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FACTS For FORCES DEFINITIONS Review and learn the definitions of the special words used in physics. By Rebecca K. Fraker, 2010 Force A push or a pull on an object. Mass The amount of matter that makes up an object Under normal circumstances the amount of matter in an object does not change. Gravity The pull of matter on matter. The greater the mass of an object the greater its gravitational pull on another object. • Small mass = small pull • Large mass = large pull Gravity continued… • Small mass = small pull • Large mass = large pull You and everything that has mass (your dog, your pencil, your lunch) has gravity. Weight Weight is a measure of the force of gravitational attraction. MASS does not change BUT…WEIGHT does. Weight depends on the gravitational attraction. Weight continued… Weight depends on the masses of the objects, and the distance between the centers of the masses. Systems of Measurement Mass and Weight The Standard International System, known as SI, is based on the metric system. The English System is based on pounds and ounces. Measuring Systems • SI System: Mass is measured by kilograms, Weight by Newtons. • English System: Mass is measured by slugs, Weight by pounds. Remember! Weight is the measure of the gravitational pull on the mass of an object. Shout it out! Can you answer this question? Here on Earth, your mass is 32 kilograms. What is your mass on the planet Quagga? ANSWER: Your mass remains the same on Quagga as on Earth, because mass is the amount of matter in an object and that does not change when you travel. Shout it out! On the planet Zero you weigh 100 pounds. The gravity on planet Zero is higher than that of Earth. What happens to your weight when you go to Earth? You will weigh less on Earth because the pull of gravity is less. But your mass remains the same. Friction The rubbing of one object or surface against another. A force on objects or substances in contact with each other that resists motion of the objects or substances relative to each other.. Static Friction • Static friction arises between two objects that are not in motion with respect to each other: for example between a cement block and a wooden floor. • To counterbalance forces that would move the objects, it increases up to a certain maximum level of force. At this point the objects will begin moving. • It is measured as the maximum force the bodies will sustain before motion occurs. Kinetic friction Kinetic friction arises between bodies that are in motion with respect to each other: for example the force that works against sliding a brick along a wooden floor. Between two hard surfaces, the kinetic friction is usually somewhat lower than the static friction, meaning that more force is required to set the objects in motion than to keep them in motion. Displacement A rabbit is being chased by a fox. It is running for its hole. But it does not run in a straight line. Instead it zigzags. It runs ten feet to the right, then ten feet to the left and then 20 feet straight and another 5 to the right. Altogether it ran 45 feet to get to safety. Displacement But if the rabbit had run straight, it would have only had to run 28 feet. That measurement is the displacement. Momentum The quantity of motion in a moving body is its momentum. It is always proportioned to the quantity of matter multiplied into the velocity. Another word for it is impetus. Inertia It takes an extra strong push to start an object moving because of inertia. It wants to stay where it is. It takes an extra strong force to STOP something once it is moving because of inertia. It wants to keep on moving. Inertia This is stated in Newton’s First Law of Motion: A body at rest tends to stay at rest, and a body in motion tends to stay in motion unless acted upon by an outside force. Acceleration You are driving in at 30 kilometers per hour. Then you speed up and go 50 kilometers per hour. This upward change in speed is called acceleration. Happy Learning!