Transcript Physics Definition

Physics
Newton’s Laws of Motion
Teacher: Luiz Izola
Chapter Preview
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Force and Mass
Newton’s First Law of Motion
Newton’s Second Law of Motion
Newton’s Third Law of Motion
Forces in Two Dimensions
Weight
Normal Forces
Learning Objectives
 Learn Newton’s Three Laws.
 Learn How to Solve Problems related
to the laws
 Understanding their impact on our
daily lives.
Introduction
 We are all subject to Newton’s laws of
motion.
 Our lives are constrained and regulated by
these three fundamental statements
concerning matter and its motion.
 Newton’s Laws are recognized as the
indispensable foundation for Physics.
Force and Mass
 A force is a PUSH or a PULL applied to an
object.
 Two quantities characterize a force: magnitude
and direction.
 Due to these two quantities, Force is considered
a vector.
 An object’s mass is a measure of how difficult is
to change its velocity. How much force must be
applied to take/bring an object from/to rest. Also,
how much effort is needed to change its direction.
 Mass is measured in Kg (kilograms).
Newton’s First Law
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A force is required to change an object’s motion.
Why an object that receives a initial push, usually
stops after some time?
Newton’s First Law
An object at rest remains at rest as long as no net force
acts on it.
An object moving with constant velocity continues to
move with the same speed and same direction as long
as no net force acts on it.
Newton’s First Law
 Newton’s First Law is also known as the
“Law of Inertia”
 According to Newton’s first law, being at
rest and moving with constant velocity are
actually equivalent.
 If the net force of an object is zero, its
velocity is constant. (Discuss it!)
Newton’s Second Law
 When you hold any object in your hand, you
have to exert an upward force equivalent to the
force of gravity applied on the object.
 If you remove your hand, the object would
accelerate downward.
 Newton’s second law basically states that
UNBALANCED FORCES CAUSE ACCELERATIONS.
Newton’s Second Law

Acceleration is Proportional to Force
Newton’s Second Law
 Acceleration is Inversely Proportional to Mass
Newton’s Second Law
 Combining these results, we find that in the simple case
where with just one force in just one direction, the
acceleration is given by:
Acceleration = Force / Mass
 Rearranging the equation we arrive at Newton’s Second
Law:
ΣF = ma
m = mass F = force a= acceleration
 Because this is a vector relationship, the direction of a
vector’s acceleration is the SAME as the NET FORCE
acting on it.
Newton’s Second Law
 In terms of vectors components, we can derive the second
law in terms of x, y, and z axis.
ΣFx = max
ΣFy = may
ΣFz = maz
If the sum of all forces acting on an object is zero, the
acceleration of the object must be zero. This proves the first
law of Newton. Explain
Force unit of measurement if newton (N).
Practice Session
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In a grocery store you push a 12.5kg cart with a force of
14.0N. If the cart starts at rest, how far does it move in
3.00 seconds?
A 0.53Kg billiard ball is given a speed of 12m/s during a
time interval of 0.04s. What force acted on the ball?
A baseball pitcher can accelerate a 0.15Kg ball from rest
to 28 m/s in a distance of a 1.50 meters. (a) What is the
average force exerted on the ball during the pitch? (b) If
the mass of the ball is increased, is the force required of
the pitcher increased, decreased, or the same on (a)
A catcher stops a 92 m/s pitch in his glove, bringing it to
a rest in 0.15 meters. If the force exerted by the catcher
is 803N, what is the mass of the ball?
Practice Session
5.
Foamcrete is a substance designed to stop an airplane
that has run off the end of a runaway, without causing
injury to passengers. It is solid enough to support a
car, but crumbles under the weight of a large airplane.
By crumbling, it slows the plane down to a safe stop.
Suppose that a 747 with a mass=1.75 x 105 Kg and an
initial speed of 26.8 m/s is slowed to a stop in 122
meters. What is the magnitude of the retarding force F
exerted by the Foamcrete on the plane?
Practice Session
6. A jetliner lands and begins to slow to a
stop as it moves along the runaway. If its
mass is 3.50x105 Kg, its speed is 27 m/s,
and the net braking force is 4.30 x 105 N.
(a) What is the speed 7.50 secs later?
(b) How far it traveled in this time?
The Gravitational Force and Weight
 All objects are attracted to earth. This attractive
force is called gravitational force Fg. It is
directed towards the center of the earth and its
magnitude is called the weight of the object.
 Fg = mass (m) x acceleration of gravity (g)
 Because it depends on g, weight varies with
geographic location. Objects weight less at
higher altitudes
 Kilogram is a unit of mass not weight.
Newton’s Third Law
“ If two objects interact, the force F12 exerted
by object 1 on object 2 is equal in magnitude
and opposite direction to the force F21
exerted by object 2 on object 1.”
F12 = - F21
“The action force is equal in magnitude to the
reaction force and opposite in directions.”
Newton’s Laws Applications
1) A hockey puck having a mass of 0.30kg slides
on the horizontal, frictionless ice surface. Two
hockey sticks strike the ball at the same time,
exerting the following forces on the puck:
F1 = 5.0N, north of east at 60o .(stick 1)
F2 = 8.0N, south of east at 20o. (stick 2)
Determine the magnitude and direction of the
puck’s acceleration.
Newton’s Laws Applications
2)
A large man and a small boy standing facing
each other on frictionless ice. They put their
hands together and push against each other
so that they move apart. (a) Who moves away
with the higher velocity? (b) Who moves
farther?
Newton’s Laws Applications
3) A traffic light weighing 122N hangs from a
cable T3 tied to two other cables T1 and T2
fastened to a support. T1 and T2 make angles
of 37o and 53o with the horizontal. These two
cables as not as strong as the vertical cable
and they will break if the tension in them
exceeds 100N. Will the traffic light remain
hanging in this situation, or one of the cables
will break?
Newton’s Laws Applications
4) A car of mass m is on an icy inclined driveway
with a angle Θ with the horizontal. (a) Find the
acceleration of the car assuming the driveway
is frictionless. (b) Suppose the car is release
from rest at the top of the incline, and the
distance from the car’s front bumper to the
bottom of the incline is d. How long it takes for
the front bumper to reach the bottom?
Homework
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2.
Driving home from school one day, you spot a ball
rolling out into the street. You brake for 1.20 seconds,
slowing your 1.2 ton car from 80 miles/hour to 40
miles/hour. (a) What was the average force exerted on
your car during the brake? (b) How far did you travel
during the brake?
A drag racer crosses the finish line doing 212
miles/hour and prompt deploys her braking parachute.
(a) What force must the chute exert on the 885
kilograms car to slow it to a 40 miles/hour in a distance
of 165 meters? (b) Describe your solving strategy.
Homework
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4.
You hold a brick at rest on your hand (a) Identify the
forces acting on the brick. (b) Is the brick in
equilibrium? Explain. (c) Describe the Newton’s Law
that supports your explanation.
On your vacation , your 1300-kg car pulls a 540-kg
trailer away from a stop light with an acceleration of
1.90 meters/second2. (a) What is the net force exerted
by the car on the trailer? (b) What is the net force
acting on the car?
Homework
5.
A force of magnitude 7.5 N pushes three boxes with
masses m1 = 1.30kg, m2 = 3.20kg and m3 = 4.9kg, as
shown in the picture
Homework
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A 6.0-kg block of ice resting over a table is acted on by
two forces: F1 = 13N, west of north with a 60o angle
and F2 = 11N, east of north with a 30o angle. These
two forces are applied by two men pushing the block of
ice against the table. Think about the direction of the
forces. Find (a) The ice’s acceleration. (b) the normal
force exerted on it by the table.
A farm tractor tows a 4.3 ton trailer up a 26o incline at a
steady speed of 15 miles/hour. What force does the
tractor exert on the trailer? Ignore friction.
You pull upward a suitcase with a force of 115N, and it
accelerates upward at 0.075 meters/second2. (a) What
is the mass and the weight of the suitcase?
Homework
A shopper pushes a 7.5-kg cart up a 13o incline. Find
the magnitude of the horizontal force needed to give
the cart an acceleration of 1.41 meters/second2.
10. Before practicing his routine on the rings, a 67-kg
gymnast hangs motionless, with one hand grasping
each ring and his feet touching the ground. Both arms
make a 24o angle with the vertical. (a) If the force
exerted by the rings on each arm is 290N, what is the
magnitude of the force exerted by the floor on his feet?
(b) If the angles are greater than 24o, would the force
exerted by the floor be greater than or less than the
value found in (a)? Explain
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