Energy and Simple and Compound Machines

Objective 4.07

Determine how people use simple machines to solve problems.

• Since a body can do work for different reasons it can have energy for different reasons. It is useful to classify energy according to the reasons that bodies have it.

Kinetic Energy

• Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or horizontal motion - has kinetic energy. There are many forms of kinetic energy - vibrational (the energy due to vibrational motion), rotational (the energy due to rotational motion), and translational (the energy due to motion from one location to another).

Potential Energy• An object can store energy as the result of its position.

For example, the heavy ball of a demolition machine is storing energy when it is held at an elevated position. This stored energy of position is referred to as potential energy. Similarly, a drawn bow is able to store energy as the result of its position. When assuming its usual position (i.e., when not drawn), there is no energy stored in the bow. Yet when its position is altered from its usual equilibrium position, the bow is able to store energy by virtue of its position. This stored energy of position is referred to as potential energy. Potential energy is the stored energy of position possessed by an object.

• A cyclist with Kinetic energy is moving up the hill and transforming the  energy into Potential energy.

Simple and Compound Machines

• The forces at work on simple machines can be balanced or unbalanced. Remember, when the forces are balanced, you can also say that the forces are in equilibrium.

• Look at the pulley on the left. When the masses are equal, then the forces on those masses are balanced. The masses do not move, and the system is in equilibrium.

• Now look at the pulley on the right. The force of 2kg mass is twice that of the force on the 1kg mass. The forces are not equal, so the 2kg mass drops and the 1kg mass rises. The masses will move faster and faster, so the system is not in equilibrium.

Fixed Pulleys

• A fixed pulley is a pulley in which the wheel does not move. Fixed pulleys change the direction of the effort force. It does not increase the size of the effort force. The effort force is equal to the resistance force in a fixed pulley; therefore, the mechanical advantage (MA) of a fixed pulley is equal to 1.

Movable Pulleys

• May be single or combination • A pulley that moves with the load ( both the load and the

pulley move) • The load moves in the same direction as the applied

force • The movable pulley allows the effort to be less than the

weight of the load • It takes less force to raise an object than if you used only

your hands • The amount of force required depends on the number of

supporting ropes • The greater the number of pulleys and supporting ropes,

the smaller the force required.

Compound Machines• A compound machine is a machine that is

made up of two or more simple machines. The simple machines in a compound machine work together to make a task easier. An example of a compound machine is a wheelbarrow, which is made of two simple machines: a lever and wheel and axle.

• The large machines we are all familiar with are made up of many simple and compound machines. Such things would be a car or an airplane. With all the simple machines working together, cars and airplanes can overcome the forces of gravity and moving air. The car overcomes gravity when it is going up hill, and an airplane overcomes gravity when it begins a flight. The engines of the car and airplane provide the energy the simple and compound machines need to move the car and airplane.

Review

• Simple machines are used to reduce the amount of force it takes to do work.

• Compound machines are made of two or more simple machines.