Physics 4A Lecture 5: Jan. 22, 2015 Sunil Sinha UCSD Physics Quick Ques@on: •  What happens when you jump in the air? Which is the most accurate statement? •  A) It is the upward force exerted by the ground that pushes you up, but this force can never exceed your weight •  B) You are able to spring up because the earth exerts a force on you that is stronger than the downward force you exert on earth •  C) Since the ground is sta@onary, it cannot exert the upward force necessary to propel you into the air. Instead, it is the internal forces of your muscles ac@ng on your body itself that propels your body into the air. •  D) When you push down on the earth with a greater force than your weight, the earth will push back on you with the same magnitude and thus propel you into the air. Pop quiz: •  A steel rod A of mass 2 kg is rigidly aTached to a metal block B of mass 5 kg and is pulling it up so it accelerates ver@cally at 2 m/s2. The force in the link between A and B is: A)  7g N B)  7g + 14 N C)  5g + 10 N D)  5g N Quick ques@on •  Which one of these statements is NOT true: •  (A) It easier to keep a block on a horizontal surface moving than to get it to move. •  (B) A heavier object will start to slide down a slippery slope before a light one •  (C) Fric@on will exactly cancel a horizontal force trying to slide a body on a horizontal surface un@l it exceeds µW (W= weight of body, µ =coeff of sta@c fric@on) Ques@on 1 •  Of everything covered in this course so far, I have the most trouble with: •  (A) Vectors •  (B) projec@les •  (C) Ropes, pulleys and sliding blocks •  (D) Circular mo@on Ques@on 2 •  I find that the material in this course is being covered; •  (A) Too fast •  (B) Too slowly •  (C) At about the right speed.