Newton’s Second Law of Motion WE ARE GOING TO USE THE CORNELL NOTE-TAKING TECHNIQUE TODAY, SO BE READY!!

Weight or Mass? PROBLEMS 70 kg (9.81 N/kg) = (C) 687 N 70 kg (1.62 N/kg) = (A) 113.4 N 55 kg (3.73 N/kg) = (C) 205 N

Use the Cornell-Notetaking-Technique!

Write the following questions into the “cue column.” What is Newton’s Second Law? How do you convert from pounds to Newtons? How do you convert from Newtons to pounds? What is an application of Newton’s Second Law? What is the Normal Force? What is the acceleration due to gravity on the Earth and on the Moon?

Acceleration depends on Force!

Force depends on mass!

Therefore, the product of mass and acceleration is Force!

Net External Force = massacceleration Mass – a measurement of inertia, depends on the amount of Matter in an object. Change in Motion ALL forces acting on an object

UNITS OF FORCE: 1 kilogram * 1 meter/second2 = 1 Newton 1 Newton (N) = 0.225 pounds (lbs) 1 pound (lb) = 4.45 Newtons (N) Did you know? The abbreviation for the pound (lb) comes from libra, the Latin word for pound!

Example #1 You slide your physics book towards your friend. If the net external force is 2.6N to the right on the 2.2 kilogram textbook, then what is the book’s acceleration? Given: F = 2.6 N m = 2.2 kg Find: a Solution: a = F / m = (2.6 kg m/s2) / 2.2 kg = 1.2 m/s2

Weight, an application of Newton’s Second Law! Weight is a measure of the force exerted by the Earth on an object. Weight is also called the force of gravity. Weight depends on the mass of an object and on the acceleration due to gravity.

Mass or Weight “POP” Quiz ___________________ measures the downward pull of gravity. ___________________ is the amount of matter. ___________________can change depending on the strength of gravity’s pull. ___________________ is measured using a bathroom scale. An object has a mass of 2 kg on Earth. Will it have the same mass on the moon? Will it have the same weight on the moon? Explain your answer by correctly using the following terms: mass, weight, gravity, & force

Mass or Weight “POP” Quiz ANSWERS WEIGHT measures the downward pull of gravity. MASS is the amount of matter. WEIGHT can change depending on the strength of gravity’s pull. WEIGHT AND MASS is measured using a bathroom scale. An object has a mass of 2 kg on Earth. Will it have the same mass on the moon? Will it have the same weight on the moon? Explain your answer by correctly using the following terms: mass, weight, gravity, & force YES, BECAUSE THE MASS DOES NOT CHANGE IF YOU GO FROM EARTH TO THE MOON. NO, BECAUSE WEIGHT DEPENDS ON THE FORCE DUE TO GRAVITY WHICH IS DIFFERENT DEPENDING IF YOU ARE ON THE MOON OR ON THE EARTH.

ANSWERS TO NEWTON’S 2ND LAW SET #1 50 m/s2 50 Kg 1.6 m/s2 4 m/s2 and 2 m/s2 3.2 Kg 7.5 m/s2 30 m/s2 285 N 9.81 m/s2

3-BEFORE-”G” RULE TRY THE PROBLEM YOURSELF. ASK A CLASSMATE FOR HELP. ASK ANOTHER CLASSMATE FOR HELP.

Weight Formula Where “g” is the acceleration due to gravity. This acceleration is known as free-fall. On Earth gEARTH = 9.81 m/s2 The value of “g” will decrease as the distance from the Earth’s surface increases. On Earth’s moon gMOON = 1.635 m/s2

Example Problem #2 What is the weight of a kitten, in Newtons that is made up of 0.015 kilograms of matter? Fg = m g Fg = (0.015 kilograms) (9.81 m/s2) Fg = 0.14715 kg m/s2 = 0.14715 N A VERY IMPORTANT NUMBER TO MEMORIZE!

Example Problem #3 An adult cat’s weight is 15 Newtons, what is the mass of the cat in kilograms? Fg = m g 15 Newtons = m (9.81 m/s2) m = Fg / g m = (15 kg m/s2 ) / (9.81 m/s2) m = 1.53 kg

Example Problem #4 What is the acceleration due to gravity, in m/s2 of the “dwarf” planet of Pluto if an adult cat weighs 18 Newtons and has a mass of 10 kilograms? Fg = m g 18 N = (10 kg) g g = Fg / m g = (18 kg m/s2) / (10 kg) = 1.8 m/s2

The “Normal” Force, FN The force exerted by an object on the Earth (or other objects) is known as the “normal” force. This force is perpendicular to the surface of contact. The word “normal” is used because the direction of the contact force is perpendicular to the surface (in geometry, “normal” means perpendicular).

The Normal Force Formula FN Same as the Weight, just in the opposite direction! These are known as opposite-pairs forces. Force of the table on the TV Force of the TV on the table Fg

Answer the questions from the cue section into the summary section Answer the questions from the cue section into the summary section. You have 3 ½ minutes!

Example #5 A 2.20 kg physics textbook is placed on a desktop. Calculate the weight (on the Earth) of the textbook and the normal force. Draw a free-body diagram showing the FN and the Fg. Given: m = 2.20 kg g = 9.81 m/s2 Find: Fg and FN Solution: Fg = mg = 2.20 kg (9.81 m/s2) = 21.58 N FN = mg = 2.20 kg (9.81 m/s2) = 21.58 N

Example #5 – Free-Body Diagram FN = 21.58 N Fg = 21.58 N

Example #6 Same conditions as example #2, yet calculate it for a physics textbook on the Moon!

Answer to example #6 Fg = 3.597 N FN = 3.597 N FN = 3.597 N