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Transcript
Newton’s Laws of Motion Newton’s 1st Law In an inertial reference frame, an object at rest will remain at rest and an object in motion will remain in motion with constant velocity unless acted on by an external force. Newton’s 1st Law Inertial Reference Frame - Non-accelerating - Classroom (mostly...) Expanded meaning of velocity Train Example Car on turn example Newton’s 1st Law Internal vs. External Forces Multiple External Forces Implications for problem solving - a = 0 m/s2 - v0 = vf - xf = x0 + v0t Newton’s 2nd Law The acceleration of an object is proportional to the vector sum of the external forces acting on the object and inversely proportional to its mass. Newton’s 2nd Law Newton’s 2nd Law Units of force Meaning of “vector sum” Interaction with first law Example 1 A 5 kg bowling ball is dropped out of a window that is 4 meters above the ground. a. What is the acceleration of the bowling ball? b. What force must have been applied to the bowling ball in order for it to have this acceleration? Weight vs. Mass Mass is the amount of matter in an object, measured in kilograms. Weight is the gravitational force on an object, measured in newtons. Example 2 A 75 kilogram person stands still on a floor. What force must the floor exert on the person? Normal Force Force of a surface on an object in contact with the surface. “Normal” = Perpendicular to surface Often (but not always!) equal to weight. Apparent Weight = Normal Force Example 3 A 75 kilogram person stands in an elevator. The elevator begins to move upward, with an acceleration of 2.0 m/s2. a. What must be the apparent weight of the person? b. When the elevator reaches its destination, it slows down with the same acceleration. What is the person’s apparent weight in this situation? Contact & Field Forces Contact Forces require contact. Examples: - Normal Force - Tension - Applied Forces Field forces do not require contact. Examples: - Gravity - Electric Force - Magnetic Force Four Fundamental Forces Image © Randall Munroe, used with permission. Link: https://xkcd.com/1489/ Normal Force is really electromagnetic force. Newton’s 3rd Law When an object exerts a force on a second object, the second object exerts a force on the first object with equal magnitude and opposite direction. Newton’s 3rd Law Intuitive Examples - Skating Rink - Rocket Propulsion Less Intuitive Examples - Gravity - Motorcycle vs. Truck - Jumping / Normal Force How do things change if it’s always balanced? Equal and Opposite Reactions always occur on different objects. Different masses yield different accelerations. Example 4 You throw a 5 kg bowling ball out of a second story physics classroom window. It accelerates downward at 9.8 m/s2. What is the upward acceleration of the Earth due to the bowling ball? The mass of the Earth is 5.972e24 kg.