Do Now 2.10.14
Grab a Calculator
If a table is being pushed across the floor at 20N
and friction is 5 N. What is the net force on the
chair?
Newton's Second Law of Motion
Week of 2.10.14
• The acceleration of an object is directly
proportional to the net force acting on the object,
in the direction of the net force, and is inversely
proportional to the mass of the object
- in short: A = net force or F = ma
mass
- symbols: a = acceleration
Fnet = force
m = mass
nd
Newtons 2 Law
• A boy pushed forward a cart of groceries
with a total mass of 40.0 kg. What is the
acceleration of the cart if the net force on
the cart is 60.0 N?
• An automobile with a mass of 1200 kg
accelerates at a rate of 3.0m/ s2 in the forward
direction. What is the net force acting on the
automobile?
• A 25 N force accelerates a boy in a wheelchair at
0.5m/s2. What is the mass of the boy and the
wheelchair?
You Try!
• A = F/m
60.0 N = 1.50 m/s2
40.0 kg
• F = ma
1200 kg x 3.0 m/s2 = 3600 N
• M = F/a
25 N = 50 kg
0.50 m/s2
Answers
Q: Would it be easier to lift a huge truck on
the Earth or on the Moon?
Weight vs. Mass
Q: Would it be easier to lift a huge truck on the
Earth or on the Moon?
A: On the moon, when you lift an object you
are dealing with weight, since weight is the
gravitational pull on the most massive body,
and the moon has less gravity than the earth
Weight vs. Mass
• Weight : The force of gravity acting on an object
• Mass: the measure of inertia on an object
Weight vs. Mass
• Weight = Mass x Acceleration due to gravity
• W= mg
• What is the acceleration of gravity?
Weight vs. Mass
• What is the weight of a person with a mass of 29 kg on
earth? If the acceleration of gravity on Mars is 3.7 m/s/s
what would be the weight of this person on Mars?
Practice Problems
• The gravitational pull on an unknown planet in
20 m/s/s. Ms. Miller goes to visit the aliens on
this planet. If her mass is 40 kg on earth what
would her weight be on this new planet?
• A person weighs 56 kg on earth. What is their
mass?
You Try!
• If you would like to earn 15% back on your last
Exam this is what you can do.
•
•
•
•
Pick up an Exam
Take it home
Answer the questions
YOU MUST DO IT ON A SEPARATE SHEET OF
PAPER
• YOU MUST TELL ME WHY YOU GOT IT
WRONG THE FIRST TIME!!!!!
• If you do not do what it bolded and underlined you will
not get credit.
• Rewrite the questions in your notes
• Underline what the question is asking
for
• Write the equation you will be using
• Solve
Practice Problems
• What is the acceleration of a helicopter
with a mass of 5000 kg and a force of
10,000N?
• A boy kicks a soccer ball that is at rest and
it reaches a speed of 10 m/s 1 second after
being kicked. What is its acceleration?
Practice Makes Perfect!
• During a test crash an airbag inflates to
stop the dummies forward motion. The
dummy has a mass of 75 kg. If the net
force is 825 N towards the rear of the car,
what is the dummies deceleration?
Practice Makes Perfect!
• A boy has a weight of 220 kg on a planet with a
gravitational pull of 36 m/s/s. What would his weight be
on Earth?
• A car is going 90 km/hr at a constant speed. How far will
he travel in 3 hours?
Practice Makes Perfect!
• A bicycle takes 8 seconds to accelerate at a
constant rate from rest to a speed of 4 m/s.
If the mass of the bicycle and rider
together is 85 kg, what is the net force
acting on the bicycle? (hint: first calculate
acceleration)
Practice makes Perfect!
•
•
•
•
Speed
Acceleration
Mass vs Weight
Net Force
• You can make an equations sheet!
• One page ONLY equations
Test Thursday
• Action Reaction Forces
- whenever on object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object.
ex. produces motion:
car tires push backwards against the
road the road pushes forward against the
car tires
ex. no motion:
hand pushes against the wall the wall pushes
against your hand
Newtons Third Law
- if we drop a sheet of tissue paper in front
on the heavy weight champion of the world
and challenge him to hit it in midair with
50 lbs of force, his best punch couldn’t
even come close
Why?
- a force is not a thing in itself but makes
up an interaction between itself and
another (system)
- the action force and the reaction force
- net force only equals 0 when force
are equal and opposite
Q: One cold and rainy day, you car battery is
dead and you must push the car to get it
started. Why can’t you push the car by
remaining comfortably inside pushing
against the dash?
Q: One cold and rainy day, you car battery is dead and you
must push the car to get it started. Why can’t you push
the car by remaining comfortably inside pushing against
the dash?
A: Pushing again the dash creates a internal force to the
system, in order to accelerate the car you must have an
external force to the system
• Definition
- a quantity defined as the product of an object’s
mass and its velocity
• Formula
M=mxv p=mxv
• Units
Kg x m/s
Momentum
• Define Momentum
• What is the formula?
• What are the units?
• Why do these units make sense?
Do Now!!!!
• Which object has more momentum: a car
traveling at a speed of 10 km/hr or a
baseball pitched at 150 km/hr. Explain
your answer
Think about it!
• What is the momentum of an 0.30 kg blue jay
flying at 17 m/s?
• What is the mass of the train car moving at 14
m/s with a momentum of 140 kg-m/s? of the car
moving at 10 m/s with a momentum of 100kgm/s? What is the total momentum of the system?
Practice Makes Perfect!
• Notes for this week (40 points)
• Participation (20 points)
Tribal Council