Physics of Everyday Life Class 2 Force Causes Acceleration Friction Mass and Weight Mass Resists Acceleration Newtons Second Law of Motion Free Fall NonFree Fall Net force is the combination ID: 617362
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PHY205H1F Summer Physics of Everyday LifeClass 2
Force Causes AccelerationFrictionMass and WeightMass Resists AccelerationNewton’s Second Law of MotionFree FallNon-Free FallSlide2
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.
Net Force
Slide3
The Force of Gravity – a.k.a.
“The Earth exerts
on
the
ball.”
Weight =
mg
g
= 10 m/s
2
The direction of the weight is toward the centre of the earth.
Weight is measured in .Slide4
Normal Force – a.k.a. Support Force
“The diving board exerts
on
the dog.”Slide5
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
wooden floor is less than that on a
floor.Slide6
Sliding Friction
“The ground exerts a
on
Suleyman
.”Slide7
Static Friction
“The ground exerts a
on the shoe.”Slide8
The Clickers
Power Light
Status Light
When I start asking clicker questions:
Status light will
flash
green
when your response is
registered on my computer. Status will flash red if your response is not registered.
On/Off Switch
Please turn on your clicker now
8Slide9
A car is parked on flat, horizontal pavement.Which of the following forces are acting on the car?GravityNormal
Static frictionAll of the aboveA and B, but not C
Multiple Forces on a Single ObjectSlide10
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?
Up
Down
The net force is zero
The Net ForceSlide11
Mass and Weight1 kilogram weighs
(9.8 newtons to be precise).Relationship between kilograms and pounds:1 kg weighs lb = N at Earth’s surface1 lb = N4.54 kg weighs lbsSlide12
Mass Resists AccelerationThe same force applied toTwice the mass produces half the acceleration.
3 times the mass, produces 1/3 the acceleration.Acceleration is inversely proportional to mass.Slide13
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.
2Slide14
A fan attached to a cart causes it to accelerate at 2 m/s
2. 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
16 m/s
2
.
8 m/s
2
.
4 m/s
2
.
2 m/s
2. 1 m/s2
.Slide15
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?Slide16
Free FallThe greater
the mass of the object…the its force of attraction toward the Earth.the its tendency to move i.e., the greater its inertia.So, the acceleration is .It is equal to the acceleration due to gravity: m/s
2
(precisely 9.8 m/s
2
).Slide17
Free FallWhen acceleration is g—free fallNewton’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 .Slide18
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?
The force of gravity is 10 times greater on the basketball than on the tennis ballThe force of gravity is the same on both ballsThe force of gravity is slightly larger on the basketball than on the tennis ballFree Fall
CHECK YOUR NEIGHBOR
Slide19
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?
The acceleration of the basketball is 10 times greater than the acceleration of the tennis ballThe acceleration of both balls is the sameThe acceleration of the basketball is slightly larger than the acceleration of the tennis ballFree Fall
CHECK YOUR NEIGHBOR
Slide20
Non-Free FallWhen 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 objectSlide21
Terminal SpeedR 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 changeSlide22
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 .Slide23
PHY205H1F Physics of Everyday LifeChapter 5
Forces and InteractionsNewton’s Third Law of MotionVectorsSlide24
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.
3Slide25Slide26
A Mack Truck drives North on the highway, and collides head-on with a mosquito. Which is true?
The Mack Truck exerts a greater force on the mosquito than the mosquito exerts on the Mack Truck.The mosquito exerts a greater force on the Mack Truck than the Mack Truck exerts on the mosquito.The Mack Truck exerts the same force on the mosquito as the mosquito exerts on the Mack Truck.Impossible to determine without knowing the speeds of the truck and mosquito.Don’t know or none of the aboveSlide27
A Mack Truck drives North on the highway, and collides head-on with a mosquito. Which is true?
The Mack Truck does more damage to the mosquito than the mosquito does to the Mack Truck.The mosquito does more damage to the Mack Truck than the Mack Truck does to the mosquito.The Mack Truck does the same amount of damage to the mosquito as the mosquito does to the Mack Truck.Impossible to determine without knowing the speeds of the truck and mosquito.Don’t know or none of the aboveSlide28
F = maora = 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 .Slide29
Action and reaction forcesone 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 .Slide30
Identifying Action / Reaction PairsConsider an accelerating car.
Action: tire pushes on road.Reaction:Slide31
Identifying Action / Reaction Pairs
Consider a rocket accelerating upward.Action: rocket pushes on gas.Reaction:Slide32
Identifying Action / Reaction Pairs
Action force: man pulls on rope to the left.Reaction force?
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.Slide33
Identifying Action / Reaction Pairs
Consider a stationary man pulling a rope.Action: man pulls on ropeReaction:Slide34
Identifying Action / Reaction Pairs
Consider a basketball in freefall.Action force: gravity of Earth pulls ball down.Reaction force?Feet push ground down.
Ground pushes feet up.
Gravity of Earth pulls man down.
Gravity of ball pulls Earth up.
Air pushes ball up.Slide35
Identifying Action / Reaction Pairs
Consider a basketball in
freefall
.
Action:
Earth pulls on ball
Reaction:
a =
F
m
a =
F
mSlide36
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!Slide37
Chapter 5, Problem 1A 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?Slide38
Chapter 5, Problem 1A 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?Slide39
Chapter 5, Problem 1A 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?Slide40
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.Slide41
Consider the orange and the apple pulling on it.Action and reaction do not cancel (because they act on ). by apple accelerates the orange.Slide42
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.Slide43
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.Slide44
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 .Slide45
A bird flies by
A. 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
Slide46
Slightly tilted wings of airplanes deflect
A. oncoming air downward to produce lift.oncoming air upward to produce lift.Both A and B.Neither A nor B.
Newton’s Third Law
CHECK YOUR NEIGHBOR
Slide47
Vectors & Scalars
Vector quantityhas and direction.is represented by an arrow. Example: velocity, force, acceleration Scalar quantity
has .
Example:
mass, volume, speedSlide48
Vector AdditionThe 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 otheruse Theorem: R2 = V 2
+
H
2
.Slide49
Which figure shows
?Slide50
Vector components
Vertical and horizontal components of a vector are to each other
The components add to give the actual vectorSlide51
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. 0.45. 53.90.
Vectors
CHECK YOUR NEIGHBOR
Slide52
Chapter 5, Problem 6You 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?Slide53
Chapter 5, Problem 6You 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?Slide54
Before Class 3 next Wednesday
Please read Chapters 7 and 8, or at least watch the 20-minute pre-class video for class 3Pre-class reading quiz on chapters 7 and 8 is due Wednesday May 22 by 10:00am Something to think about: [image downloaded Jan.16 2013 from http://campbellpost.wordpress.com/2012/01/26/canoe/ ]
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?