2When one object applies a force to a second, we call this force the action. The force the second object returns to the first is called the reaction.Newton realized that while an action and reaction act in opposite directions, they have the same strength. These ideas are summarized in Newton’s third law of motion: For every action, there is an equal but opposite reaction.
3When one object applies a force to another, both objects feel force When one object applies a force to another, both objects feel force. The second object feels the action force, while the first object feels the reaction force.
4Scientifically, work is done when a force moves an object Scientifically, work is done when a force moves an object. Force is any push or pull exerted on matter.Forces give energy to an object. And energy can be defined as the ability to do work. Whenever you push, lift, or throw an object, you are doing work.
5An additional requirement for work is that the force applied must overcome a resistance force, such as gravity or friction. Whenever two surfaces touch, a force called friction is created. Friction acts in an opposite direction to the motion of the moving object.Friction will cause a moving object to slow down and eventually come to a complete stop. Gravity is the force that pulls an object downward. Therefore, for work to be done, it must move an object in such a way that it resists the pull of gravity.
6Another requirement for work to be done is that the object must be moved in the same direction as the force applied to the object. If the applied force is upward, then the direction of movement would also need to be upward in order for work to be done.Normally, holding a heavy box for an hour would be considered hard work. But according to the scientific definition of work, no work is being done because the box is not moving. Carrying the heavy box while walking is not scientific work, either, because the direction of the force is up, but the box is moving forward.
7Simple MachinesSimple machines are devices with few moving parts that make work easier to do. They lower the force needed to move and lift heavy objects and loads. There are six types of simple machines—levers, wheels and axles, pulleys, inclined planes, wedges, and screws.
81. LeversA lever is a bar that pivots around a fixed point called a fulcrum. The lever turns up or down around the fulcrum.The part of the lever you apply an input effort force to is the effort arm. The resistance arm of the lever produces an output force to lift the load. If the effort arm is longer than the resistance arm, the lever changes a small input force into a larger output force.
9Three Types of Levers1. First-class levers have the fulcrum placed between the load and the effort, as in the seesaw, crowbar, and balance scale. These levers change the direction of the force.
102. Second-class levers have the load between the effort and the fulcrum. A wheelbarrow is a second-class lever. The wheel’s axle is the fulcrum, the handles take the effort, and the load is placed between them. The effort always travels a greater distance and is less than the load.
113. Third-class levers have the effort placed between the load and the fulcrum. The effort always travels a shorter distance and must be greater than the load. A hammer acts as a third-class lever when it is used to drive in a nail: the fulcrum is the wrist, the effort is applied through the hand, and the load is the resistance of the wood.
122. Wheels and AxlesA wheel with a rod, called an axle, through its center lifts or moves loads. A small amount of input force on the wheel becomes a large force on the axle. Doorknobs and screwdrivers use wheels and axles in this way.
133. PulleysA pulley makes moving objects easier by changing the direction in which you have to apply force. A pulley is made from a rope that fits into the groove of a wheel. As the wheel turns, the rope moves. If you hook an object to the rope, you can move the object up by pulling down on the rope. In effect, you are cutting your work in half. Pulley systems can combine fixed and movable pulleys.
144. Inclined PlaneThe inclined plane, or ramp, is the simplest machine of all. The longer the incline, the easier work is. With an inclined plane, one must push the weight a further distance but can use less force to do so. Therefore, the inclined plane decreases the size of the input force.
155. WedgesA wedge is an inclined plane that is used as a tool for cutting or separating things. Some wedges consist of two inclined planes put together back to back so their slanted sides face outward. The thinner the wedge, the greater is the output force. Needles, ax blades, knife blades, and log splitters are examples of wedges.
166. ScrewsA screw is an inclined plane wrapped in a spiral around a cylinder or cone. Screws and bolts are used as fasteners for wood or metal. Drills, corkscrews, and jar lids are also examples of screws.