CHAPTER 12

Free-Write

Can an object move without anyone or anything acting on it?

What is a Force?

Any action that can change the state of motion of an object

Every change that you observe or feel is caused by a force

How can Forces Be Explained?

Newton’s Laws:

An object at rest remains

at rest and an object in motion maintains its velocity unless it experiences a net force > zero

Also known as the

How can Forces Be Explained?

Newton’s Laws:

The unbalanced

force acting on an object equals the object’s mass times its acceleration

𝐹𝑛𝑒𝑡 = force (𝑁𝑒𝑤𝑡𝑜𝑛𝑠)

𝑚 = mass (usually in 𝑘𝑔)

𝑎 = acceleration (𝑚/𝑠²)

How can Forces Be Explained?

Newton’s Laws:

For

every action there is an equal and opposite reaction force

What are Some Types of Forces That Affect Objects?

Friction

Gravity

Just to Name a Couple… There Are More!

Motion & Force Review

Fundamental Forces- natural forces such as electromagnetic, strong nuclear, weak nuclear and gravity

Vary in strength

Force of Friction-opposes motion

Can be harmful or helpful

Motion & Forces Review

Balanced Force-net force is zero, no motion changeof the object

Unbalanced Force-net force is greater than zero, the object is accelerating in the direction of the greater force

Newton’s Laws Free-Write

If you are sitting on your bike on level ground, what do you need to do to make it move?

If you are coasting on your bike, what do you need to do to make your bike stop?

Do people who race bicycles use heavy bikes or light bikes? Why?

Forces-

Free-Write

Why do you think astronauts float in space when they leave the Earth’s atmosphere?

Weight & Mass

Weight – the force on an object due to gravity

𝑊𝑒𝑖𝑔ℎ𝑡 = 𝑚𝑎𝑠𝑠 ∙ 𝑓𝑟𝑒𝑒−𝑓𝑎𝑙𝑙 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛

Earth’s gravity = 9.8 Τ𝑚𝑠2

Weight is measured in Newtons (because it is a force)

(𝑔𝑟𝑎𝑣𝑖𝑡𝑦)

Weight is Different from Mass

Mass – the amount of matter in an object

Weight – the gravitational force that an object experiences because of itsmass

Mass is always the same, but weight is not always the same

Weight influences shape (Example: skeletons of land animals)

Law of Universal Gravitation

All objects in the universe attract each other through the force of gravity

Gravitational force increases as one or both of the objects’ masses increase

Gravitational force can also decrease as the distance between the two massesincreases

Our Solar System and Weight

Although your mass will stay the same on a different planet, your weight will not

Depending on the mass of the planet and its diameter, you may have a different weight because of its gravitational force

Earth’s gravity = 9.8 Τ𝑚𝑠2

Mars’s gravity = 3.7 Τ𝑚𝑠2

Moon’s gravity = 1.7 Τ𝑚𝑠2

Free Fall

Free fall – the motion of a body when only the force of gravity is acting on the body

Without air resistance, all objects falling near the Earth’s surface accelerate at the same rate regardless of their mass

Acceleration is constant (law of universal gravitation)

Astronauts in orbit are in free-fall

(appears like weightlessness)

Air Resistance

A falling object stops accelerating when the force of air resistance becomes equal to the gravitational force of an object

Air resistance acts in the opposite direction to the weight of an object

Terminal velocity – When air resistance and weight are equal to one another, an object reaches its maximum velocity

Projectile Motion

A curved path that an object follows when thrown, launched, or otherwise projected near the surface of the Earth

Has a horizontal component

(force of throw)

Has a vertical component

(force of gravity)

Orbiting the Earth is projectile motion

Momentum

The product of an object’s mass and its velocity

𝑝 = 𝑚 ∙ 𝑣

All moving objects have momentum

Forces cause momentum to change

Can have a direction (because of velocity)

Increase as mass & velocity increase

Conservation of Momentum

The total amount of momentum in an isolated system is conserved

Totals are the same before and after a collision

Can be transferred to other objects