Chapter 3 Forces

Section 1 Newton’s Second Law

Force, Mass and Acceleration Compare hard thrown ball vs. gently tossed ball Compare hard thrown ball vs. gently tossed ball Force? Velocity? Acceleration? Force? Velocity? Acceleration? Throw hard a baseball vs. softball Throw hard a baseball vs. softball Who goes faster? Why? Who goes faster? Why? Velocity and Acceleration depend on mass Velocity and Acceleration depend on mass

Newton’s 2 nd Law of Motion Def: acceleration is in the same direction as net force Def: acceleration is in the same direction as net force

Net Force Net Force = mass x acceleration F net = ma Net Force = mass x acceleration F net = ma kg x m/s 2 = N (Newton) kg x m/s 2 = N (Newton)

Practice! Rocket has 28,913 N of thrust and a mass of 2,350 kg. What is the acceleration? (frictionless surface) Rocket has 28,913 N of thrust and a mass of 2,350 kg. What is the acceleration? (frictionless surface) What is the mass of a rocket that has a thrust of 105 N and an acceleration of 70 m/s 2 ? What is the mass of a rocket that has a thrust of 105 N and an acceleration of 70 m/s 2 ?

Friction Roll a ball in the parking lot… Roll a ball in the parking lot… Newton’s 1 st law says ball will keep constant speed Newton’s 1 st law says ball will keep constant speed Stops because of friction (negative acceleration – force is friction) Stops because of friction (negative acceleration – force is friction) Def: force that opposes the sliding motion Def: force that opposes the sliding motion

Microwelds Def: points where two surfaces come in to contact Def: points where two surfaces come in to contact Larger force = stronger microwelds Larger force = stronger microwelds Break microwelds = need force Break microwelds = need force

Static Friction Push box = doesn’t move Push box = doesn’t move Acceleration = 0 so net force = 0 Acceleration = 0 so net force = 0 What’s canceling your force? What’s canceling your force? Static Friction: force prevents two surfaces from sliding past each other Static Friction: force prevents two surfaces from sliding past each other

Sliding Friction Def: force that opposes the sliding motion of two surfaces Def: force that opposes the sliding motion of two surfaces Stop pushing = box stops Stop pushing = box stops

Rolling Friction Car stuck in mud… Car stuck in mud… Def: frictional force between rolling object and surface. Def: frictional force between rolling object and surface.

Air Resistance Falling object pulled by gravity – opposed by air resistance Falling object pulled by gravity – opposed by air resistance Def: friction like force opposed motion as moves through air Def: friction like force opposed motion as moves through air No air = objects fall with same acceleration bowling ball vs. feather No air = objects fall with same acceleration bowling ball vs. feather

Terminal Velocity Def: highest speed a falling object will reach Def: highest speed a falling object will reach When air resistance balances downward force When air resistance balances downward force Depends on size, shape, mass Depends on size, shape, mass Sky diver no parachute vs. with parachute Sky diver no parachute vs. with parachute

Section 2 Gravity

What is gravity? How would you throw a long pass with a football? How would you throw a long pass with a football? Anything that has mass is attracted by gravity Anything that has mass is attracted by gravity Gravity: attractive force between two objects that depends on masses and distance Gravity: attractive force between two objects that depends on masses and distance Stronger force? Stronger force? More mass/closer together More mass/closer together

Gravity We only feel earth’s gravitational force We only feel earth’s gravitational force 1 of 4 Basic Forces 1 of 4 Basic Forces Others: Others: electromagnetic force – electricity and magnetism electromagnetic force – electricity and magnetism strong and weak nuclear force – inside nuclei of atoms strong and weak nuclear force – inside nuclei of atoms

Law of Universal Gravitation Sir Isaac Newton Sir Isaac Newton

Earth’s Gravitational Acceleration When only force acting on two things is gravity – they fall at the same rate When only force acting on two things is gravity – they fall at the same rate Close to earth’s surface acceleration due to gravity is 9.8 m/s 2 Close to earth’s surface acceleration due to gravity is 9.8 m/s 2 Newton’s second law  F = ma Newton’s second law  F = ma F gravity (N) = mass (kg) x acceleration of gravity (m/s 2 ) F gravity (N) = mass (kg) x acceleration of gravity (m/s 2 ) F = mg F = mg

Weight Def: gravitational force on a object Def: gravitational force on a object Calculate your weight in N Calculate your weight in N

Weightlessness and Free Fall When you free fall gravity pulls everything at same rate = no gravity When you free fall gravity pulls everything at same rate = no gravity Spaceship in orbit experiences 90% of earth’s gravity Spaceship in orbit experiences 90% of earth’s gravity Ship is in free fall in orbit = no gravity Ship is in free fall in orbit = no gravity

Projectile Motion Projectile: anything thrown or shot through air Projectile: anything thrown or shot through air Gravity causes path to be curved Gravity causes path to be curved Horizontal motion – throwing Horizontal motion – throwing Vertical motion – gravity Vertical motion – gravity

Horizontal and Vertical Distance Same vertical distance = same mass = same acceleration = same time Same vertical distance = same mass = same acceleration = same time Different horizontal distance Different horizontal distance

Centripetal Force Centripetal acceleration: acceleration towards the center of a curved path. Centripetal acceleration: acceleration towards the center of a curved path. Newton’s 2 nd law – acceleration in direction of force  centripetal force Newton’s 2 nd law – acceleration in direction of force  centripetal force Anything moving in circle is because cent. force is accelerating towards center Anything moving in circle is because cent. force is accelerating towards center

Gravity

Section 3 Newton’s 3 rd Law of Motion

Newton’s 3 rd Law Def: when one object exerts a force on another, the 2 nd object exerts an equal but opposite force Def: when one object exerts a force on another, the 2 nd object exerts an equal but opposite force Action – Reaction Action – Reaction Jumping on trampoline Jumping on trampoline

Action/Reaction Do NOT Cancel Forces are equal, but acting on different objects Forces are equal, but acting on different objects Act on water – reaction pushes forward = unbalanced force Act on water – reaction pushes forward = unbalanced force

Rocket Propulsion Rocket exerts force on gas backwards – gas exerts force on rocket forwards Rocket exerts force on gas backwards – gas exerts force on rocket forwards

Momentum How much force is needed to change motion? How much force is needed to change motion? Def: product of mass and velocity Def: product of mass and velocity

Practice! What is the momentum of a 1,300 kg car traveling 28 m/s? What is the momentum of a 1,300 kg car traveling 28 m/s? What is the velocity of a 0.15 kg baseball that has a momentum of 6 kg m/s? What is the velocity of a 0.15 kg baseball that has a momentum of 6 kg m/s?

Force and Changing Momentum Catch a foul ball bare handed? Catch a foul ball bare handed?

Force and Changing Momentum F = maa = (v f – v i ) F = maa = (v f – v i ) t Put the two together: Put the two together: When that baseball hits – calculate the force When that baseball hits – calculate the force

Force and Changing Momentum What is the force of a 0.15 kg baseball with a velocity of 40 m/s when it comes to a stop in 1 second? What is the force of a 0.15 kg baseball with a velocity of 40 m/s when it comes to a stop in 1 second?

Law of Conservation of Momentum Momentum doesn’t change unless mass, velocity or both change Momentum doesn’t change unless mass, velocity or both change Can be transferred – pool balls Can be transferred – pool balls Def: group of objects exerts forces only on each other, their total momentum doesn’t change Def: group of objects exerts forces only on each other, their total momentum doesn’t change

When Objects Collide 1 st puck will give more momentum to the 2 nd puck in the same direction 1 st puck will give more momentum to the 2 nd puck in the same direction Moving towards each other same speed = 0 momentum Moving towards each other same speed = 0 momentum Collide = 0 momentum – moving same speed opposite direction Collide = 0 momentum – moving same speed opposite direction