Transcript Conceptual Physics 2.2 PP

Section 2.2
Acceleration and Newton’s Second
Law
Review
What was Newton’s First Law of Motion?
What is Inertia?
Review
What was Newton’s First Law of Motion?
An object at rest will stay at rest and an object
in motion will continue in motion with the same
speed and direction UNLESS acted
on by a
force.
What is Inertia?
The property of an object that resists
changes in its motion.
The First Law
Newton said that a force is required to
change the motion of an object, but how
does a force change an objects motion?
Answer: Acceleration
Acceleration
Acceleration is the rate at which speed
increases.
It is the change in an objects speed
(velocity) over time.
Time to Think
Can you do it???
Which has a greater acceleration?
A car that accelerates from 0 mph to 40
mph in 10 s
Or
A car that accelerates from 0 mph to 60
mph in 10 s
Time to Think
Which skier has a greater acceleration?
Skier on the bunny hill
Or
Skier on a double black diamond
WHY?
We Can Calculate Acceleration
Remember, acceleration is the change in speed
over time
SI units of acceleration are m/s2
Try it!
1.
If a car accelerates from 0 m/s to 40 m/s
in 10 s, what is its acceleration?
Try it!
1.
If a car accelerates from 0 m/s to 40 m/s
in 10 s, what is its acceleration?
a = v = 40m/s = 40m x 1 = 4m/s2
t
10s
s
s
What about this one?
A car is traveling 40 m/s and accelerates to
60 m/s in 10 s, what is its acceleration?
REMEMBER: Acceleration is determined
by the change in velocity
over time
How to Calculate it
Let’s do it!
A car is traveling 40 m/s and accelerates to
60 m/s in 10 s, what is its acceleration?
a = v2 – v1 = 60m/s – 40m/s = 20m/s =2m/s2
t
10s
10s
You try it
Webster is running to his car after school.
His speed is 7m/s, but he gets tired quickly
and slows down to 4m/s over 3s. What is
Webster’s acceleration?
You try it
Webster is running to his car after school.
His speed is 7m/s, but he gets tired quickly
and slows down to 4m/s over 3s. What is
Webster’s acceleration?
a = v2 – v1 = 4m/s – 7 m/s = -3m/s = -1m/s2
t
3s
3s
What’s the dilly with the negative?
A negative acceleration means an object
slows down; positive means it speeds up.
Positive acceleration = acceleration
Negative acceleration = deceleration
Back to Newton
He discovered a relationship between the
force applied on an object, the mass of the
object, and the resulting acceleration.
It was called:
Newton's Second Law of Motion
(probably because it was his second law? IDK)
Newton’s Second Law
To use Newton’s second law properly, keep the
following important ideas in mind. They are a
good guideline for how to apply the second law to
physics problems.
1. The net force is what causes acceleration.
2. If there is no acceleration, the net force must be
zero.
3. If there is acceleration, there must also be a net
force.
4. The force unit of newtons is based on kilograms,
meters, and seconds.
Ways to write the
►a
= F/m
► m = F/a
► F = ma
nd
2
Law
Test
1. What is the acceleration of a 1,500kilogram car if a net force of 1,000 N is
exerted on it?
Test
1. What is the acceleration of a 1,500kilogram car if a net force of 1,000 N is
exerted on it?
Answer: 1.5 m/sec2
Test
2. As you coast down the hill on your
bicycle, you accelerate at 0.5 m/sec2. If the
total mass of your body and the bicycle is
80 kg, with what force is gravity pulling you
down the hill?
Test
2. As you coast down the hill on your
bicycle, you accelerate at 0.5 m/sec2. If the
total mass of your body and the bicycle is
80 kg, with what force is gravity pulling you
down the hill?
Answer: 40 kg·m/sec2 or 40 N
Test
3. You push a grocery car with a force of 30
N and it accelerates at 2 m/sec2. What is its
mass?
Test
3. You push a grocery car with a force of 30
N and it accelerates at 2 m/sec2. What is its
mass?
Answer: 15 kg
Vocabulary
► Acceleration
► Deceleration
► Newton’s
Second Law
2.2 Section Review
1. List three units in which acceleration can
be measured.
2. According to Newton’s second law, what
causes acceleration? What resists
acceleration?
2.2 Section Review
3. An 8,000 kg helicopter’s speed increases
from 5 m/sec to 25 m/sec in 5 seconds.
Calculate its acceleration and the net force
acting on it.
4. Define the term “net force.”