Physics Chapter 2: Force and Newton’s Laws Section 1: Newton’s First Law

Vocab Force Net force Balanced forces Unbalanced forces Newton’s 1 st Law Friction Static friction Sliding friction Rolling friction

Force Force: A push or a pull –Force = mass x acceleration OR f = ma –Units = Newtons (N) f am

Combining Forces More than one force can act on an object at the same time When more than one force is acting on an object, the net force determines the motion of an object Net force: combination of all forces acting on an object If the forces are in the same direction  Add forces together If the forces are in opposite directions  Subtract the forces, the object will move in the direction of the larger force

Balanced and Unbalanced Forces If two or more forces are exerted on an object they can be balanced or unbalanced forces Balanced forces: The effects of the two forces cancel each other out –The object will not move –Net force = 0 Unbalanced forces: The effects of the two forces do NOT cancel each other out –The object will move in the direction of the greater force –The net force is NOT equal to zero

What type of force is acting upon the door? Will the door move? What is the net force? Balanced No 0

What type of force is acting upon the door? Will the door move? Will the door open or close? Unbalanced Yes Close

Isaac Newton 1643 – 1727 English physicist, mathematician, astronomer, natural philosopher, alchemist and theologian In 1687 he published his Philosophiæ Naturalis Principia Mathematica –It is said to be the greatest single work in the history of science

Newton’s First Law of Motion Newton’s First Law of Motion: Objects at rest remain at rest until a force acts upon it

Friction Friction: Force that acts to resist sliding between two touching surfaces To keep an object constantly moving force needs to be applied to overcome the effects of friction

3 Types of Friction 1.Static friction: Type of friction that prevents an object from moving when a force is applied –Ex. Trying to push a heavy refrigerator –Caused by the attraction between the two surfaces when they are in contact –To move the object, you have to exert a force large enough to break the bonds holding the surfaces together

2.Sliding friction: Type of friction that slows down an object that slides –Ex. Bicycle brakes sliding against a tire –Friction between the two rough surfaces causes the object to slow down

3.Rolling Friction: Type of friction needed to make a wheel or tire turn –Ex. Keeps a bicycle tire from slipping on the ground, allows you to walk –Friction exerts force on a tire/your foot that pushes the bike/you forward

Physics Chapter 2: Force and Newton’s Laws Section 2: Newton’s Second Law

Vocab Newton’s 2 nd Law Gravity Weight Centripetal force Air Resistance Terminal Velocity

Force and Acceleration Newton’s 2 nd Law connects force, acceleration, and mass Newton’s 2 nd Law of Motion: An object acted upon by a force will accelerate in the direction of the force –Acceleration = net force mass

Units of Force Force is measured in Newtons (N) –1 N = 1 kg  m/s 2 Because force deals with mass (kg) and acceleration (m/s 2 )

Gravity Gravity: A force that pulls objects toward each other –Exists between any two objects that have mass –Depends on the mass of the objects and the distance between them More mass/less distance = stronger gravitational force

Weight The force of gravity causes all objects near Earth’s surface to fall with an acceleration of 9.8 m/s 2 This gravitational force is also called weight Weight: Gravitational force between an object and the Earth

Weight and Mass Weight and mass are different Weight is a force –When you stand on a scale you are measuring the pull of Earth’s gravity (a force) Mass is the amount of matter in an object –NOT a force

Using Newton’s Second Law The second law tells you how to calculate the acceleration of an object if its mass and the forces acting on it are known Speeding Up –An object will speed up if the force applied to it is in the direction of motion Ex. Applying force (kick) to the ball in the same direction it was previously moving will cause it to speed up

The girl speeds up because the force is being applied in the direction of motion

Slowing Down –An object will slow down if a force is applied in the opposite direction of the object’s motion Ex. Sliding friction preventing a book from sliding across the table after you stop applying force with your hand

The boy slows down because the force is being applied in the opposite direction of motion

Turning –Forces and motion are not always in a straight line –If a net force acts at an angle to the direction of motion the object will follow a curved path Ex. If you shoot a basketball, the ball will NOT continue to move in a straight line after it leaves your hand Instead it will curve downward as the force of gravity pulls the ball down The ball’s motion is a combination of its original motion and downward motion due to gravity

Gravity exerts a force downward that makes it move in a curved path

Circular Motion If you are traveling in a circular motion you are constantly accelerating because you are constantly changing direction –According to Newton’s 2 nd Law, if you are constantly accelerating, there must be a force acting on you the entire time –The force that keeps the object moving in a circle is called centripetal force

Satellite Motion Objects that orbit the Earth are satellites of Earth –Gravity is the centripetal force If gravity is pulling satellites towards the Earth then why don’t the satellites fall straight down? –Satellites are being pulled vertically towards the Earth’s surface. –They are moving so fast horizontally that the Earth’s surface curves away from the satellite as fast as it falls downwards

Air Resistance If you are running, walking, or parachuting, air is pushing against you Air Resistance: Form of friction that acts to slow down any object moving in the air –Air resistance gets larger as the object moves faster –When an object falls it speeds up as gravity pulls it downward –At the same time air resistance pushes upward on the object and increases as the object falls faster –At one point the upward air resistance will equal the downward force of gravity –This is called terminal velocity When this happens the net force will be 0 (balanced)

Air Resistance and Shape Air resistance depends on the shape of an object as well as the speed Moving at the same speed, the air resistance on a pointed, narrow object is less than a broad, flat object

Which person would experience the most air resistance? Because this person has increased their surface area they will experience more air resistance and fall more slowly

Physics Chapter 2: Force and Newton’s Laws Section 3: Newton’s Third Law

Vocab Newton’s 3 rd Law Action-reaction force pair Weightlessness

Action and Reaction Newton’s first two laws explain how the motion of a single object changes Newton’s 3 rd law explains what happens when an object exerts a force on another object Newton’s 3 rd Law: For every action, there is an equal but opposite reaction

The car jack is pushing up on the car with the same amount of force with which the car is pushing down on the jack

Action and Reaction Forces Don’t Cancel Forces exerted by two objects on each other are often called action-reaction force pairs –Either force could be considered the action or the reaction force Action-reaction forces are opposite but they do NOT cancel each other out –This is because they act on different objects

When two bumper cars collide, each car pushes on the other car. According to Newton’s 3 rd law each car pushes on the other with the same force but in opposite directions. This force causes you to slow down.

When the child pushes against the wall, the wall pushes against the child.

Large and Small Objects Sometimes it is difficult to notice action- reaction force pairs because one object is so large it appears to remain motionless –It has so much inertia that it appears to remain still when a force acts upon it –Ex. Walking When you walk you exert a force upon the ground The Earth pushes back on your shoe but the Earth has so much mass compared to you that it appears not to have moved at all

A Rocket Launch The launching of a rocket is a great example of Newton’s 3 rd Law 3 rocket engines supply the force, called thrust, that lift the rocket When the rocket fuel is ignited, a hot gas is produced The gas molecules collide against the walls, the walls exert a force that pushes them out the engine –This downward push is the action force There is an upward push on the engine by the gas molecules which propels the rocket forward –This is the reaction force

Weightlessness Astronauts appear to float around inside of a space shuttle as it orbits the Earth The astronauts are said to be weightless, as if Earth’s gravity were no longer pulling on them While this is happening the pull of Earth’s gravity is still 90% as strong

Measuring Weight When you stand on a scale, your weight pushes down on the scale causing the springs to compress The scale pointer moves from zero and points to your weight At the same time the scale pushes up on you with a force equal to your own weight (according to Newton’s 3 rd Law) –This force balances the downward pull of gravity on you

Free Fall and Weightlessness Imagine that scale is now inside of an elevator that is free falling An object is in free fall when the only force acting on it is gravity Inside the free falling elevator both you and the scale are in free fall Because the only force acting on you is gravity, the scale is no longer pushing up on you –The scale pointer would stay on zero and you would appear to be weightless

Weightlessness: The condition that occurs in free fall when the weight of an object seems to be zero –You are not really weightless in free fall because Earth is still pulling down on you –With nothing to push up on you, you would have no sensation of weight

A: When you stand on a scale on Earth, the reading on the scale is your weight B: If you were to stand on a scale in a falling elevator, the scale would read zero

Weightlessness in Orbit To understand how objects move in the orbiting space shuttle, imagine you were holding a ball in the free falling elevator If you let go of the ball its position relative to you and the elevator wouldn’t change because you are all moving at the same speed If you push on the ball you are adding force to it in addition to the downward pull of gravity so the ball will drift around until it hits a surface