Chapter 3 Forces

Section 3.1: Newton’s 2 nd Law  2 nd Law of Motion: describes how force, mass, and acceleration are related Formula: force = mass x acceleration F = m x a Newtons (N): unit for force Example Problem: How much force is needed to accelerate a 1000 kg car 3 m/sec/sec?

2 nd Law Practice Problems 1. How much force is needed to accelerate a 1400 kg car 2 m/s/s? 2. If a 70 kg swimmer pushes off a wall with a force of 250 N, at what rate will the swimmer accelerate from the wall? 3. A dancer lifts his partner above his head with an acceleration of 2.5 m/s/s. The dancer exerts a force of 200 N. What is the mass of the partner?

 Friction: force that opposes motion Microwelds: source of friction

 Static Friction: no movement

 Sliding Friction: solid objects slide over each other

 Rolling Friction: friction between rolling object and surface it’s rolling on.

Air Resistance : opposes the force of gravity Makes objects slow down

Terminal Velocity The downward force of gravity equals the upward force of air resistance. When these forces are balanced, object no longer accelerates, it falls at a constant speed. Because forces are balanced, skydiver has reached terminal velocity.

Section 3.2: Gravity  Newton’s Law of Gravitation: force of attraction that exists between all objects in the universe  The size of the gravitational force between 2 objects depends on… Masses Distances between them

Falling Objects  All objects fall at the same rate: 9.8 m/s/s  Example Problem: A student drops a penny from the top of the school building. If it takes 5 seconds for the penny to hit the ground, what is the speed of the penny just before impact?

 Multiflash photography shows that each ball has the same acceleration downward (9.8 m/s/s) whether it’s thrown or dropped.

Weight & Mass  Weight: gravitational force exerted on an object (N)  Mass: measure of how much matter is in an object (kg)  Formula: weight(N) = mass (kg) x acceleration due to gravity (m/s/s)

Example Problem  What is the weight of a person with a mass of 50 kg? Remember: Earth’s acceleration of gravity is 9.8 m/s/s.

weight(N) = mass x gravity 1.A man has a mass of 75 kg on the Earth. What is his weight? 2.Find the acceleration of gravity on a planet if a person with a mass of 66 kg weighs N on that planet. 3.A person weighs 500 N on the Earth. What is the person’s mass?

Projectile Motion  The path of a projectile is curved because of gravity.

Centripetal Force  When an object moves in a circular path, it accelerates towards the center of the circle because of centripetal force.

Facts About Gravity  The size of the gravitational force between 2 objects depends on… Masses Distances between them  As you go farther from earth, weight decreases.  With no air resistance, a penny & a feather fall at the same rate.  Objects in free fall seem to be weightless.

Elevator is in free fall. Scale shows boy’s weight Scale shows zero weight

Lab: Finding Acceleration Due to Gravity TrialTime for 20 swings (sec) T Time of 1 swing Find average value of T Length = 1.84 m

Formula Problem Set-Up Answer + Label

Section 3.3: Newton’s 3 rd Law  3 rd Law of Motion: every action has an equal and opposite reaction All forces come in pairs.  Examples: Walking: feet push floor, floor pushes back Flight of a bird: bird exerts force on air and air pushes back Rocket: gases move down and rocket moves up

Momentum  Formula: p = mass x velocity  Unit for momentum: kg-m/s  Law of Conservation of Momentum: momentum isn’t created or destroyed Momentum can be transferred from one object to another in a collision

Momentum Practice Problems 1. An athletic has a mass of 100 kg and a velocity of 8 m/s. What’s the athlete’s momentum? 2. What is the mass of a person walking at a speed of 1 m/s if their momentum is 60 kg-m/s? 3. What is the velocity of an object that has a mass of 175 kg and a momentum of 1750 kg-m/s?