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Today Re-Teaching Newtons First and Second Laws

of Motion

Balanced vs. Unbalanced Forces

- Balanced Forces

- Unbalanced Forces

- Cancel each other out
- NO net force acting on object (Fnet 0 N)

- Do not cancel out
- YES net force acting on object (Fnet gt 0 N)

Check For Understanding

- Which statement below is true?
- When two unbalanced forces are applied to an

object, the net force is 0 N. - When two balanced forces are applied to an

object, the net force is 0 N. - Two unbalanced forces cancel each other out.
- When balanced forces act on an object, net force

is negative.

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What is the net force acting on this box?

Fnet12 N 8 N 4 N to the right

Practice Finding the Net Force

Check For Understanding

You must have a magnitude and a direction for

Fnet.

Balanced or unbalanced? Fnet

Balanced

0 N

Check For Understanding

Balanced or unbalanced? Fnet

Unbalanced

5 N to the left

Check For Understanding

Balanced or unbalanced? Fnet

Balanced

0 N

Check For Understanding

Balanced or unbalanced? Fnet

Unbalanced

15 N up

Answer Balanced and Unbalanced Forces questions

on your sheet. 5 minutes!

Check For Understanding

- When balanced forces act on an object, what is

the net force acting on the object?

Check For Understanding

- If the net force on an object is greater than

zero, what can you conclude about the forces

acting on the object? - It is being acted on by unbalanced forces.
- It is being acted on by balanced forces.
- It is being acted on by no forces at all.
- It is moving at a constant speed.

Newtons First Law

- Objects at rest remain at rest unless acted upon

by a net force. - Objects in motion remain in motion at a constant

speed unless acted upon by a net force. - Law of Inertia

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FR

F

f

F FR f

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Newtons Second Law

- When a net force acts on an object, the object

accelerates in the direction of the force. - Acceleration is directly proportional to net

force and inversely proportional to mass.

Forces are either balanced, or unbalanced

When forces are equal, no acceleration occurs.

Forces are either balanced, or unbalanced

But when they are unbalanced, thats when objects

accelerate

Forces are either balanced, or unbalanced

But when they are unbalanced, thats when objects

accelerate

Forces are either balanced, or unbalanced

But when they are unbalanced, thats when objects

accelerate

Forces are either balanced, or unbalanced

But when they are unbalanced, thats when objects

accelerate

Forces dont cause motion, forces cause

ACCELERATIONS!

Accelerations cause MOTION!

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FR

F

f

F FR f

FR

f

F

The car will accelerate if F gt FR f

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Objects mass (kg)

Objects acceleration (m/s2)

Net force acting on object (N)

F ma

Check For Understanding

- A car is accelerating at 5 m/s2. The car has a

mass of 1000 kg. What is the force that is

pushing the car?

F

1000 kg

5 m/s2

Check For Understanding

- Kristopher pushes with 150 N of force on a box

with a mass of 50 kg. At what rate does the box

acclerate?

150 N

a

50 kg

Check For Understanding

- Tylesia pushes Mr. D with a force of 240 N. Mr. D

accelerates forward at a rate of 3 m/s2. What is

Mr. Ds mass?

240 N

m

3 m/s2

Relationships in F ma

- ? net force (Fnet) ? ? acceleration (a)
- ? mass (m) ? ? acceleration (a)
- ? mass (m) ? ? net force (Fnet)

? net force (Fnet) ? ? acceleration (a)

How fast will this rock accelerate?

100 N

a

50 kg

How fast will this rock accelerate?

200 N

a

50 kg

? net force (Fnet) ? ? acceleration (a)

? mass (m) ? ? acceleration (a)

? mass (m) ? ? acceleration (a)

20 N

a

5 kg

? mass (m) ? ? acceleration (a)

20 N

a

20 kg

? mass (m) ? ? acceleration (a)

? mass (m) ? ? acceleration (a)

- Kristopher pushes with 150 N of force on a box

with a mass of 50 kg. At what rate does the box

acclerate?

150 N

a

50 kg

? mass (m) ? ? acceleration (a)

- Kristopher pushes with 150 N of force on a box

with a mass of 100 kg. At what rate does the box

acclerate?

150 N

a

100 kg

Who will be harder to move?

How much force is needed to accelerate this

wrestler at a rate of 2 m/s2?

F

200 kg

2 m/s2

How much force is needed to accelerate this

wrestler at a rate of 2 m/s2?

F

20 kg

2 m/s2

? mass (m) ? ? net force (Fnet)

Check For Understanding

- A car is accelerating at 5 m/s2. The car has a

mass of 2000 kg. What is the force that is

pushing the car?

F

2000 kg

5 m/s2

Check For Understanding

- A car is accelerating at 5 m/s2. The car has a

mass of 5000 kg. What is the force that has to be

pushing the car?

F

5000 kg

5 m/s2

Balanced vs. Unbalanced Forces

- Balanced Forces

- Unbalanced Forces

- Cancel each other out
- NO net force acting on object (Fnet 0 N)
- NO acceleration (a 0 m/s2)
- Object keeps same speed

- Do not cancel out
- YES net force acting on object (Fnet gt 0 N)
- YES acceleration (a gt 0 m/s2 or a lt 0 m/s2)
- Object changes speed

Balanced Forces Unbalanced Forces

Fnet is

Acceleration is

Object will

Described by which of Newtons Laws?

Balanced Forces Unbalanced Forces

Fnet is Fnet 0 N Fnet gt 0 N

Acceleration is

Object will

Described by which of Newtons Laws?

Balanced Forces Unbalanced Forces

Fnet is Fnet 0 N Fnet gt 0 N

Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2

Object will

Described by which of Newtons Laws?

Balanced Forces Unbalanced Forces

Fnet is Fnet 0 N Fnet gt 0 N

Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2

Object will remain at rest or maintain constant speed accelerate (change speed)

Described by which of Newtons Laws?

Balanced Forces Unbalanced Forces

Fnet is Fnet 0 N Fnet gt 0 N

Acceleration is a 0 m/s2 a gt 0 m/s2 or a lt 0 m/s2

Object will remain at rest or maintain constant speed accelerate (change speed)

Described by which of Newtons Laws? First Law Second Law

Challenge Question

- A car moves along a road at a high speed. The

driver pushes the gas pedal down farther,

generating a forward force of 3000 N on the car.

A strong gust of wind hits at the exact moment

the driver pushes the gas pedal, exerting a

backward force of 2500 N on the car. The road

also applies a backward force of 500 N due to

friction. - What is the acceleration of the car at the exact

moment the gust of wind hits the car? - 0 m/s2
- 500 m/s2
- 2500 m/s2
- 3000 m/s2