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Motion and Forces Section 2-3 What is a force? Push or pull one body exerts on another Push a door Stretch a rubber band Shove a book across the table Some are obvious (car hits tree), others are not (floor pushing on feet). Effects of Forces on Objects Does the object move when a force is exerted on it? Balanced forces: equal in size but opposite in direction (tug of war) Unbalanced Forces: (Net forces) change the velocity (speed or direction) of the object Inertia and Mass Inertia: A moving object keeps moving and a stationary object remains stationary until a force acts on it. Compare the inertia of bowling ball with tennis ball More mass = more inertia Newton’s first law An object in motion remains in motion unless a net force acts on it. An object at rest remains at rest unless a net force acts on it. (INERTIA) Wheelbarrow with rake on top strikes a rock. What happens? Why do cars have seatbelts? Friction Force that opposes motion between two surfaces that are touching. Amount of friction depends on: The kinds of surfaces, and the force pressing the surfaces together.