Potential and Kinetic Energy Potential energy (PE) is stored energy. Kinetic energy (KE) is energy in the form of motion. The total amount of potential energy and kinetic energy in a system is known as mechanical energy.

Increasing the height of an object increases potential energy Increasing the speed of an object increases kinetic energy

If the total mechanical energy of the system is 100 J:

Where does the pendulum have 100 J of kinetic energy?

Where does the pendulum have 100 J of potential energy?

Where does the pendulum have 50 J of potential energy? Changes in Force and MotionAll changes in motion require the input of a force. The simplest way to think about motion is a change in position. Forces are measured in units called newtons (N).

When two equal forces are acting on an object from opposite directions, the forces are balanced. The ball will not move.

An object in motion will continue moving—at the same speed and in the same direction— until an unbalanced force acts upon it. This means if you kick a soccer ball straight across the ground, it should continue moving in a straight line, at the same speed, forever! However, you know this doesn't happen. The ball slows and eventually stops moving. A force must act on it as it moves.

Speed describes rate of movement, or how fast something moves. Anything that moves across a distance over time has speed. It is

calculated using the formula distance ÷ time: A straight line like this means an object is moving at a constant

speed. Object A is not speeding up, slowing down, or changing direction. You can calculate an object's speed by finding the slope of the line. The steeper the slope, the faster the speed.

A line that changes directions like this means an object is changing speed or direction as it moves. Object B begins by moving 25 meters at a constant speed for 6 seconds. (The red line's slope is less steep than the green line's, so Object B is moving more slowly than Object A.) Between 6 seconds and 14 seconds the line is horizontal. This means Object B has stopped and is at rest. After 14 seconds, Object B moves 25 meters at a constant speed back toward its starting position.

A line that curves up like this means that an object is getting faster. Every second, Object C is moving a greater distance from its starting point.

When forces are unbalanced the ball will move. If the force is continually applied, the ball will move left at increasing speed. If the force is applied once, the ball will stay at constant speed.

100 J PE0 J KE

100 J PE0 J KE50 J PE

50 J KE50 J PE50 J KE

0 J PE100 J KE

Applied force is 75 N to the left.

Applied force is 0 N so there is no motion.

Remember:

Potential Energy + Kinetic Energy = Total Energy

Each position of the pendulum adds up to the total energy of 100 J.

Remember: An upward line means that you are increasing your distance (farther away from start). Downward means your distance is decreasing (getting closer to start). The graphs must always move right because time never stops.

Inclined Planes and PulleysAn inclined plane is a sloping surface like a ramp or wedge. A pulley—usually called a pulley system—consists of one or more wheels and a rope.

Inclined planes decrease the amount of force needed to move something. However, you have to apply the force over a greater distance. The longer the inclined plane is, the less force you use. All simple machines involve a trade-off between force and distance.

To make a pulley system, a rope is looped around a wheel and an object (called a load) is attached to the rope. Pulling on the free

end of the rope lifts the load. Adding wheels to a pulley system can make it easier to lift a load. Pulley systems can reduce the amount

of force needed to lift a load. They can change the direction of force applied to a load. They can even do both at the same time.

Pulley systems make it easier to lift a load, because each part of the system supports part of the load's weight.

Because System 3 contains the most pulleys, you use the least force to lift the load. However, you also lift the load over the greatest

distance.

Pulley 1 - Fixed pulley, Pulley 2 – Moveable Pulley,

Pulley 3 - Compound pulley

In Pulley 1, there is no change in effort force. It only

changes the direction of the force.

In Pulley 2, the force is reduced by half.

In Pulley 3, the force is reduced by half and the direction of

the force is changed.

Remember, pulling down allows you to use your own body

weight to pull the rope through the pulley.

3

The left truck ramp will require less force over a greater distance. If I raise the truck height, I will increase the force needed to move a load.