Ch 4 Forces And Newton s Laws What is a Force A Push or a Pull Two types of forces Contact and Non contact Non Contact Action at a distant force Is the Force a Vector or a Scalar Units and Dimensions Mass Measure of Mass is inertia How does it differ from Weight Higher mass harder to change the motion i e higher inertia 1 Newton s First Law Of Motion An object continues in a state of rest or in a state of motion at a constant speed along a straight line unless compelled to change that state by a net force What is the net force If so why will a ball rolling on a flat surface stop 2 Vector sum of all the forces acting on the object is called the net force i e both magnitude and direction needs to be considered What is the net force on the car 3 Inertial Reference Frame Newton s laws are valid in an inertial frame i e the acceleration of the frame should be zero Constant Velocity Ideally we do not have such a frame but earth is a very good approximation 4 Newton s Second Law First law no net force no change in velocity Second law What happens if there is an external force net Acceleration of the object Net Force 1 Acceleration inversely proportional to mass m 5 F a m or F ma Only two factors determine the acceleration Force and Mass Force Units Newton N kg m s2 What is 1 Newton 6 Newton s Second Law Of Motion When a net external force F acts on an object of mass m the acceleration a results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass The direction of the acceleration is the same as the direction of the net force F a m or F ma SI Unit of Force kg m s2 newton N 7 Units for Mass Acceleration Force System Mass Acceleration Force SI kilogram kg meter second2 m s2 newton N CGS gram g centimeter sescond2 dyne dyn cm s2 BE slug sl foot second2 ft s2 pound lb 8 Example 1 Pushing A Car Frictional force 560 N Push 275 N 395 N 670 N Net force 670 N 560 N N 110 N F ma 110 N 1850 kg a a 110 N 1850 kg 0 059 m s2 what is the direction 9 Forces In 2 D Fx max Fy may Two forces are applied onto an object 1 12N towards North and 2 5N towards East what is the direction of the motion of the object Resultant force 12N 12 2 5 2 13 tan 12 5 2 4 5N tan 1 12 5 67 38 0 10 1300 kg raft Example 2 P Force by the man In 65 seconds where will the boat be if v0x 0 15 m s a A Force of wind ax ay d 11 Resultant Force And Acceleration Force X component Y Component P 17 N 0N A 15cos67 N 15sin67 N Resultant F x 17 15 cos 67 23N F 23N 0 018 m s 2 m 1300 kg x ax F 15 sin 67 0 14 N F 14 N a 0 011 m s y y y m 1300 kg 2 12 Displacement Of The Boat In 65 sec For x v0x 0 15 m s ax 0 018 m s2 t 65 sec x x v0x t axt2 0 15 65 1 2 0 018 65 2 48 m For y v0y 0 ay 0 011 m s2 t 65 sec y v0yt ay t2 0 65 0 011 65 2 23 m 13 Which of these will cause the acceleration to be doubled a All of the forces acting on the object doubles b The net force acting on the object doubles c Both the net force and the mass doubles d The mass of the object is reduced by a factor of two a b d 14 Newton s Third Law of Motion Whenever one body exerts a force on a second body the second body will exert an oppositely directed force of equal magnitude on the body Action reaction law Acting on which object 15 MS 11000 kg mA 92 kg Force by Astronaut on Space Ship P Force on Astronaut by Space ship P What will be the acceleration of the two objects For the astronaut If P 36 N mA 92 36 N 92 aA i e aA 0 39 m s2 36 N 11000 kg as i e as 0 0033 m s2 16 Types Of Forces Fundamental Forces Gravitational force Strong Nuclear Electroweak Electromagnetic force All other forces are Non Fundamental can be explained by a fundamental force Maxwell Unification theory 17 Gravitational Force Newton s law of Universal gravitation m1 m2 F G 2 r r center to center G 6 673 x 10 11 N m2 kg2 Always attractive Planetary motion Satellites 18 Weight Of An Object Mass m on the surface of the earth W mg Me Radius of earth Re Distance Re approx m1 m2 F G 2 r W G mM e 2 mg Re So Me g G 2 Re 6 67 10 11 N m 2 kg 2 5 98 10 24 kg 6 38 10 6 m 2 9 799 m s2 19 Hubble Telescope The mass of Hubble telescope is 11600 kg Re 6 38 106 m Me 5 98 1024 kg i What is the weight when resting on earth surface M e M h 6 67 10 11 5 98 10 24 11600 W G 2 r 6 38 106 2 1 14 105 N ii and in orbit 598 km above r Re 598 km R 6 38 106m 598 103m 6 98 106m 20 Check Your Understanding 2 m2 m1 and the net gravitational force acting on the third object is zero Which of the drawings correctly represents the locations of the object d 21 The Normal Force Newton s 3rd Law Table exert a force on the block FN Net Force on the block FN W Normal Force The force component that a surface exerts on the object Normal to the contact surface How can the table exert a force Consider the springs in a mattress 22 What happens if I push or pull on the block What happens to the normal force if the rope has a pull 15N 23 Balancing Act 24 Apparent Weight How do you feel when the elevator suddenly starts going up FN Feel heavy FN W ma FN Apparent weight W True weight 25 Elevator Accelerating Up With a Applying 2nd law vertically upward FN W ma FN W ma Apparent weight is larger than the true weight W mg g 9 8 m s2 26 Elevator Accelerating Down With a FN mg ma if downward a negative FN mg ma What if a g FN 0 27 Static And Kinetic Friction Two surfaces touching each other Contact surface is not smooth In addition to the normal force …