Chapter 6 Forces and Motion Section 1-3

Alabama Course of Study 1.3 Measuring dimension, volume, and mass using Système International d'Unités (SI units) 8.) Identify Newton's three laws of motion. 8.1 Defining terminology such as action and reaction forces, inertia, acceleration, momentum, and friction 8.2 Interpreting distance-time graphs

Aristotle 1 Was the first to question gravity and thought that the rate at which an object falls was dependent on its mass This is incorrect and later proven Galileo Galileo questioned Aristotle and argued that mass did not matter in gravity He dropped two cannonballs with different masses and the two cannonballs landed at the same time

Forces 2 Force- push or pull on an object Similar to velocity and acceleration, a force is described by its strength and by it’s direction Force is measured by Newtons or N. Net force- combination of all the forces acting on an object Note- An object must move in order for it to be considered a force

Forces in Same Direction 3 A piano is being pulled by Sam with 20N to the right & pushed by Billy Bob with a force of 20N to the right. What is the total force used? If the force is going in the same direction, you can add the forces to get the net force. Add 25N and 20N to get 45 N to the right Remember you must have the direction its moving 3 25N 20N

Forces in Different Directions 4 John and Ben are playing tug of war. John is pulling with 50N of force to the left and Ben is pulling with 65N of force the right. What is the net force? Since Ben is pulling with more force, the force is moving to the right and you must subtract John’s force 65N – 50N= 15N to the right. 50N 65N

Think time 5 John, Mary, and Charlie is on the left side of the rope and Ben, Sara, and Connor are on the right side. With the information given below what is the net force? Left- John-50N Mary-53N Charlie-55N Right- Ben-65N Sara-48N Connor-43N Answer: 50N+53N+55N=158N to the left 65N+48N+43N=156N to the right 158N -156N= 2N to the left 50N 53N 55N 65N 48N 43N

Forces 6 Balanced forces-when the forces on an object produce a net force of 0N. Balanced forces will not move Ex. A chandelier is being pulled down by gravity but a chain is holding it up Because the chandelier is not falling, it is balanced. Unbalanced forces- when the net force is not 0N When we did the problem on the tug-of-war the net force was not 0N, so therefore it was unbalanced and was moving. Unbalanced forces is necessary when a object must be moved

Friction 7 Friction- force that opposes motion between two surfaces that are in contact Amount of friction depends on the force pushing the surfaces together Weight increases the amount of friction The rougher the surface the greater the friction EX. Hockey puck slides better over ice than snow

Types of Friction 8 Static Friction- friction that acts objects that are not moving This is why a desk is not flying across the room when you sit in it. Sliding Friction- two solid surfaces slide over each other Why the desk shifts across the room?

9 More Types of Friction Rolling Friction- when an objects rolls across a surface Skateboard going down the street Fluid Friction- occurs when a solid object moves through a fluid Surfer moving through the water

Gravity 10 Acceleration due to gravity is the same for all objects Why? The heavier the object is the harder it is to accelerate because it has more mass. All objects travel towards the earth at -9.8 m/s2 So after three seconds what is the acceleration? 9.8 • 3= -29.4 m/s2

Gravity 11 Weight-the force of gravity on a person or object on the surface of a planet Weight is also measured in Newtons Two factors that affect gravity: mass and distance The more mass present, the greater the gravity The more distance between objects the less gravitational pull on an object

Velocity of Falling Objects 12 Velocity of Falling Objects ∆velocity= Gravity X Time In the equation above the symbol ∆, means change. So to get this you must subtract the starting velocity with the ending velocity. Also in the equation above you must know the acceleration due to gravity is 9.8 m/s2

13 Math Time A stone at rest is dropped from a cliff, and the stone hits the ground after a time of 3s. What is the stone’s velocity when it hits the ground? ∆v= -9.8 m/s2 X 3s ∆v= -29.4 m/s ∆velocity= Gravity X Time ∆V G T

Problems 14 What is the ∆Velocity of an object that fall for 20s? How long does it take for an object to hit the ground if it’s ∆Velocity is 200m/s? What is the ∆Velocity of an object that fall for 15s? How long does it take for an object to hit the ground if it’s ∆Velocity is 320m/s?

Free Fall (not the ride) 15 Free Fall (not the ride) Free fall- an object that is falling without air resistance or any other force besides gravity 2 places that have no air is a vacuum or in space Vacuum is a place with no matter If a feather and an apple are dropped they fall at the exact same rate in a vacuum.

Air Resistance and Falling Objects 16 Air resistance- force that opposes the motion of objects through air Ex. A crumbled paper falls faster than a flat sheet Air resistance depends on the size, shape, and speed of the object

17

Projectile Motion 17 Projectile Motion- curved path that an object follows when thrown or launched near the surface of the earth Two components make up projectile motion- horizontal motion and vertical motion

18 Centripetal force Centripetal force- a force that keeps an object moving in a circle. Ex. Why you do not fall out of a rollercoaster, when going through a loop

Newton’s First Law of Motion 19 Newton’s First Law of Motion In 1686, Sir Isaac Newton explained motion with his three laws of motion. 1ST Law- an object at rest remains at rest, and an object in motion remains in motion unless acted upon by an unbalanced force.

1st Law: Part 1-Objects at rest 20 1st Law: Part 1-Objects at rest If something is not moving, it will continue not to move if nothing bothers it If you push or pull the object (an unbalanced motion), it causes it to move Ex. A golf ball on a tee will remain there until a push by the club causes it to move

1st Law: 2nd Part- Objects in motion 21 1st Law: 2nd Part- Objects in motion An object stays in motion unless acted upon by another force Ex. Bumper cars- you stay in motion until you hit another car which either stops you or slows you down. The car will stay in motion until acted upon by another force (another car)

22 1st Law and Inertia What is inertia? Inertia is the tendency of all objects to resist any change in motion Inertia is why objects in motion stays in motion Ex. When a car turns fast, you may tap the side of the car. This is because of inertia Mass is a measurement of inertia Greater the mass, the greater the inertia

Newton’s 2nd Law of Motion 23 Newton’s 2nd law of Motion- the acceleration of an object depends on the mass of the object and the amount of force applied Part 1: Acceleration depends on mass If you push an empty shopping buggy, you use a small amount of force to accelerate it. If the buggy is full, it is harder to push. Acceleration of an object decreases as its mass increases

2nd Law: Pt 2- Accel. depends on Force 24 Going back to the shopping cart. You must increase your force to make it go faster. As force increases the acceleration increases. As mass increases the acceleration decreases.

Math and Newton’s 2nd Law 25 Math and Newton’s 2nd Law Acceleration= Force Or Force= Mass X Acceleration Mass What is the acceleration of a 3 kg mass if a force of 14.4N is used to move the mass? (note: 1 N is equal to 1 kgXm/s²) Acceleration= 4.8 m/s² Acceleration= 14.4 kgXm/s² 3 kg F A M

Newton’s 3rd Law of Motion 26 Newton’s 3rd Law of Motion- whenever one object exerts a force on a second object, the second objects exerts an equal and opposite force on the first Summary: if a force is exerted, another force occurs that is equal in size and in opposite direction Ex. Jumping on a trampoline What is pushing down? What is pushing up?

Action and Reaction Pairs 27 Example: Jumping into the air Action- you jumping Reaction- ground pushes you up Example: Knocking on a door Action: you hit the door Reaction: the door pushes back on your hand Example: Rowing a boat Action: moving the oar in the water Reaction- water pushing back on the oar

Action and Reaction 28 Both objects can move Example: Volleyball hitting your arms Action: volleyball moving down Reaction: volleyball changes direction Action: Your arms move up Reaction: Volleyball pushes down on arms

29 Action and Reaction You try: Example: Baseball hitting a bat Action: baseball_________________ Reaction: baseball________________ Action: Bat__________________ Action: Bat___________________

The Law of Conservation of Momentum 30 The Law of Conservation of Momentum Law of conservation of Momentum- total momentum stays the same or is conserved, unless outside forces act on the object The momentum either stays with the object or is transfer to the next object Example: When you hit a pool ball, it transfers it’s momentum to the ball it hits.

31 Momentum Momentum- a quantity defined as the product of the mass and velocity of an object The higher the momentum the harder it is to stop an object Momentum (p)= Mass X Velocity

Math Time 32 What is the momentum of an ostrich with a mass of 120 kg that runs with a velocity of 16m/s north? Momentum= 19,200 kgXm/s north Momentum (p)= Mass X Velocity Momentum (p)= 120 kg X 16 m/s north P m a

Collisions w/2 moving objects 33 2 objects moving in the same direction collide and the object switch momentums 4m/s 2m/s 2m/s 4m/s

Collisions w/ 1 moving Object 34 When an object hits a stationary object, it transfers its momentum to the stationary object 4 m/s Not Moving 4 m/s Not Moving

Collisions when the objects connect together 35 Collisions when the objects connect together When two cars collide and stay connected, momentum is transferred evenly to both objects 4 m/s Not Moving 2m/s 2m/s