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.
Chapter 11 and Section 1 of Chapter 13
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!)
Examples: the low dive vs. the high dive at a swimming pool. Throwing a penny off a chair vs. the Empire State Building!
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.
On Earth the 3 natural forces are
- The mass of the objects, and
- The distance between them.
In other words, the bigger the objects, the closer they are, the more gravitational pull!
2 sec 19. 6 m/s downward
3 sec 29.4 m/s downward
4 sec 39.2 m/s downward
And so on…..
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.
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 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!
We 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.
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.
- It is traveling forward at a constant
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.
So, if you were to try and hit a bull's-eye with an arrow, where should you aim the arrow before letting go?
All moving objects encounter friction, an opposing force to motion
Without it most motion would be impossible
4 types, static, sliding, rolling, fluid
1. Normal friction is the outward force from the surface. This creates static force that keeps an object from moving (outward force from a surface and bottom of object.)
- It is the largest frictional force
- Always opposite direction of the
applied force (pushing a cart,
Once the object is in motion it experiences sliding friction
Opposite direction from applied force
Less than static friction so less force is needed to keep it in motion
It opposes the motion in the LIQUID or GAS
Like swimming, it is hard to move
If you are in the air, fluid friction is called air resistance
At higher speeds it is very noticeable.
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.
“An object at restremains at rest and an object in motionremains 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 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!)
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 the force. You can make up the difference if you decrease one, but increase the other.
P(momentum) = mass x velocity
F = mass x acceleration
“ 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!