find the net
force. If the net force is zero, the body is in equilibrium. If there is
a net force, a second force equal and opposite to this net force will
put the body in equilibrium.
Find the net force:
Find the net force:
Determine the force on the ball.
Determine the mass of the ball.
Determine the force of the cable(s) on the ball in
case A and case B.
Case A 1 cable
Case B 2 Cables
5 kg
5 kg
Determine force of each cable on the 7 kg ball.
7 kg
4 kg
Inertia
• Inertia is the tendency of an object to resist
being moved or, if the object is moving, to resist a
change in speed or direction.
• Newton’s first law is often referred to as the
law of inertia because it states that in the
absence of a net force, a body will preserve its
state of motion.
• Mass is a measure of inertia.
Newton’s 3rd Law
Newton’s Third Law
• If two objects interact, the magnitude of the
force exerted on object 1 by object 2 is equal to
the magnitude of the force simultaneously
exerted on object 2 by object 1, and these two
forces are opposite in direction.
• In other words, for every action, there is
an equal and opposite reaction.
• Because the forces coexist, either force can be
called the action or the reaction.
Hammer Striking a Nail
• What are the action/reaction pairs for a hammer
striking a nail into wood?
• Force of hammer on nail = force of nail on hammer
• Force of wood on nail = force of nail on wood
• Which of the action/reaction forces above act on the
nail?
• Force of hammer on nail (downward)
• Force of wood on nail (upward)
Does the nail move? If so, how?
Fhammer-on-nail > Fwood-on-nail
so the nail accelerates downward
Action and Reaction
Forces
• Action-reaction pairs do not imply that the
net force on either object is zero.
• The action-reaction forces are equal and
opposite, but either object may still have a
net force on it. Consider driving a nail into wood with
a hammer. The force that the nail
exerts on the hammer is equal and
opposite to the force that the hammer
exerts on the nail. But there is a net
force acting on the nail, which drives
the nail into the wood.
Action-Reaction: A Book on a Desk
Action Force
• The desk pushes
up on the book.
Reaction Force
• The book pushes
down on the desk.
Action-Reaction: A Falling Book
Action
• Earth pulls down on the
book (force of gravity).
• What is the result of the
action force (if this is the
only force on the book)?
• Unbalanced force produces 2
an acceleration of -9.81 m/s .
Reaction
• The book pulls up on Earth.
• What is the result of the
reaction force?
• Unbalanced force produces a
very small upward
acceleration (because the
mass of Earth is so large).
Momentum
1. 70,000 kg x m/s
2. 35,000 kg x m/s
3. 2 m/s
4. 0.50 kg
5. 40,000 kg x m/s
6. 29 m/s
7. The 4 kg more requires more force to stop.
8. 1,225 kg
9. 4.2 kg x m/s
10. 15 m/s
11. 0.01 kg x m/s
Momentum Conservation
1.
2.
3.
4.
5.
6.
7.
8.
9.
6.7 kg
400 m/s
3.38 kg x m/s
-0.50 m/s
100 kg
A) 480 kg x m/s B) 6.0 m/s
A) 60,000 kg x m/s
A) 8.4x1011 kg x m/s
A) 360 kg x m/s
10. Since the gun and bullet are stationary before being fired, the momentum
of the system is zero. The “Kick” is the momentum of the gun that is
equal to that of the bullet.
11. It means that momentum is transferred without loss.
Momentum
• Momentum measures a moving object’s
resistance to changes in its motion. Momentum
includes the effects of speed and direction as
well as mass.
Newton’s Second Law
The acceleration of an object is directly
proportional to the net force acting on the
object and inversely proportional to the
object’s mass.
SF = ma
net force = mass  acceleration
SF represents the vector sum of all external forces
acting on the object, or the net force.
Normal Force
• The normal force acts on a surface in a
direction perpendicular to the surface.
• The normal force is not always opposite in
direction to the force due to gravity.
– In the absence of other forces, the
normal force is equal and opposite
to the component of gravitational
force that is perpendicular to the
contact surface.
– In this example, Fn = mg cos q.
Fundamental Forces
• There are four fundamental forces:
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▫
▫
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Electromagnetic force
Gravitational force
Strong nuclear force
Weak nuclear force
• The four fundamental forces are all field
forces.