Presentation Transcript

1. Key Concepts: Newton’s 1st law Newton’s 2nd law Weight Newton’s 3rd law

   Physics 221, January 26

2. Newton's first law, also called the law of inertia, defines a special class of reference frames, called inertial frames.  It states that, when viewed in an inertial reference frame, an object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless it is acted on by an external net force. Note: Newton's second and third laws are valid in all inertial reference frames . Newton's second lawstates that the acceleration of an object is directly proportional to the net forceacting on it, and inversely proportional to its mass, F = ma. Newton's third lawstates that for every force that an object exerts on a second object, there is a force equal in magnitude but opposite in direction exerted by the second object on the first object. Newton's third law is also called the law of action and reaction.
3. As I apply the brakes in my car, books on the passenger seat suddenly fly forward. That is most likely because the car is not an inertial reference frame. the seat supplies a forward push to make the books accelerate. there is a strong gravitationalfield generated by the brakes. there is a strongmagnetic field generated by the brakes. 30
4. Suppose an ice skater is moving on the surface of a frozen lake at constant velocity. What is true about the external (outside) forces acting on the skater? There are none. There could be some but they all cancel out. Gravity can be ignored. The all are perfectly horizontal. 30
5. Remember! constant velocity  no acceleration  no net force
6. To cause a 25 kg object to experience an acceleration of 2 m/s2, the net force that needs to be applied to the object is 5 N. 12.5 N. 50 N. 100 N. 30
7. Remember! a = ∆v/∆t acceleration = change in velocity/change in time A change in speed and/or a change in direction produces a change in velocity.
8. The positions of two blocks at successive 0.20-second time intervals are represented by the numbered squares in the figure below. The blocks are moving towards the right. Are the blocks accelerating? No Yes, both blocks are accelerating. Only the upper block is accelerating. Only the lower block is accelerating. 30
9. Extra Credit: The positions of two blocks at successive 0.20-second time intervals are represented by the numbered squares in the figure below. The blocks are moving towards the right. Do the blocks ever have the same speed? No Yes, at instant 2 Yes, at instant 5. Yes, at instants 2 and 5. Yes, at some time during the interval 3 to 4. 30
10. Weight The force of gravity ,Fg, acting on an object is called its weight. Near the surface of Earth Fg= mg, pointing downward. g = 9.8m/s2 pointing downward.
11. Suppose you are standing on a bathroom scale when you are flying in a jet airplane. For a moment the scale reads less than your actual weight. During that moment, the scale is exerting an upward force on you that is greater than your weight. equal than your weight. less than your weight. zero. 30
12. Suppose you are standing on a bathroom scale when you are flying in a jet airplane. For a moment the scale reads less than your actual weight. During that moment, you are accelerating upward. accelerating downward. not accelerating. accelerating horizontally. 30
13. Extra Credit: Suppose you are standing on a bathroom scale while riding in an elevator. For a moment the scale reads 155 lb = 689 N while your actual is 175 lb = 778 N. What is your (and the elevator’s) acceleration? (Let g = 10 m/s2.) -1.14 m/s2 -1.29 m/s2 -10 m/s2 -20 m/s2 -89 m/s2 30
14. Solution: Your actual weight: 778 N Your mass: 77.8 kg Let the upward direction be the positive direction. Net force acting on you: Ffloor – mg = 689 N – 778 N = -89 N a = Fnet/m = -89 N/77.8 kg = -1.14 m/s2
15. A woman exerts a constant horizontal force on a large box. As a result, the box moves across a horizontal floor at a constant speed v0. The constant horizontal force supplied by the woman has the same magnitude as the weight of the box. is greater than the weight of the box. has the same magnitude as the total force which resists the motion of the box. is greater than the total force which resists the motion of the box. is greater than either the weight of the box or the total force which resists its motion. 30
16. If the woman in the previous question doubles the constant horizontal force that she exerts on the box, and the force resisting the motion of the box does not change, the box then moves with constant speed, that is double the speed v0 in the previous question. with a constant speed that is greater than the speed v0 in the previous question, but not necessarily twice as great. for a while with a speed that is constant and greater than the speed v0 in the previous question, then with a speed that increases thereafter. for a while with increasing speed, then with decreasing speed. with constantly increasing speed. 30
17. If the women suddenly stops applying a horizontal force to the box, then the box will immediately come to a stop. continue moving at a constant speed for a while and then slow to a stop. immediately start slowing to a stop. continue at a constant speed. increase its speed for a while and then start slowing to a stop. constantly increasing speed. 30
18. A car has a maximum acceleration of 3.0 m/s2. What would be its maximum acceleration while towing a second car twice its mass? 2.5 m/s2 2 m/s2 1.5 m/s2 1 m/s2 0.5 m/s2 30
19. Extra Credit: If you push on a friend and he is accelerating away from you, how will the force you exert on your friend compare to the force your friend exerts on you? You push harder, since the friend is moving away from you. Your friend pushes harder, he has to push himself off. The forces are equal in magnitude. 30