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Newton’s Laws of Motion Force • A force is a push or a pull. • Net force refers to what you get when you consider the total effect of all the forces acting on an object. • If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. • If the forces are not equal and opposite, then the forces are unbalanced and the motion of the object changes. Acceleration • Acceleration is any change in speed and/or direction (a change in velocity) over some period of time. • A net force acting on an object causes it to accelerate. Object accelerates Inertia • It’s hard to change the motion of an object that has lots of inertia & it’s easy to change the motion of an object that has little inertia. • Mass can be defined as the measure of an object’s inertia. • The more mass an object has, the more inertia it has, and the harder it is to change its motion. Applying a larger net force to an object results in a larger acceleration. A soccer ball sitting at rest takes an unbalanced force of a kick to change its motion. leads to small force small acceleration leads to large force large acceleration If you apply equal net forces to two objects, the one with the smaller mass will accelerate more. leads to small mass large acceleration leads to large mass small acceleration If you want to cause two different objects to have the same acceleration, the object with the larger mass will require a larger force. can lead to small force same acceleration can lead to large force same acceleration Newton’s 2 nd net force acting on mass of acceleration of = an object object object F = ma Those three letters, plus the equals sign, are a statement of Newton’s 2nd Law of Motion. Newton’s 3rd Law of Motion Imagine you are in a football game. You line up facing your opponent, the ball’s snapped, and the two of you crash together. Who feels the force of the crash? Action & Reaction • Newton was the first to realize that all forces occur in pairs. – There is no such thing as an isolated force existing by itself! • Right now, gravity is pulling you down in your seat, but Newton’s 3rd Law says your seat is pushing up against you with equal force. • You are not moving because there is a balanced force acting on you. – Gravity is pulling down, your seat is pushing up. Newton’s 3rd Law • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • Simply put – – For every action (force) there is an equal, but opposite, reaction. Review • Newton’s 1st – – Inertia- an object will resist a change in its motion. (If not moving, it doesn’t want to move. If moving, it wants to keep moving in a straight line at a constant speed.) • Newton’s 2nd – – F = ma – force is equal to mass times acceleration • Newton’s 3rd – – For every action there is an equal and opposite reaction.
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