Presentation on theme: "Example inclined plane #2 The box from a previous problem will slide down the incline as soon as the rope is cut. Over the length of 0.50m, the 102-kg."— Presentation transcript:
Example inclined plane #2 The box from a previous problem will slide down the incline as soon as the rope is cut. Over the length of 0.50m, the 102-kg box reaches a speed of 1.2 m/s. a.Find the acceleration of the box. b.Which forces are important for this motion? c.Determine the force of friction on the box 20º
Reading Assignment: (4.3) According to the book, which of the following are explanations for mass? A.The quantity of matter in a body B.Related to the number of protons, neutrons and electrons in a body C.Quantitative measure of inertia D.The ratio of the magnitude of the net force to the magnitude of the acceleration
Falling object Determine the force of gravity on a 0.5-kg mass. m = 0.5 kg Acceleration?
Weight = force of gravity Weight F g Mass m Unit: Newtons Has direction toward center of planet Weight is a force vector Unit: kilograms Has no direction Measures inertia “amount of matter” Mass is a scalar
Experiment Pulling slowly Pulling fast Will the bottom or the top string break?
Experiment T1T1 T2T2 FgFg Slow pull: T 1 ruptures fast pull: T 2 ruptures
A hollow tube forms three-quarters of a circle. It is lying flat on a table. A ball is shot through the tube at high speed. As the ball emerges from the other end, does it follow path A, path B, or path C? Explain. A B C
Reading Assignment: (4.5) A.An action-reaction pair is a pair of forces of same magnitude and opposite direction. B.F(A on B) = -F (B on A) C.Weight and normal force are an action/reaction pair. D.Action and reaction pairs occur only for contact forces. Which of the following states Newton’s third law?
Newton’s third law For every force acting on an object there is an equal and opposite reaction force on the object causing the force. http://www.jrcompton.com/photos/The_Birds/J/Mar-07.html
Action-reaction pairs Act on different objects Generally along the same line Force on nail by hammer F NH Force on hammer by nail F HN
Windshield A fly of mass 1 g is hitting the windshield of a car of 1000 kg. a)Which of the two experiences the greater force? b)Which of the two experiences the greater acceleration?
Action-reaction pairs FgFg FNFN What are their reaction forces? Force of gravity on block by planet Reaction: Force of gravity on planet by block Force of contact on block by table Reaction: Force of contact on table by block
Reaction force of weight A mass of 1 kg experiences a force of gravity equal to 10 N, causing it to accelerate downward at 10 m/s 2 in the absence of other forces. Analyze the influence of the reaction force on the Earth.
Multiple objects in contact Two blocks with different masses are lying on the table so that they are in contact. If one block is pushed against the other, find the acceleration of the system. Analyze the contact force between them. M F m
Someone of mass 100 kg is standing on top of a vertical cliff with only an old rope that he knows will support no more than 500 N. His plan is to slide down the rope using friction to keep from falling freely. At what minimum rate can he accelerate down the rope in order not to break it?
A horse pulling a wagon exerts a force on the wagon. The wagon thus exerts a reaction force of equal magnitude and opposite direction on the horse. So, if they start to pull on each other while at rest, they never should go anywhere. How can we get out of this dilemma?