Lecture 7: Forces & The Laws of MotionPowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Lecture 7: Forces & The Laws of MotionQuestions of Yesterday1) A ball is thrown vertically upwards in the air by a passenger on a train moving with a constant velocity. To a stationary observer outside the train, is the velocity of the ball at the top of its trajectorya) greater than b) Less thanc) Equal tothe velocity observed by the passenger?2) The hang-time of a basketball player who jumps a vertical distance of 2 ft is about 2/3 second. What will the hang-time be if the player reaches the same height while jumping 4 ft horizontally?a) less than 2/3 sb) greater than 2/3 sc) equal to 2/3 sForcesContact ForcesForce applied to an object by direct contactA physical push or pullEx. Stretching a spring,Hitting a baseballTug-of-warField ForcesForce applied to an object without direct contactApplied over a range of spaceEx. Gravitational force,Electrical force,Magnetic forceNewton’s First LawWhat happens when… you push your physics book sitting on your desk?You roll a basketball across the floor?You hit a hockey puck in an ice rink?Why?An object at rest will remain at rest unless acted on by an external force.Also, an object in motion will remain in its original state of motion unless acted on by an external force.Newton’s First LawWhat happens when… you push your physics book sitting on your desk?You roll a basketball across the floor?You hit a hockey puck in an ice rink?Why?An object moves with a velocity that is constant in magnitude and direction unless acted on by a nonzero net force.Net force = sum of all external forces acting on an objectNewton’s First LawWhy is Newton’s first law true?What makes an object resist changing its state of motion?An object moves with a velocity that is constant in magnitude and direction unless acted on by a nonzero net force.INERTIAtendency of an object to continue in its original state of motionNewton’s First LawAn object moves with a velocity that is constant in magnitude and direction unless acted on by a nonzero net force.Which ball do you want to have a game of toss with? Why?MASSmeasure of object’s resistance to changes in its motion due to an external forceNewton’s Second LawThe acceleration a of an object is directly proportional to the net force acting on it and inversely proportional to its massThe acceleration of an object is inversely proportional to its massThe acceleration of an object is directly proportional to the net force acting on itNewton’s Second LawThe acceleration a of an object is directly proportional to the net force acting on it and inversely proportional to its massa = F/mF = maSI Unit of force = Newton1 N = 1 kg*m/s2F and a arevectors!Vector equation!Fx = maxFy = mayFz = mazNewton’s Second LawI apply a force F1 to my physics book to push it across the desk with a velocity of 10 m/s. If instead I want to push the book at a velocity of 20 m/s is the force I need to applygreater than, less than, or equal to F1?F = maIf an object is accelerating does that mean that there has to be a net force on it?If an object is not accelerating does that mean that no forces are acting on it?Gravitational ForceForce of attraction between any two objects in the Universe. Gravitational force causes….Objects in free fall near the Earth’s surface to accelerate towards the Earth the moon to orbit the earth & the planets to orbit the sunAn astronaut to be able to jump higher on the Moon than on EarthGravitational ForceNewton’s Law of Gravitational Force m1,m2= mass of objects attracting each otherr = distance between the objectsUniversal gravitational constant = G = 6.67*10-11 N*m2/kg2Fg = Gm1m2 r2WeightWhat is the magnitude of the gravitational force acting on object with mass m near Earth’s surface?ME = 5.98*1024RE = 6.38*106 Fg = GMEm RE2 Fg = m*(9.8m/s2) = mgWeightMagnitude of gravitational force acting on an object near Earth’s surfacew = mgNewton’s Third LawForces in nature always exist in pairs!If object 1 and object 2 interact, the force F12 exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force F21 exerted by object 2 on object 1For every force there is an equal and opposite reaction forceAction-reaction force pairs always act on different objects!F12 = -F21Newton’s Third LawWhich vehicle experiences the greater acceleration?F12 = -F21If a Mack Truck and Honda Civic have a head-on collision, upon which vehicle is the impact force greater?Newton’s Third LawWhat force(s) acts on the object during free fall?F12 = -F21What exerts this force on the object?Is there a nonzero net force on a freely falling object? Why?What is the corresponding reaction force?What does it act on?Newton’s Third LawWhy doesn’t the Earth accelerate towards us as we accelerate towards it?F12 = -F21m2a2 = -m1a1aE = -(m0a0 )/ME m0 << MEaE ≈ 0 m0a0 = -MEaEFE0 = -F0ENormal ForceIs there a nonzero net force acting on the block?What about the gravitational force?FgNNormal ForceFgNN’Normal Force force exerted by surface on object in contact with the surfaceAre N and Fg action-reaction pairs? Why?Is the magnitude of N greater than, less than, or equal to Fg?Fg’Free Body DiagramsFgNN’Diagram of a single object with all the forces acting ON itDO NOT INCLUDE:-Forces that the object exerts on other objects or the Earth-Other objects that the object is in contact withFg’Free Body DiagramsTennis ball undergoing projectile motionFgFree Body DiagramsTennis ball while a tennis player is serving it. The player swings the racket at a 30o angle with the horizontal when in contact with the ball.FgIs the ball accelerating?How do you know?In what direction is it accelerating?FA30oFree Body DiagramsA block sliding down a frictionless inclined plane.FgIn what direction?NIs the block accelerating?Free Body DiagramsA block sliding down a frictionless inclined plane.FgN+ y+ x- x- yChoose coordinate system to be in direction of incline plane! FgyFgxFgx = FgsinFgy = FgcosQuestions of the Day1) You must apply a force F1 to begin pushing a crate from rest across the floor, you must apply a force F2 to keep the crate moving at a constant velocity once its in motion. Which statement is true?a) F1 = F2 b) F1 > F2c) F1 <