Energy and Forces in Motion Physical Science Chapter 11 and Section 1 of Chapter 13.

Energy and Forces in Motion

Physical Science

Chapter 11 and Section 1 of Chapter 13

What is Energy?

Energy is the ability to do work.

The Law of Conservation of Energy says:Energy cannot be created or destroyed, it can only

change form.

An example of energy conservation is when potential energy becomes kinetic energy (and sometimes back into potential energy!)

Potential Energy

Potential Energy is the energy an object has because of its position or shape. The object is ready to move, but not yet in motion.

Examples: A stretched rubber band, a wind-up toy, a kid at the top of a slide.

Potential Energy Due to Gravity

Gravitational Potential Energy occurs when gravity is the force used to create the potential energy. The more force you build UP against gravity, the greater the GPE.

Examples: the low dive vs. the high dive at a swimming pool. Throwing a penny off a chair vs. the Empire State Building!

Chemical Potential Energy

Remember that a chemical change occurs when 2 or more different elements are combined to create a new substance.

Chemical potential energy is greatest just before the actual chemical change. Example: The side of the marshmallow is completely brown just before it begins burning!

Kinetic Energy

Kinetic energy is the energy of motion. All moving objects have kinetic energy.

Kinetic energy depends on speed and mass. The faster the object is moving, the more kinetic energy. Kinetic energy increases as the mass of the object increases.

From Potential to Kinetic

An object’s greatest potential energy is just before it begins to move (winding up a toy).

Once the object begins moving, it has kinetic energy. The object has it’s greatest kinetic energy just before it begins to slow down or stop.

Everyday Use of Potential and Kinetic Energy

http://www.youtube.com/watch?v=Jnj8mc04r9E&safety_mode=true&persist_safety_mode=1&safe=active

Forces

A force is a push or pull

A net force is the combination of all forces involved.

For example, if you and a friend were each pushing a heavy box, then the force would be the combination of the force used by both you and your friend.

Balanced vs Unbalanced Forces

A balanced force means that the opposing forces are the same, so they cancel each other out. When you have a balanced force, you have a net force of 0.

An unbalanced force is when the forces are not equal (one is stronger). Net force = greater force – lesser force.

3 Natural Forces

On Earth the 3 natural forces are- Gravity- Friction- Air Resistance (friction of the air)

Centripetal Force

Centripetal force is the force needed to move an object in a circle (i.e., an ice skater)

Because an object (moving with centripetal force) is always changing direction, it is always accelerating.

Centripetal Force Demo

http://www.youtube.com/watch?v=56QCI4Ig4EY&safety_mode=true&persist_safety_mode=1&safe=active

Gravity and Motion

Aristotle believed that the rate an object falls to Earth depends on the object’s mass (the larger the mass, the faster it would fall).

Galileo believed that objects fall at the same rate because the rate of gravity is the same (9.8m/s/s). He was correct. It is hard to prove on Earth because of air resistance.

Acceleration at a Constant Rate

the rate of acceleration on Earth is 9.8 m/s2

When an object is dropped on Earth, it is falling at a rate of 9.8 m/s faster than the second before (no matter the size)

1 sec 9.8 m/s downward2 sec 19. 6 m/s downward3 sec 29.4 m/s downward

4 sec 39.2 m/s downward And so on…..

Slowing Down Acceleration

Air Resistance is fluid friction, which slows down the acceleration of gravity a force that acts against a falling object. The longer an object falls, the more force of air resistance is built up.

Slowing Down Acceleration

Terminal Velocity - When the force of an object falling and the force of the air resistance pushing up on that object are the same (net force of 0), then the object’s velocity towards the ground will stop accelerating (falls at a constant speed.)

Terminal Velocity

Terminal velocity is a good thing. If hailstones didn’t have terminal velocity, they would cause a great deal of harm and damage by the time they hit the ground. Because (most) hailstones are small, their terminal velocity is between 5 m and 40 m/s. If there was no terminal velocity, the hailstones would be hitting us at a velocity of up to 350 m/s!

Terminal Velocity

http://www.youtube.com/watch?v=kNED5RzqxOo&safety_mode=true&persist_safety_mode=1&safe=active

Creating Terminal Velocity and

Free FallWe use items to help us achieve terminal velocity faster by increasing the force of air resistance. An example would be a parachute.Skydivers say they are in free fall before the parachute opens, but that isn’t correct. Free fall means that there is no other force acting upon the falling object except gravity, and that means you can’t have free fall if there is any air resistance.

Free Fall in Space

There is no such thing as weightlessness, even in space. That’s because gravity always exists, and weight is dependent on gravity.

When you see astronauts “floating” in space, they still have weight, because there are still objects around you (planets, stars, the space craft). The amount of gravity is so slight, this is why you appear to float.

Free Fall from the Fringes of Space

http://www.youtube.com/watch?v=TamthU7wKJc

Orbiting Objects in Free Fall

When the shuttle is orbiting the Earth, it has two motions:

- It is traveling forward at a constant

speed, - It is being pulled by gravity downward towards

the Earth. This is called Satellite Motion.

The reason why astronauts don’t hit their heads on the ceiling of the shuttle during free fall is because the astronauts are also in free fall towards the Earth.

The Role of Gravity and Orbiting

All orbiting objects move in a circular path (the moon around Earth, the Earth around the Sun, etc.).

Any object moving in a circle is constantly changing direction. Any object in motion must be acted upon by an unbalanced force. The unbalanced force that causes objects in orbit to move in a circular motion is called centripetal force.

Projectile Motion andGravity

Projectile motion is the curved path an object follows when thrown or propelled near the surface of the Earth.

Projectile motion can be vertical or horizontal. When they are combined, they form a curved path. The horizontal velocity remains constant, but the vertical velocity slows down because of gravity.

Projectile Motion andGravity

Example: throw a baseball. When the ball goes forward, it starts to fall to Earth. This motion goes in a curved path along the surface of the Earth.

So, if you were to try and hit a bull's-eye with an arrow, where should you aim the arrow before letting go?

Projectile Motion Demo

http://www.youtube.com/watch?v=sFcsMDzW1Yg&safety_mode=true&persist_safety_mode=1&safe=active

FrictionFriction

All moving objects All moving objects encounter friction, encounter friction, an opposing force an opposing force to motionto motion

Without it most Without it most motion would be motion would be impossibleimpossible

4 types, static, 4 types, static, sliding, rolling, fluidsliding, rolling, fluid

Static FrictionStatic Friction11. Normal friction is the outward force from . Normal friction is the outward force from the surface. This creates static force keeps the surface. This creates static force keeps an object from moving (outward force from a an object from moving (outward force from a surface and bottom of object.)surface and bottom of object.)

-- It is the largest frictional forceIt is the largest frictional force

-- Always opposite direction of the Always opposite direction of the

applied force (pushing a cart, applied force (pushing a cart,

walking)walking)

2. Sliding Friction2. Sliding Friction

Once the object is in motion it Once the object is in motion it experiences sliding frictionexperiences sliding friction

Opposite direction from applied Opposite direction from applied forceforce

Less than static friction so less force is Less than static friction so less force is needed to keep it in motionneeded to keep it in motion

3. Rolling Friction3. Rolling FrictionAs something rolls, the As something rolls, the object and floor bend object and floor bend slightly. This bend slightly. This bend causes causes rolling frictionrolling friction

It is a much smaller It is a much smaller force than static frictionforce than static friction As much as 1000 As much as 1000

times smallertimes smaller

Allows you to move Allows you to move heavy objectsheavy objects

Ball bearings reduce Ball bearings reduce frictionfriction

4. Fluid Friction4. Fluid Friction

It opposes the motion It opposes the motion in the LIQUID or GASin the LIQUID or GAS

Like swimming, it is Like swimming, it is hard to movehard to move

If you are in the air, If you are in the air, fluid friction is called fluid friction is called

air resistanceair resistanceAt higher speeds it is At higher speeds it is very noticeable.very noticeable.

Isaac Newton

Remember Newton and the apple? What is a unit of force called?

Isaac Newton wrote a book about his observations on motion (Principia). He didn’t actually come up with the official laws, but his findings led to the laws we call Newton’s Laws of Motion.

Newton’s First Law of Motion

“An object at rest remains at rest and an object in motion remains in motion at a constant speed and in a straight line unless acted upon by an unbalanced force.”

An unbalanced force doesn’t just mean someone stopping the object. Gravity, air resistance and friction are all unbalanced forces.

Inertia and Mass

Inertia is a resistance to change in motion. If there were no gravity, air resistance or friction, then the object would continue to move at the same speed and in the same direction (Earth around the Sun, etc.).

The more mass of an object, the greater its inertia (try stopping a car in the same way you stop a bicycle!)

Newton’s 2nd Law of Motion

“The acceleration of an object depends on the mass of the object and the amount of force applied.”

Force = mass x acceleration (F = ma)

The amount of force depends on the amount of mass and the acceleration rate. If you increase either mass or acceleration, you increase the force. If you decrease one, you will decrease he force. You can make up the difference if you decrease one, but increase the other.

Momentum vs ForceMomentum is how difficult it is to stop a moving object (so the object is in motion):

P(momentum) = mass x velocity

Force is how much force the object would have at the moment it collides with another object:

F = mass x acceleration

Law of Conservation of Momentum

As with all conservation laws, momentum is not created or destroyed, but is transferred.When you bowl, the momentum of (the rolling bowling ball transfers to the pins.)This law illustrates Newton’s 3rd law. When a moving object strikes another object, the momentum of the moving object (action) transfers and causes the 2nd object to move (reaction).

Newton’s 3rd Law of Motion

“ Whenever one object exerts a force on second object, the second object exerts an equal and opposite force on the first.”

Forces work in pairs (action/reaction). Reaction is not always evident on falling objects (such as a bouncing ball). Action/Reaction occurs at the same time!

Normal Force

When you stand on the floor, the floor pushes back on your feet. The normal force is the outward force from the surface. The stronger the surface, the more normal force. (Ex: Which would have more normal force: a concrete wall or a wall of marshmallows?)