PHY205 Physics of Everyday Lifejharlow/teaching/everysum13/lec02... · 1 PHY205H1F Summer Physics of Everyday Life Class 2 • Force Causes Acceleration • Friction • Mass and

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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 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

𝐹 1

𝐹 2 𝐹 1 + 𝐹 2 = 𝐹 𝑛𝑒𝑡

The Force of Gravity –

a.k.a.

“The Earth exerts

on the ball.”

• Weight = mg

• g = 10 m/s2

• The direction of the weight is

toward the centre of the earth.

• Weight is measured in

.

Normal Force –

a.k.a. Support Force

“The diving board exerts

on the dog.”

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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.

Sliding Friction

“The ground exerts a on

Suleyman.”

Static Friction

“The ground exerts a

on the shoe.”

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

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• 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

Multiple Forces on a Single Object

• 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

The Net Force

Mass and Weight

1 kilogram weighs (9.8 newtons to be precise).

Relationship between kilograms and pounds:

• 1 kg weighs lb = N at Earth’s surface

• 1 lb = N

• 4.54 kg weighs lbs

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.

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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.

2

m

Fa net

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 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/s2

(precisely 9.8 m/s2).

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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

.

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. The force of gravity is 10 times greater on the

basketball than on the tennis ball

B. The force of gravity is the same on both balls

C. 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. The acceleration of the basketball is 10 times greater

than the acceleration of the tennis ball

B. The acceleration of both balls is the same

C. The acceleration of the basketball is slightly larger than

the acceleration of the tennis ball

Free Fall

CHECK YOUR NEIGHBOR 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

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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

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.

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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 .

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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:


• Consider a rocket accelerating upward.

• Action: rocket pushes on gas.

• Reaction:


• Action force: man

pulls on rope to the left.

• Reaction force?

A. Feet push on ground to the right.

B. Ground pushes on feet to the left.

C. Rope pulls on man to the right.

D. Gravity of Earth pulls man down.

E. Gravity of man pulls Earth up.

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• Consider a stationary man pulling a rope.

• Action: man pulls on rope

• Reaction:


• Consider a basketball in

freefall.

• Action force: gravity of

Earth pulls ball down.

• Reaction force?

A. Feet push ground down.

B. Ground pushes feet up.

C. Gravity of Earth pulls man down.

D. Gravity of ball pulls Earth up.

E. Air pushes ball up.


• Consider a basketball in freefall.

• Action: Earth pulls on ball

• Reaction:

a = F m

a = F

m

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!

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• 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?





punched?

• (b) If the mass of the kleenex is 0.003 kg, what force does

the boxer exert on it?





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.

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• 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 .

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A bird flies by

A. flapping its wings.

B. pushing air down so that the air pushes it upward.

C. hovering in midair.

D. inhaling and exhaling air.


CHECK YOUR NEIGHBOR

Slightly tilted wings of airplanes deflect

A. oncoming air downward to produce lift.

B. oncoming air upward to produce lift.

C. Both A and B.

D. Neither A nor B.


CHECK YOUR NEIGHBOR

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.

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Which figure shows ? A A A1 2 3

Vector components

• Vertical and horizontal components of a

vector are to each other

• The

components

add to give

the actual

vector

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.

B. 45.

C. 53.

D. 90.

Vectors

CHECK YOUR NEIGHBOR 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?

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• 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:

[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?

http://campbellpost.wordpress.com/2012/01/26/canoe/

http://campbellpost.wordpress.com/2012/01/26/canoe/

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PHY205 Physics of Everyday Lifejharlow/teaching/everysum13/lec02... · 1 PHY205H1F Summer Physics of Everyday Life Class 2 • Force Causes Acceleration • Friction • Mass and