Transcript PHY205 Physics of Everyday Life

PHY205H1F Summer
Physics of Everyday Life
Class 2
• Force Causes
Acceleration
• Friction
• Mass and Weight
• Mass Resists
Acceleration
• Newton’s Second
Law of Motion
• Free Fall
• Non-Free Fall
Net Force
• Net force is the combination of all forces that
change an object’s state of motion.
• Net force is the
of all the forces acting
on an object.
𝐹1
𝐹2
𝐹1 + 𝐹2 = 𝐹𝑛𝑒𝑡
The Force of Gravity –
a.k.a.
• Weight = mg
• g = 10 m/s2
• The direction of the weight is
toward the centre of the earth.
• Weight is measured in
.
“The Earth exerts
on the ball.”
Normal Force –
a.k.a. Support Force
“The diving board exerts
on the dog.”
The Force of Friction
•
•
depends on the kinds of material and how
much they are pressed together.
is due to
and to “stickiness”
of the atoms on a material’s surface.
Example: Friction between a crate on a
floor is less than that on a
floor.
wooden
Sliding Friction
“The ground exerts a
Suleyman.”
on
Static Friction
“The ground exerts a
on the shoe.”
The Clickers
Status Light
Power Light
When I start asking
clicker questions:
• Status light will
flash green when
your response is
registered on my
computer.
• Status will flash red
On/Off Switch
if your response is
Please turn on your clicker now
not registered.
8
Multiple Forces on a Single Object
• A car is parked on flat, horizontal pavement.
• Which of the following forces are acting on
the car?
A.Gravity
B.Normal
C.Static friction
D. All of the above
E. A and B, but not C
The Net Force
• A car is parked on flat, horizontal pavement.
• The “net force” is the vector sum of all the
forces on the car.
• What is the direction of the net force on the
car?
A. Up
B. Down
C. The net force
is zero
Mass and Weight
1 kilogram weighs
(9.8 newtons to be precise).
Relationship between kilograms and pounds:
• 1 kg weighs
• 1 lb =
lb =
N
• 4.54 kg weighs
lbs
N at Earth’s surface
Mass Resists Acceleration
The same force applied to
• Twice the mass produces half the acceleration.
• 3 times the mass, produces 1/3 the acceleration.
• Acceleration is inversely proportional to mass.
2
Newton’s Second Law
The acceleration of an object is directly
proportional to the net force acting on it, and
inversely proportional to its mass.

 Fnet
a
m
A fan attached to a cart causes it to accelerate
at 2 m/s2.
Suppose the same fan is attached to a second
cart with smaller mass.
The mass of the second cart plus fan is half the
mass of the first cart plus fan. The
acceleration of the second cart is
A. 16 m/s2.
B. 8 m/s2.
C. 4 m/s2.
D. 2 m/s2.
E. 1 m/s2.
Chapter 4, Problem 7
•
•
•
A rock band’s tour bus of mass M is accelerating away from
a stop sign at a rate of 1.2 m/s2.
Suddenly a piece of heavy metal, mass M/6, falls onto the
top of the bus and remains there.
What is the acceleration of the bus + metal?
Free Fall
The greater the mass of the object…
• the
• the
inertia.
its force of attraction toward the Earth.
its tendency to move i.e., the greater its
So, the acceleration is
.
It is equal to the acceleration due
to gravity:
m/s2
(precisely 9.8 m/s2).
Free Fall
When acceleration is g—free fall
• Newton’s second law provides an explanation
for the
accelerations of freely falling
objects of various masses.
• Acceleration is
when air resistance is
negligible.
• Acceleration depends on
(weight) and
.
Free Fall
CHECK YOUR NEIGHBOR
A 600 g basketball and a 60 g tennis ball are dropped from
rest at a height of 3 m above the ground. As they fall to the
ground, air resistance is negligible.
Which of the following statements is true for the balls as
they fall?
A.
B.
C.
The force of gravity is 10 times greater on the
basketball than on the tennis ball
The force of gravity is the same on both balls
The force of gravity is slightly larger on the basketball
than on the tennis ball
Free Fall
CHECK YOUR NEIGHBOR
A 600 g basketball and a 60 g tennis ball are dropped from
rest at a height of 3 m above the ground. As they fall to the
ground, air resistance is negligible.
Which of the following statements is true for the balls as
they fall?
A.
B.
C.
The acceleration of the basketball is 10 times greater
than the acceleration of the tennis ball
The acceleration of both balls is the same
The acceleration of the basketball is slightly larger than
the acceleration of the tennis ball
Non-Free Fall
When an object falls downward through the air it
experiences:
• force of gravity pulling it
.
• air drag force acting
.
• R depends on the
of
the object relative to the air,
and the
of the object
Terminal Speed
• R increases with
• Net force goes to
when the object is
moving fast enough so
that R = mg (air
resistance = weight)
• Then no net force
 No acceleration
 Velocity does not
change
Non-Free Fall—
Example
• A skydiver jumps from plane.
• Weight is the only force until
acts.
• As falling speed increases, air resistance on
diver builds up, net force is reduced, and
acceleration becomes
.
• When air resistance equals the diver’s weight,
net force is
and acceleration terminates.
• Diver reaches terminal velocity, then continues
the fall at
.
PHY205H1F
Physics of Everyday Life
Chapter 5
• Forces and Interactions
• Newton’s Third Law of Motion
• Vectors
3
Newton’s Third Law
If object 1 acts on object 2 with a force, then
object 2 acts on object 1 with an equal force
in the opposite direction.
A Mack Truck drives North on the highway,
and collides head-on with a mosquito. Which
is true?
A. The Mack Truck exerts a greater force on the
mosquito than the mosquito exerts on the Mack
Truck.
B. The mosquito exerts a greater force on the Mack
Truck than the Mack Truck exerts on the mosquito.
C. The Mack Truck exerts the same force on the
mosquito as the mosquito exerts on the Mack Truck.
D. Impossible to determine without knowing the speeds
of the truck and mosquito.
E. Don’t know or none of the above
A Mack Truck drives North on the highway,
and collides head-on with a mosquito.
Which is true?
A. The Mack Truck does more damage to the mosquito
than the mosquito does to the Mack Truck.
B. The mosquito does more damage to the Mack Truck
than the Mack Truck does to the mosquito.
C. The Mack Truck does the same amount of damage
to the mosquito as the mosquito does to the Mack
Truck.
D. Impossible to determine without knowing the speeds
of the truck and mosquito.
E. Don’t know or none of the above
F = ma
or
a=F/m
• If the force is equal on the truck and the
mosquito, is the acceleration equal?
• Acceleration is
if m is lower ( F
divided by m)
• Mosquito accelerates more, so it receives
more
.
Action and reaction forces
• one force is called the action force; the other
force is called the
.
• are co-pairs of a single interaction.
• neither force exists without the other.
• are equal in
and opposite in direction.
• always act on
.
Identifying Action / Reaction Pairs
• Consider an accelerating car.
• Action: tire pushes on road.
• Reaction:
Identifying Action / Reaction Pairs
• Consider a rocket accelerating upward.
• Action: rocket pushes on gas.
• Reaction:
Identifying Action / Reaction Pairs
• Action force: man
pulls on rope to the left.
• Reaction force?
A.
B.
C.
D.
E.
Feet push on ground to the right.
Ground pushes on feet to the left.
Rope pulls on man to the right.
Gravity of Earth pulls man down.
Gravity of man pulls Earth up.
Identifying Action / Reaction Pairs
• Consider a stationary man pulling a rope.
• Action: man pulls on rope
• Reaction:
Identifying Action / Reaction Pairs
• Consider a basketball in
freefall.
• Action force: gravity of
Earth pulls ball down.
• Reaction force?
A.
B.
C.
D.
E.
Feet push ground down.
Ground pushes feet up.
Gravity of Earth pulls man down.
Gravity of ball pulls Earth up.
Air pushes ball up.
Identifying Action / Reaction Pairs
a=
a=
• Consider a basketball in freefall.
• Action: Earth pulls on ball
• Reaction:
F
m
F
Ride the MP Elevator!
• In the corner of every elevator in the tower
part of this building, there is a mass hanging
on a spring.
• If you look closely at the spring, it has a scale
which reads Newtons.
• This is how much upward force is needed to
support the hanging mass.
• In your next tutorial you will be going with
your team to look more carefully at this scale,
and record how it changes as the elevator
accelerates!
Chapter 5, Problem 1
• A boxer punches a piece of kleenex in midair and brings it
from rest up to a speed of 25 m/s in 0.05 s.
• (a) What acceleration does the kleenex have while being
punched?
Chapter 5, Problem 1
• A boxer punches a piece of kleenex in midair and brings it
from rest up to a speed of 25 m/s in 0.05 s.
• (a) What acceleration does the kleenex have while being
punched?
• (b) If the mass of the kleenex is 0.003 kg, what force does
the boxer exert on it?
Chapter 5, Problem 1
• A boxer punches a piece of kleenex in midair and brings it
from rest up to a speed of 25 m/s in 0.05 s.
• (a) What acceleration does the kleenex have while being
punched?
• (b) If the mass of the kleenex is 0.003 kg, what force does
the boxer exert on it?
• (c) How much force does the paper exert on the boxer?
Defining Your System
• Consider a single
enclosed orange.
– Applied
causes the orange to
accelerate in accord
with Newton’s second
law.
– Action and reaction pair
of forces is not shown.
• Consider the orange and the apple pulling
on it.
– Action and reaction do not cancel
(because they act on
–
).
by apple accelerates
the orange.
• Consider a system comprised of both the
orange and the apple
– The apple is no longer
to the system.
– Force pair is internal to system, which doesn’t
cause acceleration.
– Action and reaction within the system
– With no
, there is no
acceleration of system.
.
• Consider the same system, but with
external force of friction on it.
– Same internal action and reaction forces
(between the orange and apple) cancel.
– A second pair of action-reaction forces
(between the apple’s feet and the
)
exists.
– One of these acts by the system (apple
on the floor) and the other acts
the
system (floor on the apple).
– External frictional force of floor pushes
on the system, which
.
– Second pair of action and reaction forces
do not
.
Newton’s Third Law
CHECK YOUR NEIGHBOR
A bird flies by
A.
B.
C.
D.
flapping its wings.
pushing air down so that the air pushes it upward.
hovering in midair.
inhaling and exhaling air.
Newton’s Third Law
CHECK YOUR NEIGHBOR
Slightly tilted wings of airplanes deflect
A.
B.
C.
D.
oncoming air downward to produce lift.
oncoming air upward to produce lift.
Both A and B.
Neither A nor B.
Vectors & Scalars
Vector quantity
• has
and direction.
• is represented by an arrow.
Example: velocity, force, acceleration
Scalar quantity
• has
.
Example: mass, volume, speed
Vector Addition
The sum of two or more vectors
• For vectors in the same direction,
arithmetically.
• For vectors in opposite directions,
arithmetically.
• Two vectors that don’t act in the same
or opposite direction:
– use parallelogram rule.
• Two vectors at right angles to each other
– use
Theorem: R2 = V 2 + H 2.
Which figure shows A1  A2  A3 ?
Vector components
• Vertical and horizontal components of a
vector are
to each other
• The
components
add to give
the actual
vector
Vectors
CHECK YOUR NEIGHBOR
You run horizontally at 4 m/s in a vertically falling rain that
falls at 4 m/s. Relative to you, the raindrops are falling at an
angle of
A.
B.
C.
D.
0.
45.
53.
90.
Chapter 5, Problem 6
• You are paddling a canoe at a speed of 4 km/h directly
across a river that flows at 3 km/h, as shown.
• (a) What is your resultant speed relative to the shore?
Chapter 5, Problem 6
• You are paddling a canoe at a speed of 4 km/h directly
across a river that flows at 3 km/h, as shown.
• (a) What is your resultant speed relative to the shore?
• (b) In approximately what direction
should you paddle the canoe so that
it reaches a destination directly
across the river?
Before Class 3 next Wednesday
• Please read Chapters 7 and 8, or at least watch the
20-minute pre-class video for class 3
• Pre-class reading quiz on chapters 7 and 8 is due
Wednesday May 22 by 10:00am
• Something to think about:
• There are two seemingly identical mouse
traps sitting on the floor. They have the
same mass, size, colour, shape and
smell.
• One has been set by bending the spring
back and hooking it, the other is not set.
• What is the physical difference between
the two traps? Why is one so much
scarier than the other?
[image downloaded Jan.16 2013 from
http://campbellpost.wordpress.com/2012/01/26/canoe/ ]