Aim: How do we apply Newton’s 2nd Law to systems that move up and down?

Weight Fg = mg The weight of an object is equal to the mass of an object multiplied the acceleration due to gravity (gravitational field value) (9.81 N = 2.20 lb) Fg=weight m= mass g = 9.81m/s2 = gravitational acceleration due to gravity = 32.2 ft/s2

Thought Question 1 Suppose you are talking by interplanetary telephone to your friend who lives on the Moon. He tells you that he has just won a piece of gold weighing one newton in a contest. Excitedly, you tell him that you entered the Earth version of the same contest and also won a newton of gold! Who’s richer? The person on the moon is richer because it takes more mass to equal one newton on the moon than on the earth.

Thought Question 2 A baseball of mass m is thrown upward with some initial speed. If air resistance is ignored, what is the magnitude of the force on the ball (a) when it reaches half its maximum height and (b) when it reaches its peak? Mg down

Thought Question 3 In the absence of air friction, it is claimed that all objects fall with the same acceleration. A heavier object is pulled to the Earth with more force than a light object. Why does the heavier object not fall faster? Suppose the heavier object has double the mass. Well, then it is true it has double the weight but it also requires double the force to accelerate this mass at the same rate as the lighter object.

Normal Force The normal force is the force that the ground exerts on a direction perpendicular to the surface.

Problem 1 Bob with mass of 80 kg stands at rest on the surface of the earth. a) What is Bob’s weight? b) What is the normal force acting on Bob?

Questions about Normal Force Think of some scenarios where the normal force is not upwards Think of some scenarios where the weight and normal force are not equal in magnitude

Normal Force and Apparent Weight Does the bathroom scale read your weight or the normal force acting on you?

Normal Force and Its Application to Elevators (Problem 2) A person of 70 kg stands on a scale in an elevator. Calculate the force that the scale reads when the elevator is: a) At rest b) Accelerating upwards at 2m/s2 c) Accelerating downwards at 2 m/s2

Elevator Summary

Post Malone has a mass of 100 kg Post Malone has a mass of 100 kg. He steps inside an elevator that is accelerating upwards at 4 m/s2 . What his apparent weight?

President Obama has a mass of 80 kg and steps on a scale inside an elevator. The elevator accelerates downward at 4 m/s2 . What does the scale read for Obama’s weight?

George W. Bush weighs 885 N. When he steps on a scale inside a moving elevator, it reads 780 N. What is the acceleration of the elevator?