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Thursday, September 3
• Word of the Day
– Newton’s second law of motion: acceleration is
equal to the force divided by the mass of the
object
• Warm-Up:A horizontal force of 100 N pushes
a box across a floor at a constant speed
– What is the net force acting on the box?
– What is the force of friction on the box?
• Phil Physicer weighs 600 Newtons and
stands on 2 bathroom scales. One scale
reads twice as much as the other, what are
the scale readings?
Monday, May 4
• Word:
– Terminal speed: the max speed a person in
free fall reaches where there is no more
acceleration
• Warm-Up:
– What is the difference between velocity
and acceleration?
– The moon has 1/6 the gravity of earth. If
an object has a mass of 120 kg. What is its
mass on the moon?
Tuesday, May 5
• Word:
– Newton’s Third Law: to every action there
is an equal and opposite reaction
• Warm-Up:
– A wedge of cheese has a mass of 0.6 kg.
What is its weight in Newtons?
– A girl pulls a 10-kg wagon with 30 N of
force. What is the acceleration on the
cart?
Chapter 3
Newton’s Second Law of Motion –
Force and Acceleration
Acceleration
• Bubble Gum Physics Lab
• What is acceleration, how would you
define it?
• How did the bubble gum lab show
acceleration?
Galileo’s Definition
• Change in Velocity can be any of 3
things:
– Change in speed
– Change in direction
– Change in both speed and direction
• A car has 3 things that change the
velocity. What are they?
– Gas
– Brake
– Steering Wheel
Examples
1. Suppose you are driving 30 miles per hour,
then in the next second you are up to 35
miles per hour, and the next second 40
miles per hour. What is your acceleration?
a. 5 miles per hour per second
2. Now you slow down and go from 40 miles
per hour to 35 to 30 each second. What is
the acceleration? What is this type of
acceleration often called?
a. -5 mph/sec
b. deceleration
• An object falling without air resistance is in
free fall.
• The force acting to accelerate the object is
gravity.
• The acceleration on all objects is 9.8 (10)
m/s2.
• Draw a picture of an object being dropped
from the side of the building. Draw where
you think the object will be at 1s, 2s, and 3s.
Acceleration and Force
• Acceleration is directly proportional to
net force
– Greater Force = Greater Acceleration
– If you push a shopping cart it accelerates.
If you apply four times the net force, how
much greater will the acceleration be?
• 4 times greater
Mass vs. Weight
• Circle which object you would rather
kick:
– A soccer ball or a bowling ball
– A tin can or a can full of rocks
– A pillow or a wall
– A piece of paper or a piece of wood
Explain
• Can you scientifically explain why you
would rather kick the lighter object?
– More massive objects have more inertia
– What does that mean?
– They want to move less so kicking them
will hurt more!
Mass vs. Weight
• Not the same thing
• Mass = amount of matter in an object;
measure of inertia
• Weight = force due to gravity that acts
on a mass
Actually 9.8
N, but for
this class
we will
round to 10
Mass vs. Weight Practice
• If you weigh 100 kg, how many
Newtons do you weigh?
– 1000 N
• If a bag of potatoes weights 100 N, how
many kg is the bag?
– 10 kg
Newton’s Second Law of Motion
•
•
Links force, acceleration, and mass
The law states:
–
•
The acceleration produced by a net force on an
object is directly proportional to the net force, is
in the same direction as the net force, and is
inversely proportional to the mass of the object.
What does this mean?
–
–
–
Acceleration is produced by force and increases
as net force increases
The object accelerates in the same direction as
the force
Acceleration decreases as the mass of an object
increases
Calculation Practice
a = F/m
Acceleration = Force divided by mass
• Practice: Fill in the blanks
1. A 5-kg bag of sand has a weight of 50 N. When
dropped its acceleration is
1. a = 50N / 5kg = 10 m/s2
2. A 10 kg bag of sand has a weight of 100 N. When
dropped its acceleration is
1. a = 100N / 10kg = 10 m/s2
3. Calculate the free-fall acceleration of a 20 kg bag of
sand.
1. a= 200N / 20kg = 10 m/s2
Calculating Acceleration
• You should have noticed a common
theme on the last slide…
– Acceleration due to gravity is a set amount
• 10 m/s2
– If we drop an object and it has been falling
for 4 seconds, what is the speed?
Which parachute will drop faster?
Why?
– Air Resistance/Drag
lessens the acceleration
– Terminal speed is
reached when there is no
more acceleration (95125 miles per hour for a
human skydiver)
– Terminal Velocity –
direction and speed
when there is no
acceleration