1. What does Net Force mean?

2. What is Newton’s 1st Law?

The Questions

Force changes motion

A force is a push or pull

BUT

IT IS THE “NET FORCE” THAT WE CARE ABOUT!!

Net ForceNet Force is the sum of

the forces acting on an object in EACH direction (x or y).

What is the Net Force acting on the golf ball??

x-Direction:

(+20N) + (-10N) = +10N

y-Direction:

(+5N) + (-5N) = 0N

-10N+20N

-5N +5N

Gravity

(+300N) + (+300N) = +600N (+Normal Up) + (-Gravity down) = 0N

Is this object in Equilibrium?

Newton’s 1st Law: The Law of Inertia

Official Version:

“An object at rest will stay at rest and an object in motion will continue in motion at a CONSTANT VELOCITY unless acted on by a NON-0 Net force.”

Newton’s 1st Law: The Law of Inertia

Newton’s 1st Law in simple English:

Objects that are not moving will not move unless pushed by a non-0 NET force. Objects that are moving will keep moving at the same velocity unless a non-0 NET force pushes on them.

If the Net Force on an object is 0N, an object at rest (velocity=0m/s) will stay at rest (velocity=0m/s).

If an object is acted upon by an unbalanced force (Net Force≠0), its motion (VELOCITY) will change.

Newton’s 1st Law: The Law of Inertia

Newton’s First Law – Simple English:

Objects that are not moving will not move unless pushed by a not-0 Net Force. Objects that are moving will keep moving at the same velocity (a = 0) unless a not-0 Net Force pushes on them.

If the Net Force on an object is 0N, an object at rest (v = 0m/s) will stay at rest (v=0m/s).

If a non-0 Net Force pushes on an object its velocity will change (acceleration will not be 0).

Inertia is a property of matter!

Who said this?? Billy Nye

Inertia is the property of an object that resists changes in motion (VELOCITY).

Inertia = mass

Inertia is the same thing as an objects mass.

Inertia is measured with kilograms.

Which ball has more inertia?

Newton’s 1st Law: The Law of Inertia

1. What does Net Force mean?

a) The sum of all of the forces acting on an object.

2. What is Newton’s 1st Law?

a) Known as the “Law of Inertia”

b) An object’s velocity will change if the Net Force is not 0.

The Answers

1. What is Newton’s 2nd Law?

2. How do we use Newton’s 2nd

Law?

3. What is weight?

The Questions

Newton’s 2nd Law: a = Fnet/m

Newton’s 1st Law says that an object’s velocity will only change if there is a non-0Net Force acting on an object.

Newton’s 2nd Law: the amount of velocity change (acceleration) an object will have depends on the Net Force acting on the object divided by the mass of the object.

a = Fnet / m

Another way to write this is: Fnet = m a

a=acceleration, Fnet = Net Force, m = mass of object

There are 3 main ideas related to Newton’s 2nd Law:

1. Non-Zero Net Forces lead to changes in the velocity of an object (acceleration).

2. A larger net force on an object causes a larger acceleration of the object.

3. Mass effects acceleration:

i. Bigger mass means smaller acceleration.

ii. Smaller mass means larger acceleration.

Newton’s 2nd Law: a = Fnet/m

The Unit of Force: Newton (N)

The S.I. unit of force is the Newton.

A newton is the amount of force needed to accelerate a 1kg object by 1m/s2.

Newton’s 2nd Law: a = Fnet/m

The 2nd Law says that a = F/m

The greater the Force, the higher the acceleration.

Lower Force, lower acceleration.

The greater the mass, the smaller the acceleration from the same net force.

Newton’s 2nd Law: a = Fnet/m

Keep the following important ideas in mind:

1. A non-0 Net Force is what causes acceleration.

2. If there is no acceleration, the Net Force must be zero.

3. If there is acceleration, there must be a Net Force.

4. The unit of Newtons is based on kilograms, meters, and seconds.

Newton’s 2nd Law: a = Fnet/m

Solving ProblemsTry to figure out these problems!!

C Level: What acceleration does a 400kg object have if a net force of 1400N is applied to that object?

Solution: 3.5m/s2

Solving ProblemsTry to figure out these problems!!

B level: What Net Force is needed to give a 850kg object an acceleration of 4m/s2?

Solution: 3400N

Solving ProblemsTry to figure out these problems!!

A Level: A 5000N car starts from rest and is moving at a velocity of 20m/s after 5 seconds. The engine produces a thrust of 4500N and there is 1000N of friction acting on the car. What is the mass of the car?

Solution: 875kg

1. What is Newton’s 2nd Law?

a = F/m

2. How do we use Newton’s 2nd

Law?

Identify the Net Force.

Plug into the equation.

The Answers

Newton’s First Law is known as:

Law

of A

ctio

n ...

Law

of C

onserv

...

Law

of I

nertia

Law

of C

onserv

...

Law

of t

he Ben...

0% 0% 0%0%

100%1. Law of Action and

Reaction

2. Law of Conserved Mass

3. Law of Inertia

4. Law of Conserved Energy

5. Law of the Benevolent Dictator

Inertia is measured in units of

Mete

rs

Lite

rs

Kilo

gram

s

Seco

nds

N/k

g

m/s

2

5%0% 0%

5%0%

89%

1. Meters

2. Liters

3. Kilograms

4. Seconds

5. N/kg

6. m/s2

What has more Inertia?

Base

ball

Skyb

all

Bow

ling B

all

Beach

Ball

Golf

Ball

Pin

g Pong B

all

6%0% 0%0%0%

94%

1. Baseball

2. Skyball

3. Bowling Ball

4. Beach Ball

5. Golf Ball

6. Ping Pong Ball

A Net Force of +100N acts on an object. That object will

Stay i

n motio

n...

Stay a

t rest

.

Have

its v

eloc..

.

17%

83%

0%

1. Stay in motion.

2. Stay at rest.

3. Have its velocity changed.

What is Newton’s 3rd Law?

Forces always occur in what?

The Questions

Newton’s 3rd Law says that for every action force there must be an equal and opposite reaction force.

The reaction force must be equal in strength and opposite in direction.

There can never be a single force, alone!!

Forces always occur in pairs.

Newton’s 3rd Law

Newton’s Third Law (action-reaction) applies when a force is pushed (or pulled) on any object, like a door.

_______ exerted a force on the door.

And

The door exerted a force on _______.

Newton’s 3rd Law

When figuring out action and reaction forces it is helpful to draw a picture.

The action force (thing that is doing something) is the ________________.

The reaction force (force that happens because of the thing that is doing something) is the _______________.

Newton’s 3rd Law

http://science.discovery.com/interactives/literacy/newton/newton.html

Newton’s 3rd Law

Solving Problems

A woman with a weight of 500N is sitting on a chair.

Describe one action-reaction pair of forces in this situation.

Give the magnitude of each force and what object the force acts on.

What is Newton’s 3rd Law?

For every action force there is an equal and opposite reaction force.

Simple English: Forces always

occur in pairs and act on different objects.

Forces occur in pairs.

The Answers