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Physics

Physics. Chapter 4 Sections 3-5. Lab Results. Part A kept the total mass constant and increased the force (mass on the string) every time Acceleration increased as force increased Acceleration α force Also note that acceleration is in the same direction as the force. Lab Results cont’d.

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Physics

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  1. Physics Chapter 4 Sections 3-5

  2. Lab Results • Part A kept the total mass constant and increased the force (mass on the string) every time • Acceleration increased as force increased • Acceleration α force • Also note that acceleration is in the same direction as the force

  3. Lab Results cont’d. • Part B kept the force constant (mass on the string) and decreased the total mass moving each time • Acceleration increased as mass decreased • Acceleration α 1/m

  4. Lab Results cont’d. • Acceleration of an object depends on both the force applied and the mass of the object • Since a α F and a α 1/m then a α F/m or a = F/m

  5. Newton’s Second Law • The acceleration of an object is proportional to the net force, in the same direction as the net force, and inversely proportional to the mass a = ΣF / m Where [a] = m/s2 [F] = N [m] = kg = ft/s2 = lb = slug

  6. Sample Problem A 10 kg block is placed on a table. A force of 6 N is applied horizontally to the block. What is the block’s acceleration?

  7. Sample Problem What horizontal force must be applied to a 15 kg body in order to give it an acceleration of 5 m/s2?

  8. Newton’s Third Law • If there are two objects, A and B, the force A exerts on B will be equal and opposite the force B exerts on A • For every action there is an equal and opposite reaction • Forces always come in pairs and occur at the same time

  9. Question A person leans against a wall. They apply 20 N of force to the wall and the wall applies 20 N of force to the person. Do these forces cancel each other out since they are equal and opposite? Why or why not?

  10. Newton’s 3rd Law cont’d. Examples – When you walk, you push on the floor so the floor pushes on you. When you swim, you push on the water so the water pushes on you. When a book sits on a table, the book pushes down on the table and the table pushes up on the book.

  11. Nonequilibrium • Object(s) is accelerating • a is not equal to zero • All the steps are the same except Σx = max and Σy = may

  12. Sample Problem A girl scout paddling a canoe pushes the water back with her paddle, exerting a backward force of 155 N on the water. Find the acceleration of the girl and the canoe if their combined mass is 90 kg.

  13. Sample Problem Three children fight over a small stuffed animal of mass 0.2 kg, pulling with the forces indicated. Find the instantaneous acceleration of the toy. 30 N 40 N 40 N

  14. Sample Problem San Francisco is famous for its hills. A car (mass = 1350 kg) is being projected up one of these hills by a drive force that is parallel to the hill and has a magnitude of 7200 N. The hill makes a 25 degree angle with the horizontal. Find the acceleration of the car and the force exerted on the car by the hill.

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