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The Laws of Motion Physics 2053 Lecture Notes The Laws of Motion

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4-01 Force 4-02 Newton’s First Law 4-03 Newton’s Second Law 4-04 Newton’s Third Law 4-05 Applications of Newton’s Laws 4-06 Forces of Friction Topics The Laws of Motion

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Types Range Gravitational Unlimited Electromagnetic Unlimited Weak Nuclear 10 12 m Strong Nuclear 10 15 m Size Force The Laws of Motion

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A force is a push or pull. An object at rest needs a force to get it moving; a moving object needs a force to change its velocity. The magnitude of a force can be measured using a spring scale. Newton’s First Law The Laws of Motion

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Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it. If no external force acts Newton’s First Law Newton’s first law is often called the law of inertia. The Laws of Motion

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When you sit on a chair, the resultant force on you is A) zero. B) up. C) down. D) depending on your weight. Newton’s First Law The Laws of Motion

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Units of Force System Mass Acceleration Force SI kg m/s 2 N = kg m/s 2 British slug ft/s 2 lb = slug ft/s 2 Newton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass. Force is a vector, so F = ma is true along each coordinate axis. Newton’s Second Law The Laws of Motion

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A man stands on a scale inside a stationary elevator. N mg Forces acting on the man Reading on scale Newton’s Second Law The Laws of Motion

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N mg v When Moving Upward With Constant Velocity Forces acting on the man Reading on scale Newton’s Second Law The Laws of Motion

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N mg a When Moving Upward With Constant Acceleration Forces acting on the man Reading on scale Newton’s Second Law The Laws of Motion

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N mg a When Moving Downward With Constant Acceleration Forces acting on the man Reading on scale Newton’s Second Law The Laws of Motion

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A constant net force acts on an object. Describe the motion of the object. A) constant acceleration B) constant speed C) constant velocity D) increasing acceleration Newton’s Second Law The Laws of Motion

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F v o = 0 m t = 5 s v = ? F = 20 N m = 5 kg Newton’s Second Law A constant force F acts on a block of mass m. which is initially at rest. Find the velocity of the block after time t. The Laws of Motion

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What average force is required to stop an 1100 kg car in 8.0 s if the car is travelling at 95 km/h? F Newton’s 2 nd Law Newton’s Second Law (Problem) The Laws of Motion

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A net force F accelerates a mass m with an acceleration a. If the same net force is applied to mass 2m, then the acceleration will be A) 4a. B) 2a. C) a/2 D) a/4 Newton’s Second Law The Laws of Motion

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The cable supporting a 2,125 kg elevator has a maximum strength of 21,750 N. What maximum upward acceleration can it give the elevator without breaking? Newton’s Second Law (Problem) T max mgm Newton’s 2 nd Law a The Laws of Motion

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How much tension must a rope withstand if it is used to accelerate a 1200 kg car vertically upward at 0.80 m/s 2. a Newton’s 2 nd Law Newton’s Second Law (Problem) The Laws of Motion

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Gravitational Force: Newton’s Second Law Gravitational Force is the mutual force of attraction between any two objects in the Universe. R m M F F Universal Gravitational Constant The Laws of Motion

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The gravitational force between two objects is proportional to A) the distance between the two objects. B) the square of the distance between the two objects. C) the product of each objects mass. D) the square of the product of each objects mass. Newton’s Second Law The Laws of Motion

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Two objects attract each other gravitationally. If the distance between their centers is cut in half, the gravitational force A) is cut to one fourth. B) is cut in half. C) doubles. D) quadruples Newton’s Second Law The Laws of Motion

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Two objects, with masses m 1 and m 2, are originally a distance r apart. The magnitude of the gravitational force between them is F. The masses are changed to 2m 1 and 2m 2, and the distance is changed to 4r. What is the magnitude of the new gravitational force? A) F/16 B) F/4 C) 16F D) 4F Newton’s Second Law The Laws of Motion

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“g” in terms of G R m M F F g Newton’s Second Law The Laws of Motion

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Mass is the measure of inertia of an object. In the SI system, mass is measured in kilograms. Gravitational mass m g m g = m i Inertial mass m i Newton’s Second Law Mass is not weight: Mass is a property of an object. Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same. The Laws of Motion

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m Weight = mg Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight is: Newton’s Second Law The Laws of Motion

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Mass and weight A) both measure the same thing. B) are exactly equal. C) are two different quantities. D) are both measured in kilograms. Newton’s Second Law The Laws of Motion

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A stone is thrown straight up. At the top of its path, the net force acting on it is A) equal to its weight. B) greater than its weight. C) greater than zero, but less than its weight. D) instantaneously equal to zero. Newton’s Second Law The Laws of Motion

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F1F1 F2F2 Action/Reaction Forces Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. Newton’s Third Law The Laws of Motion

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N mg An object at rest must have no net force on it. If it is sitting on a table, the object exerts a downward force mg on the surface of the table. The surface of the table exerts an upward force on the block, called the normal force. It is exactly as large as needed to balance the force from the object. m Newton’s Third Law The Laws of Motion

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If an upward force F is applied to the block, the magnitude of the normal force is N mg F Newton’s Third Law The Laws of Motion

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N mg F If a downward force F is applied to the block, the magnitude of the normal force is m Newton’s Third Law The Laws of Motion

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Action-reaction forces A) sometimes act on the same object. B) always act on the same object. C) may be at right angles. D) always act on different objects. Newton’s Third Law The Laws of Motion

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A 40,000 kg truck collides with a 1500 lb car and causes a lot of damage to the car. A) the force on the truck is greater then the force on the car. B) the force on the truck is equal to the force on the car. C) the force on the truck is smaller than the force on the car. D) the truck did not slow down during the collision. Newton’s Third Law The Laws of Motion

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A golf club hits a golf ball with a force of 2,400 N. The force the golf ball exerts on the club is A) slightly less than 2400 N. B) exactly 2400 N. C) slightly more than 2400 N. D) close to 0 N. Newton’s Third Law The Laws of Motion

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F v o = 20 m/s m v = 0 F = 10 N m = 5 kg xx Applications of Newton’s Laws A block of mass m moving with a speed v o is brought to rest by a constant force F. Find the distance the block moves. Newton’s 2 nd Law The Laws of Motion

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m1m1 m2m2 N m1gm1g T T m2gm2g Forces on m 1 Forces on m 2 Find the acceleration of the two block system Solving Problems with Newton’s Laws a The Laws of Motion

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m1m1 m2m2 T T m1gm1g m2gm2g Mass 1 Mass 2 Find the acceleration of the two block system Solving Problems with Newton’s Laws The Laws of Motion

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N mg cos( ) mg sin( ) y x mg Solving Problems with Newton’s Laws The Laws of Motion

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A pair of fuzzy dice is hanging by a string from your rear-view mirror. While you are accelerating from a stoplight to 24 m/s in 6.0 s, what angle does the string make with the vertical? Problem a The acceleration of the dice Newton’s 2 nd Law The Laws of Motion

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T1T1 T2T2 F m 3m2m Three mass system - find acceleration Solving Problems with Newton’s Laws The Laws of Motion

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T1T1 T2T2 F m 3m2m Three mass system - find T 2 Solving Problems with Newton’s Laws The Laws of Motion

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T1T1 T2T2 F m 3m2m Three mass system - find T 1 Solving Problems with Newton’s Laws The Laws of Motion

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T1T1 T2T2 F m 3m2m Three mass system - find T 1 Or Solving Problems with Newton’s Laws The Laws of Motion

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You are standing in a moving bus, facing forward, and you suddenly fall forward. You can imply from this that the bus's A) velocity decreased. B) velocity increased. C) speed remained the same, but it's turning to the right. D) speed remained the same, but it's turning to the left. Forces of Friction The Laws of Motion

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Friction: Force of Static Friction F fsfs m mg N Forces of Friction The Laws of Motion

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Friction: Force of Kinetic Friction F fkfk m mg v N Forces of Friction The Laws of Motion

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Steel on steel Aluminum on steel Copper on steel Rubber on concrete Wood on wood Glass on glass Waxed wood on wet snow Waxed wood on dry snow Metal on metal (lubricated) Ice on ice Teflon on Teflon Coefficients of Friction Forces of Friction The Laws of Motion

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Suppose that you are standing on a train accelerating at 2.0 m/s 2. What minimum coefficient of static friction must exist between your feet and the floor if you are not to slide? N mg a Newton’s 2 nd Law Frictional force Forces of Friction (Problem) The Laws of Motion

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The force that keeps you from sliding on an icy sidewalk is A) weight. B) kinetic friction. C) static friction. D) normal force. Forces of Friction The Laws of Motion

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m mg N F f N F The normal force x y f kk Pulling a block Pulling a block with constant speed Forces of Friction The Laws of Motion

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m mg N F f f N F The frictional force x y kk Pulling a block with constant speed Forces of Friction The Laws of Motion

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m mg N F f f N F The coefficient of friction x y kk Pulling a block with constant speed Forces of Friction The Laws of Motion

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The coefficient of static friction between hard rubber and normal street pavement is about On how steep a hill (maximum angle) can you leave a car parked? Forces of Friction (Problem) The Laws of Motion

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Drag-race tires in contact with an asphalt surface have a very high coefficient of static friction. Assuming a constant acceleration and no slipping of tires, estimate the coefficient of static friction needed for a drag racer to cover 1.0 km in 12 s, starting from rest. Newton’s 2 nd Law Forces of Friction (Problem) The Laws of Motion

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A brick and a feather fall to the earth at their respective terminal velocities. Which object experiences the greater force of air friction? A) the feather B) the brick C) Neither, both experience the same amount of air friction. D) It cannot be determined because there is not enough information given. Forces of Friction The Laws of Motion

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Which of Newton's laws best explains why motorists should buckle-up? A) the first law B) the second law C) the third law D) the law of gravitation Newton’s First Law The Laws of Motion

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