Ch. 4 Laws of Motion I. First two laws of motion “If I have seen far, it is because I have stood on the shoulders of giants.” - Sir Isaac Newton (referring

Ch. 4Ch. 4Laws of MotionLaws of Motion

I. First two laws of motion“If I have seen far, it is because I have

stood on the shoulders of giants.”- Sir Isaac Newton (referring to Galileo)

ForcesForces

Forces can change motion. Start movement, stop movement, or change the direction of

movement Cause an object in motion to speed up or slow down

Newton’s First LawNewton’s First Law Experimentation led Galileo to the idea that

objects maintain their state of motion or rest.

Newton developed the idea further, in what is now known as Newton’s first law of motion:

B. Newton’s First LawB. Newton’s First Law

Newton’s First Law of Motion “Law of Inertia”

Inertia tendency of an object to resist any

change in its motion increases as mass increases

Newton’s First LawNewton’s First Law

Which object in each pair has more inertia?A baseball at rest or a tennis ball at

rest•Answer: the baseball

A tennis ball moving at 125 mi/h or a baseball at rest•Answer: the baseball

What do you think?What do you think?

• Imagine the following two situations:– Pushing a puck across an air hockey table– Pushing a book across a lab table

• What should your finger do in each case to maintain a constant speed for the object as it moves across the table or desk? (Choose from below.)– A quick push or force, then release the object– Maintain a constant force as you push the object– Increase or decrease the force as you push the object

• Explain your choice for the puck and the book.

• Imagine the following two situations:– Pushing a puck across an air hockey table– Pushing a book across a lab table

• What should your finger do in each case to maintain a constant speed for the object as it moves across the table or desk? (Choose from below.)– A quick push or force, then release the object– Maintain a constant force as you push the object– Increase or decrease the force as you push the object

• Explain your choice for the puck and the book.

A. Newton’s Second LawA. Newton’s Second Law

F = maF: force (N)m: mass (kg)a: accel (m/s2)

1 N = 1 kg ·m/s2

a Fm

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass

Newton’s Second LawNewton’s Second Law

Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N?

• Answer: the 7 N force

A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration?

• Answer: the 3 kg airplane

Classroom Practice ProblemClassroom Practice Problem

Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?

Answer: 2625 kg•m/s2 or 2625 N

PracticePractice

1. What is the force of an object with a mass of 12 kg and an acceleration of 4 m/s2?

2. Calculate the acceleration of a 25 kg object that is moved with a force of 300 N

3. What is the mass of an object that is accelerating at 15 m/s2 when a force of 3000 N is exerted?

GravityGravity

Chapter 4 Section 2

B. GravityB. Gravity

Gravity force of attraction between any two

objects in the universe

increases as...

•mass increases

•distance decreases


Who experiences more gravity - the astronaut or the politician?

less distance

more mass

Which exerts more gravity - the Earth or the moon?


Weight the force of gravity on an object

MASSalways the same

(kg)

WEIGHTdepends on gravity

(N)

W = mgW: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)


Would you weigh more on Earth or Jupiter?

greater gravity

greater weight

greater mass

Jupiter because...


Accel. due to gravity (g) In the absence of air

resistance, all falling objects have the same acceleration!

On Earth: g = 9.8 m/s2

mW

g

elephant

m

Wg

featherAnimation from “Multimedia Physics Studios.”


What happens when you include air resistance?

Bell WorkBell Work

The attractive force between objects is called?

Weight is measured in units called ____ while mass is in____?

An object transported from the surface of the Earth to the Moon would have a smaller weight or mass?

A. Projectile MotionA. Projectile Motion

Projectile any object thrown in

the air acted upon only by

gravity

follows a parabolic path called a trajectory

has horizontal and vertical velocitiesPROJECTILE MINI-LAB


Projectile Velocities

Horizontal and vertical velocities are independent of each other!


Horizontal Velocity depends on inertia remains constant

Vertical Velocity depends on gravity accelerates

downward at 9.8 m/s2

ConcepTestConcepTest A moving truck launches a ball vertically

(relative to the truck). If the truck maintains a constant horizontal velocity after the launch, where will the ball land (ignore air resistance)?

A) In front of the truck

B) Behind the truck

C) In the truck C) In the truck. The

horizontal velocity of the ball remains constant and is unaffected by its vertical motion. Animation from “Multimedia Physics Studios.”

B. Circular MotionB. Circular Motion

Centripetal Acceleration acceleration toward the center of a

circular path caused by centripetal force


On the ground... friction provides centripetal force


In orbit... gravity provides centripetal force

ROUND LAB

C. Free-FallC. Free-Fall

Free-Fall when an object is influenced only

by the force of gravity

Weightlessness sensation produced when an object

and its surroundings are in free-fall object is not weightless!

CUP DEMO

C. Free-FallC. Free-Fall

Weightlessness surroundings are falling at the same

rate so they don’t exert a force on the object

ConcepTest 1ConcepTest 1

TRUE or FALSE:

An astronaut on the Space Shuttle feels weightless because there is no gravity in space.

FALSE!

There is gravity which is causing the Shuttle to free-fall towards the Earth. She feels weightless because she’s free-falling at the same rate.


Describe the path of a marble as it leaves the spiral tube shown below.

It will travel in a straight line since the tube is no longer exerting a net force on it.

The 3The 3rdrd Law of Motion Law of Motion

Chapter 4 Section 3

C. Newton’s Third LawC. Newton’s Third Law

Newton’s Third Law of MotionWhen one object exerts a force

on a second object, the second object exerts an equal but opposite force on the first.

A. Newton’s Third LawA. Newton’s Third Law

Newton’s Third Law of MotionWhen one object exerts a force

on a second object, the second object exerts an equal but opposite force on the first.


Problem:

How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?

NO!!!

Aren’t these “balanced forces” resulting in no acceleration?


forces are equal and opposite but act on different objects

they are not “balanced forces” the movement of the horse

depends on the forces acting on the horse

Explanation:


Action-Reaction Pairs

The hammer exerts a force on the nail to the right.

The nail exerts an equal but opposite force on the hammer to the left.


Action-Reaction Pairs

The rocket exerts a downward force on the exhaust gases.

The gases exert an equal but opposite upward force on the rocket.

FG

FR


Action-Reaction PairsBoth objects accelerate.The amount of acceleration

depends on the mass of the object.

a Fm

Small mass more accelerationLarge mass less acceleration

JET CAR CHALLENGEJET CAR CHALLENGE

CHALLENGE:

Construct a car that will travel as far as possible (at least 3 meters) using only the following materials.scissorstape4 plastic lids2 skewers

2 straws1 balloon1 tray

How do each of Newton’s Laws apply?

B. MomentumB. Momentum

Momentum quantity of motion

p = mvp: momentum (kg ·m/s)m: mass (kg)v: velocity (m/s)m

p

v

B. MomentumB. MomentumFind the momentum of a bumper car if it

has a total mass of 280 kg and a velocity of 3.2 m/s.

GIVEN:

p = ?

m = 280 kg

v = 3.2 m/s

WORK:

p = mv

p = (280 kg)(3.2 m/s)

p = 896 kg·m/s

m

p

v

B. MomentumB. MomentumThe momentum of a second bumper car

is 675 kg·m/s. What is its velocity if its total mass is 300 kg?

GIVEN:

p = 675 kg·m/s

m = 300 kg

v = ?

WORK:

v = p ÷ m

v = (675 kg·m/s)÷(300 kg)

v = 2.25 m/s

m

p

v

C. Conservation of MomentumC. Conservation of Momentum

Law of Conservation of Momentum The total momentum in a group of

objects doesn’t change unless outside forces act on the objects.

pbefore = pafter


Elastic Collision KE is conserved

Inelastic Collision KE is not conserved


A 5-kg cart traveling at 1.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision.

BEFORECart 1:m = 5 kgv = 4.2 m/s

Cart 2 :m = 2 kgv = 0 m/s

AFTERCart 1 + 2:m = 7 kgv = ?

p = 21 kg·m/s

p = 0

pbefore = 21 kg·m/s pafter = 21 kg·m/s

m

p

vv = p ÷ mv = (21 kg·m/s) ÷ (7 kg)v = 3 m/s


A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon?

BEFOREClown:m = 50 kgv = 0 m/s

Cannon:m = 250 kgv = 0 m/s

AFTERClown:m = 50 kgv = 20 m/s

Cannon:m = 250 kgv = ? m/s

p = 0

p = 0

pbefore = 0

p = 1000 kg·m/s

pafter = 0

p = -1000 kg·m/s


So…now we can solve for velocity.

GIVEN:

p = -1000 kg·m/s

m = 250 kg

v = ?

WORK:

v = p ÷ m

v = (-1000 kg·m/s)÷(250 kg)

v = - 4 m/s (4 m/s backwards)

m

p

v

Do NowDo Now

TRUE or FALSE?

The object shown in the diagram must be at rest since there is no net force acting on it.

FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.

Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.


You are a passenger in a car and not wearing your seat belt.

Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door.

Which is the correct analysis of the situation? ...


1. Before and after the collision, there is a rightward force pushing you into the door.

2. Starting at the time of collision, the door exerts a leftward force on you.

3. both of the above

4. neither of the above

2. Starting at the time of collision, the door exerts a leftward force on you.

Documents

Ch. 4 Laws of Motion I. First two laws of motion “If I have seen far, it is because I have stood on the shoulders of giants.” - Sir Isaac Newton (referring