Download Document

Newton’s Laws Guided Notes
Newton’s First Law
 An object at rest or an object in motion at constant speed
will remain at rest or at constant speed in the absence of a
resultant force.
o A glass is placed on a board and the board is jerked
quickly to the right. The glass tends to remain at rest
while the board is removed.
o Assume glass and board move together at constant
speed. If the board stops suddenly, the glass tends
to maintain its constant speed.
Inertia
 Newton's first law is often referred to as the law of inertia.
 Inertia is the resistance or unwillingness of an object to
accelerate (speed up, slow down, or change directions).
 The more mass an object has, the harder it is to
accelerate. Thus, more mass equals more inertia.
Understanding the First Law
 Discuss what the driver experiences when a car
accelerates from rest and then applies the brakes.
o The driver is forced to move forward. An object at
rest tends to remain at rest.
o Driver must resist the forward motion as brakes are
applied. A moving object tends to remain in motion.
Newton’s Second Law
 Whenever a resultant force acts on an object, it produces
an acceleration that is directly proportional to the force
and inversely proportional to the mass.
o Formula and Units:
Acceleration and Force
 Pushing a cart with twice the force produces twice the
acceleration. Three times the force triples the acceleration.
Newton: The Unit of Force
 One newton is the force required to give an acceleration of
1 m/s2 to a mass of 1 kg.
 What resultant force will give a 3 kg mass an acceleration
of 4 m/s2?
1
Additional Notes:
Newton’s Laws Guided Notes
Example
A 40 N resultant force causes a block to accelerate at 5 m/s2.
What is the mass?
Example
A net force of 4.2 x 104 N acts on a 3.2 x 104 kg airplane during
takeoff. What is the force on the plane’s 75-kg pilot?
Example
A 54-g tennis ball is in contact with the racket for a distance of
40 cm as it leaves with a velocity of 48 m/s. What is the average
force on the ball?
Weight and Mass
 Weight is the force an object applies as a result of gravity
pulling it downward. It is directed downward and it varies
with gravity.
 Mass is a universal constant which is a measure of the matter
that makes up an object. It is always constant regardless of
location
 Because weight is a force created by the downward
acceleration of gravity, we can make this substitution.
o W=mg
Example
What is the weight of a 10-kg block?
2
Additional Notes:
Newton’s Laws Guided Notes
Always Remember!!
 In Physics, the use of Newton’s second law and many
other applications makes it absolutely necessary to
distinguish between mass and weight. Use the correct
units!
o Metric SI units: Mass is in kg; weight is in N.
o Pounds should never be used!!!
Example
A resultant force of 40 N gives a block an acceleration of 8 m/s2.
What is the weight of the block near the surface of the Earth?
Newton’s Third Law
 For every action force, there must be an equal and
opposite reaction force. Forces occur in pairs.
Action and Reaction Forces
 Use the words by and on to study action/reaction forces
below as they relate to the hand and the bar:
o The action force is exerted by the _____ on the
_____.
o The reaction force is exerted by the _____ on the
_____.
Forces
 If a bar of gold that has a mass of 1 kg is sitting at rest on
a table, what forces are acting on it and to what
magnitude?
o Gravity acts on the object in the downward direction.
However, because the bar is not moving downward,
there must be another force counteracting it.
o The normal force of an object pushes up with a force
equal to the bar’s downward force. Because both
forces are present, the bar remains at equilibrium.
Free-body Diagrams
 A free body diagram is a diagram that shows all of the
forces present on a given object.
3
Additional Notes:
Newton’s Laws Guided Notes

When drawing a free body diagram, objects are always
represented as points or dots and forces are drawn as
arrows pointing away from the object.
 For now the only forces we will consider are gravity,
tension, normal, applied, and friction.
 Gravitational Force: This is the only force that will always
be present and will always point straight down.
 Tension Force: This type of force is caused by a rope,
chain, string, etc. and goes in the direction of the rope,
chain, string, etc.
 Normal Force: This type of force is present when the
object is in contact with a surface. It always points
perpendicular to the surface.
 Applied Force: This type of force is used when an outside
agent is acting on the object such as a person.
 Frictional Force: This type of force is the result of two
surfaces being drug across each other, creating friction. It
always points in the direction opposite of the movement.
 The size of the arrow should be proportional to the size of
the force. Larger forces should have larger arrows!!!
Examples
F
Example
A cart and driver have a mass of 120 kg. What force F is
required to give an acceleration of 6 m/s2 with no friction?
4
Additional Notes:
Newton’s Laws Guided Notes
Example
What is the tension FT in the rope below if the block accelerates
upward at 4 m/s2? (Draw sketch and free-body)
Example
Find the acceleration of the blocks and the tension force between
the two blocks if there is no friction on the surfaces.
5
Additional Notes: