Download Chapter 10.3-10.5

Chapter 10.3-10.5
8th Grade
• Galileo Galilei
– Italian Astronomer
– Suggested that once an object is in motion, no
force is needed to keep it moving.
• Force
– Needed to change the motion of the object.
http://www.3m.co.uk/intl/uk//3mstreetwiseuk/pupilsforce.htm
Newton’s 1st Law of Motion
• Restates Galileo’s ideas about force and
motion
• An object at rest will remain at rest, and an
object moving at a constant velocity will
continue moving at a constant velocity unless
it is acted upon by an unbalanced force.
Newton’s 1st Law of Motion
• This means that if an object is not moving, it
will not move until a force acts on it.
• If an object is already moving, it will continue
to move at a constant velocity until a force
acts to change either its speed or direction.
• Gravity and friction are unbalanced forces
that often change an object’s motion.
Inertia
• An object moving or not moving it resists
any change to its motion.
• Inertia
– The tendency of an object to resist a change in
motion
Inertia
• Newton’s 1st Law
– Also called the Law of Inertia
• The greater mass of an object, the greater
its inertia, and the greater the force needed
to change its motion.
Newton’s 1st Law
• http://www.youtube.com/watch?v=4NQbe
Z0EXZQ
Newton’s 2nd Law of Motion
• Acceleration
– depends on the object’s MASS and the NET
FORCE acting on the object.
– Net force / mass
– Units = Newtons (N)
Newton’s 2nd Law of Motion
Newton’s 2nd Law of Motion
• How do you increase acceleration?
– Change the FORCE!
• If mass is constant, acceleration and force
change in the same way.
• Decrease in mass = increase in acceleration
http://www.youtube.com/watch?v=44dMPz_DPu8
http://www.youtube.com/watch?v=WzvhuQ5RWJE
Section 10.3 Assessment
• What does Newton’s 1st Law of motion state?
– An object at rest will remain at rest and an object in
motion will remain in motion, unless acted upon by an
unbalanced force.
• Why is Newton’s 1st law of motion sometimes called the law
of intertia?
– Inertia is a measure of an object’s tendency to resist a
change in its motion.
• Use what you know about inertia to explain why you feel
pressed back into the seat of a car when it accelerates?
– Because of your inertia, your body tends to remain in
place. The car seat causes you to accelerate, therefore,
by exerting a force on your back.
Section 10.3 Assessment
• State Newton’s 2nd law of motion in your own
words.
– The greater the force= the greater the
acceleration.
– The greater the acceleration or the greater the
object’s mass = the lower the acceleration
• How could you keep an object’s acceleration
the same if the force acting on the object
were doubled?
– You could double the object’s mass
Section 10.3 Assessment
• Why does a car with a large mass might use
more fuel than a car with a smaller mass.
– A greater force is required to accelerate a more
massive car, therefore, more engine power is
required and more fuel is needed to produce that
power.
• Find the force it would take to accelerate an
800kg car at a rate of 5 m/s2
– F=ma
– 800 kg X 5m/s2= 4,000 N
Section 10.3 Assessment
• What is the net force acting on a 0.15 kg
hockey puck accelerating at a rate of 12
m/s2?
– F=ma
– (0.15 kg x 12 m/s2) = 1.8 N
Newton’s 3rd Law of Motion
• Whenever one object exerts a force on a
second object, the second object exerts a
force of equal strength in the opposite
direction on the first object
Newton’s 3rd Law of Motion
• “Action-Reaction” forces
• Ex. When a dog leaps it pushes down on the
ground. The reaction force of the ground
pushes the dog into the air.
• Do action-reaction pairs
Cancel?
• No! They are acting on
Different objects.
Momentum
• “quantity of motion”
• It is the characteristic of a moving object
related to the mass and velocity of the object
• Equation: mass x velocity
• Units = kg* m/s
Momentum
• The more mass an object has the more
momentum it has, so the harder it is to stop
the object.
• Ex. A car moving 20 m/s is harder to stop
than a baseball moving at 20 m/s because
the car has a greater mass and greater
momentum.
Momentum
• Amount of momentum objects have is
conserved when they collide.
• It can be transferred from one object to
another, but none is lost.
• Law of conservation of momentum
– The total amount of momentum of objects that
interact does not change in the absence of
outside forces.
– Ex. Friction
http://www.youtube.com/watch?v=8bTdMmNZm2M
http://www.youtube.com/watch?v=Xx9kiF00rts
Determining Acceleration Word Problems
• 1. A speedboat pulls a 55-kg water skier. The force causes
the skier to accelerate at 2.0 m/s2. Calculate the net force
that causes this acceleration.
Determining Acceleration Word Problems
• 2. What is the net force on a 1,000kg object
accelerating at 3m/s2?
Determining Acceleration Word Problems
• 3. What net force is needed to accelerate a 25
kg cart at 14 m/s2?
Determining Acceleration Word Problems
• 4. Find the force it would take to accelerate
an 800 kg car at a rate of 5 m/s2?
Determining Acceleration Word Problems
• 5. What is the net force acting on a 0.15 kg
hockey puck accelerating at a rate of 12
m.s2?
Momentum Word Problems
• Which has more momentum: a 3.0- kg
sledgehammer swung at 1.5 m/s or a 4.0-kg
sledgehammer swung at 0.9 m/s?
• Momentum = Mass X Velocity
• Smaller : 3.0 kg X 1.5 m/s = 4.5 kg x m/s
• Larger : 4.0 kg X 0.9 m/s = 3.6 kg x m/s
Momentum Word Problems
• A golf ball travels at 16 m/s, while a
baseball moves at 7 m/s. The mass of the
golf ball is 0.045 kg and the mass of the
baseball is 0.14 kg. Which has a greater
momentum?
• Golf Ball: 0.045 kg x 16 m/s = 0.72 kg x m/s
• Baseball: 0.14 kg x 7 m/s = 0.98 kg x m/s
• The baseball has greater momentum
Momentum Word Problems
• What is the momentum of a bird with a mass
of 0.018 kg flying at 15 m/s?
• 0.018 kg x 15 m/s = 0.27 kg x m/s
Momentum Word Problems
• What is the momentum of a 920 kg car
moving at a speed of 25 m/s?
• 920 kg x 25 m/s = 23,000 kg x m/s
Momentum Word Problems
• Which has more momentum? A 250 kg
dolphin swimming at 4 m/s or a 350 kg
manatee swimming at 2 m/s?
• Dolphin : 250 x 4 = 1,000 kg x m/s
• Manatee: 350 x 2 = 700 kg x m/s
• The dolphin has more momentum!