PHY205H1F Summer Physics of Everyday Life Class 2 Force Causes Acceleration Friction Mass and Weight Mass Resists Acceleration Newton’s Second Law of Motion.

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

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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.”

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 ClickersPower LightStatus 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 now8

• 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 AccelerationThe 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.

Newton’s Second LawThe acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass.

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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 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/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 .

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 FallCHECK 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 FallCHECK YOUR NEIGHBOR

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 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 Þ No accelerationÞ 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.

3

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 = maor

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:


• 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.


• 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 =Fm

a =Fm

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?




• (b) If the mass of the kleenex is 0.003 kg, what force does the boxer exert on it?




• (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 .

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.

Newton’s Third LawCHECK 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.

Newton’s Third LawCHECK YOUR NEIGHBOR

Vectors & ScalarsVector quantity• has and direction.• is represented by an arrow.

Example: velocity, force, acceleration

Scalar quantity• has .

Example: mass, volume, speed

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 other–use Theorem: R2 = V 2 + H 2.

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.

VectorsCHECK YOUR NEIGHBOR


• 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?


• 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/

PHY205H1F Summer Physics of Everyday Life Class 2 Force Causes Acceleration Friction Mass and Weight Mass Resists Acceleration Newton’s Second Law of Motion.

Documents

net force slide

force of friction

force of gravity

normal force

force of attraction

mass m6

smaller mass

single object slide