1. Objectives  Describe how the weight and the mass of an object are related.  Differentiate between actual weight and apparent weight.

2. USING NEWTON’S SECOND LAW Weight – force of gravity on an object. It is measured in Newtons like all other forces. It is a Vector quantity. It uses the symbol “W” or F w or F g and you find the weight using the equation: W = mg or F w = mg or F g = mg (new book uses F g ) The weight of any object is proportional to its mass. Weight would be a negative quantity but often times when the direction is given it is given as down instead of as a negative value. So we can say something has a weight with a negative value or with a value followed by the word “DOWN”. Because the value of g is much less on the moon than it is on Earth, you would Weigh less on the moon than on Earth, but your mass would be the same.

3. SCALES A spring scale measures Weight and not mass. Do not forget that mass is in kg. Do Example 1 p. 97 F Net = ma 1 =.3a 3.33 m/s 2 = a in direction of Anudja Do Practice Problems p. 97 # 15-18

4. APPARENT WEIGHT What would a scale read if you are not in equilibrium? If an elevator accelerates upward then the upward force on the scale is greater than the downward force of your weight and thus the scale reading is greater than your weight. If an elevator accelerates downward then you feel lighter and thus the scale has a lower reading than your weight. If someone pushes down on you then you add your weight with the force of them pushing down on you to get the Net Force. If someone pulls up on you then you subtract the force with which they pull on you from your weight to get the Net Force. Note: Need to know these conceptually not to list on the test.

5. APPARENT WEIGHT Apparent Weight – the force experienced by an object resulting from all the forces acting on it, giving the object an acceleration. Weightlessness – an object’s apparent weight of zero that results when there are no contact forces pushing up on the object. Do Example 2 p. 99 At RestWhile Accelerating F g = mg F Net = F Scale - F g F g = 75(9.8)F Scale = F Net + F g F g = 735 NF Scale = ma + mg F Scale = 75(2) + 75(9.8) F Scale = 150 + 735 F Scale = 885 N So scale reads greater value when accelerating up than when at rest. Do Practice Problems p. 100 # 19-20

6. DRAG FORCE AND TERMINAL VELOCITY Air exerts a huge force, but in most cases, it exerts a balanced force on all sides, and therefore has no net effect. Drag Force – the force exerted by a fluid on the object moving through the fluid. It is dependent on the motion of the object, the properties of an object, and the properties of the fluid that the object is moving through. As velocity increases so does the drag force. Terminal Velocity – is the constant velocity that is reached when the drag force equals the force of gravity (Weight). When Terminal Velocity is reached the object no longer accelerates to the ground.

7. DRAG FORCE AND TERMINAL VELOCITY Read p. 101 with the Terminal Velocity examples. If mass of a person on Earth is 20 kg, what will be his mass on moon? (Gravity on Moon is six times less than the gravity on Earth 20 kg mass does not change, weight does Your mass is 100 kg, and you are standing on a bathroom scale in an elevator. What is the scale reading when the elevator is falling freely? ZERO Newtons (Weightlessness) When in an elevator on a scale when will your apparent weight be greater than your real weight? When it accelerates up. Do 4.2 Section Review p. 101 # 21-27