Vocabulary: A force is an action that either pushes or pulls on an object. Force can be the result of contact. You apply force to a book when you push it across a desk. Forces can also result when objects are not in contact with each other. Objects with more mass need more force to move them. Objects with less mass need less force to move them. (S4P3b) Position is the place where an object is in relation to other things. The position where you are reading this might be a desk in the classroom. If you move to the library, your position may change to a chair in the library. (S4P3b) An object that changes position over time has motion. An object that does not change position over time is at rest. People can describe the motion of an object. They can describe how the position changes. They can describe how fast the position changes. They can describe the direction the object moves when its position changes. (S4P3b) Speed is a rate that describes the distance an object can or will move over a set amount of time. A car can move on a road at 50 miles per hour. At that speed the car will move 50 miles in one hour and 100 miles in two hours. Speed does not describe the direction an object moves. (S4P3b) Gravity is a force that causes all objects around Earth to fall toward the ground. (S4P3d) Simple machines are devices that change the direction or amount of force to do work. Simple machines do not do the work. They make the work easier to do. There are six simple machines. (S4P3a) The lever is a simple machine made up of a straight beam and a fulcrum, a point that the rod pivots on. Levers change the amount of force required to move an object. A seesaw is an example of a lever. (S4P3a) An inclined plane is a simple machine that uses a flat surface to help raise or lower a load. Inclined planes spread the amount of force required to lift a load over a distance. A wheelchair ramp is an example of an inclined plane.(S4P3a) A wedge is a simple machine made up of one or two inclined planes. Wedges change the direction of force and are usually used to push two objects apart. A wedge can be driven into a log. When the downward force of a sledgehammer is applied, the wedge will change the direction of the force outward, causing the log to split. The cutting edges of knives, axes, and chisels are wedges, as are ice picks, pins, and needles. (S4P3a) A screw is a simple machine that can be thought of as an inclined plane wrapped around an axle. Because of this, the force required to do something is spread out over a longer distance. (S4P3a) The wheel and axle is a simple machine made up of a wheel and an attached axle. The wheel and axle transfers the force from the wheel to the axle. (S4P3a) A pulley is a simple machine made up of a rope or a chain guided around a wheel or an axle. Pulleys change the direction of a force. In some situations when more than one pulley is used, it is also possible to change the amount of force required to move an object. (S4P3a) Important Tips: The more mass an object has, the more force will be needed to move it. The more speed an object has, the more force will be required to slow it, stop it, or change its direction.