Forces and Motion Chapter 12

Inquiry activity (Individual work) Follow the directions on page 355 of your textbook Answer the questions Label it as Penny Lab and turn it in.

A push or a pull exerted on an object Forces Sec. 12.1 Measured in Newton, N Examples: Wind, Gravity Represented with an arrow The length of the arrow represents the strength of the force

Forces in the same direction add together Combining Forces Forces in the same direction add together Forces in opposite directions subtract from each other Net Force = Overall Force Balanced Forces Equal in size 10 N 10 N Net Force

Result in NO movement Balanced Forces cont. 150 N Ex: Standing still Unbalanced Forces Not equal in size 5 N 10 N 15 N Net Force Ex: Tug-of-War

Result in movement Unbalanced Forces cont. 20 N 30 N 10 N Net force

Friction Force that opposes motion of objects that touch friction

Types of Friction Static friction: the force that acts on objects that are not moving *Always acts in an opposite direction Ex. Balloon, dryer sheet

2. Sliding Friction: Caused by two object sliding over each other. Types of Friction 2. Sliding Friction: Caused by two object sliding over each other. Ex. Slide, Sled, Carpet Burn

Quick Lab on sliding friction We are now going to do the quick lab found on page 360 in your textbook. You will be working in pairs that I will assign. Make a data table to record your data that includes eraser with sticky note and eraser without sticky note and 3 trials and the average distance of each eraser. Answer the questions Turn this lab in to me

Types of Friction 3. Rolling Friction: Caused by two objects rolling over each other. Ex. Roller skates, car tires, ball

Types of Friction 4. Fluid Friction: force that opposes the motion of an object through a fluid Ex. Submarine, Cake Batter

Interactive forces http://www.engineeringinteract.org/resources.htm

Air Resistance Fluid Friction acting on an object moving through the air Examples: Speed Skaters

Gravity Force of attraction between two objects that have mass. Attractive Force - pulls objects together

Gravity On Earth gravity is a constant 9.8 m/s2.

Gravitational Forces on the Planets The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared.

Falling Objects Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

Terminal Velocity Constant velocity of a falling object when the force of air resistance equals the force of gravity.

Projectile Motion The motion of a falling object after it is given an initial forward velocity. Air resistance and gravity are the only forces acting on a projectile.

Newton’s three laws of motion demonstrated http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

Ex. Resting Soccer Ball Newton’s 1st law of motion Law of Inertia Sec. 12.2 Law of Inertia “An object at rest stays at rest, and an object in motion stays in motion, unless acted upon by an outside force.” Ex. Resting Soccer Ball

Inertia Newton’s first law of motion is sometimes called the law of inertia Inertia is the tendency of an object to resist change in its motion

Youtube newton’s cradles http://www.youtube.com/watch?v=mFNe_pFZrsA http://www.youtube.com/watch?v=V87VXA6gPuE

Demos performed Quarters on a card falling into a jar when card is thumped. Marker falling into a jar when the card is thumped. Pen falling into a bottle when hoop is removed. Egg in the beaker with cookie sheet.

Newton’s 2nd law of motion “When a force is applied to an object, the object accelerates in the direction of the force applied.” Acceleration=Net Force/Mass Mass is a measure of the inertia of an object and depends on the amount of matter the object contains

Ex. How much force in needed to accelerate a 10 kg object at 2 m/s2? a = 2 m/s2 m = 10kg F = ? Formula: a = F/m 2 = F/10 F = 20 N

If a 100 newton force is applied to a 5 kg object, how quickly will it accelerate? Newton’s 2nd law of motion Formula: a = F/m a = 100/5 a= 20 m/s2 F = 100 N m = 5 kg a = ?

Weight A measure of the force of gravity acting on an object Weight = mass x acceleration due to gravity W = mg

Weight = mass x acceleration due to gravity =112 kg x 9.8 m/s2 If an astronaut has a mass of 112 kilograms, what is his weight on Earth? Weight = mass x acceleration due to gravity =112 kg x 9.8 m/s2 =1100 kg m/s2 =1100 N

Math practice problems 1. What is the acceleration of a 1,500 Kg truck with a net force of 7,500 N? 2. A runner with a mass of 60Kg accelerates at 2.2m/s2. What is the runner’s net force? 3. Find the mass of a flying discus that has a net force of 1.05N and accelerates at 3.5m/s2. 4. Ian has a mass of 58Kg. What is his weight?

Ex. Bumper Cars Newton’s 3rd law of motion Sec. 12.3 “For every action, there is an equal and opposite reaction.” Ex. Bumper Cars

The single cart accelerates eight times faster than the chain of eight carts.

Momentum The product of an object’s mass and its velocity A large momentum is hard to stop The momentum of an object at rest is zero Momentum = Mass x Velocity Units are kg m/s

Which has more momentum, a 0. 046 kg golf ball with a speed of 60 Which has more momentum, a 0.046 kg golf ball with a speed of 60.0 m/s, or a 7.0 kg bowling ball with a speed of 6.0 m/s? Momentumgolf ball = 0.046 kg x 60.0 m/s=2.8 kg m/s Momentumbowling ball = 7.0 kg x 6.0 m/s = 42 kg m/s The bowling bowl has more momentum.

Law of Conservation of Momentum If no net force acts on a system, then the total momentum of the system does not change.

The total momentum of the train cars does not change-momentum is conserved.

Practice problems How much force is needed to make a 150kg object accelerate at a rate of 1 m/s2? How much does a 50 kg person weigh if the acceleration due to gravity is 9.8 m/s2? What is the mass of a car that weighs 140,000 N? What is the acceleration of a 150 kg object when 1,500 N is applied? Calculate Dave’s acceleration knowing that in a race he starts from rest and reaches a speed of 15.3m/s in 4.71 s.

Quizzes on forces and motion http://www.neok12.com/php/watch.php?v=zX5b70020d7b510f41440767&t=Laws-of-Motion After I show the video you need to click on this link and take the 6 practice quizzes. You also need to take the vocabulary quiz on the same page.

Sec. 12.4 Universal forces Electromagnetic Forces Electric Forces-act between charged objects or particles Magnetic Forces-act on certain metals, on the poles of magnets, and on moving charges Electric and magnetic forces are the only forces that can both attract and repel

Nuclear forces Act within the nucleus to hold it together Strong Nuclear Force- a powerful force of attraction that acts only on the neutrons and protons in the nucleus Weak Nuclear Force- acts only over a short range

Gravitational Force An attractive force that acts between any two masses Newton’s Law of Universal Gravitation- “every object in the universe attracts every other object” Weakest, but most effective force

Centripetal Force A center-directed force that continuously changes the direction of an object to make it move in a circle The result of a centripetal force is a circular path of the object

The moon’s inertia and the gravitational pull of the Earth result in a circular orbit.

Satellites in Orbit Satellites need only a centripetal force provided by gravity and its inertia to stay in orbit Satellites in low orbit are slowed by friction with the Earth’s atmosphere TEST ON CHAPTER 12 Next class meeting