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Matter in Motion When an object changes position over time when compared with a reference point, the object is in motion. The rate at which an object moves is its speed. The speed of an object in a particular direction is the object’s velocity. ____ is the rate at which velocity changes. Acceleration The acceleration that occurs in circular motion is known as centripetal acceleration. A ____ is simply a push or a pull. force You have made a house of cards on top of your table. Suddenly, a gust of wind blows through an open window and your house of cards comes tumbling down. The wind applied _____ to your house of cards. an unbalanced force Two teams are having a tug of war contest. The flag in the middle is not moving because the two teams are balanced. ____ is a force that opposes motion between two surfaces that are touching. Friction When applying brakes on a car or a bicycle, you are using _____ friction to stop. sliding The treads on a car’s tires wear away due to ____ friction. rolling A sky diver’s acceleration slows down due to ____ friction. fluid A good rule to observe when around a swimming pool is “no running.” The deck around the pool becomes very slippery to walk on when wet due to fluid friction. One of the largest forces opposing the motion of a car is fluid friction. One Saturday, you go on a picnic. Although there is a slight breeze, the napkins stay on the tablecloth because of static friction. One way to increase friction is to use sand. ____ are substances that are applied to surfaces to reduce the friction between them. Lubricants ____ is a force of attraction between objects that is due to their masses. Gravity Objects of any size exert a ____ force. gravitational ____ is a measure of the gravitational force exerted on an object. Weight ____ is the amount of matter in an object, and its value does not change. Mass Mass is measured with a balance. Weight is measured with a spring scale.