Forces

Goals Construct an explanation based on experimental evidence to support the claims presented in Newton’s three laws of motion. Explain the difference in mass and weight. Relate falling objects to gravitational force Analyze and interpret data to identify the relationship between mass and gravitational force for falling objects. Apply Newton’s three laws to everyday situations by explaining the following: -Inertia -Relationship between force, mass and acceleration -Equal and opposite forces

What causes things to move? What is a force? A push or a pull that one body exerts on another.

Balanced Forces

Question: Can there be balanced forces on a object and the object still have motion?

The Body will Accelerate Unbalanced Forces If the forces acting on a body are "unbalanced" this means that there is a "net" or "resultant" force. Unequal forces pushing in opposite directions that cause a change in motion. The object will do one of the following: Speed Up Slow Down Change the Direction of its Motion In other words…. The Body will Accelerate

Questions: How do we measure force? Newtons SI unit of force is the Newton (N). Defined as the amount of force that when acting on a 1 kg object produces an acceleration of 1m/s2. Therefore, 1N = 1 kg • m s2

Newton’s First Law

What will happen? (turn and ask a friend) Abruptly stopping a cart with a suitcase on top…. You set your coffee cup on the dash and then make a sharp left turn….. You throw a tennis ball in outer space.

Newton’s First Law Inertia/Eureka Video

Newton’s First Law ?What do you mean by “Net Force”? An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force. ?What do you mean by “Net Force”? Net means: Final Sum, so net external force is the sum of all the forces acting on the object.

Newton’s First Law Newton’s First Law is also called the Law of Inertia. Inertia is the tendency of an object to resist any change in its motion. Question: Does an object at rest still have inertia?

Why does the ladder continue to move? Inertia!

Why does the person continue to move? Inertia! Clip

WHY were the distances different??? Newton’s Second Law Lab Activity Question: How will the different balls affect the distance the block moves? Be able to explain your answer… WHY were the distances different??? Roll each ball down your ramp and measure the distance the block moves.

Newton’s Second Law F=ma The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass. F=ma Clip

Newton's 2nd Law of Motion

Force = mass x acceleration Newton's 2nd Law of Motion: Force = mass x acceleration F = m x a Mass is in kg Acceleration is in m/s2 Force is in kg x m/ s2 (or N) Let’s do a sample problem…next slide 

Force Problems: What force is needed to give an object with a mass of 25.0 kg an acceleration of 15.0 m/s2 ? Formula F = ma Given: Unknown: m = 25.0 kg a = 15.0 m/sec² F = ? F = (25 kg)(15 m/sec²) F = 375 kg•m/sec² = 375 N

Newton’s Third Law of Motion …to every action force there is an equal and opposite reaction force

Newton’s Third Law "Every action has an equal and opposite reaction."

Action-Reaction forces act on different objects

Hockey Video Clip

Newton’s 3 laws

Forces that oppose motion

Friction

Forces that oppose motion -Friction is a force that opposes motion between two surfaces that are touching each other. Acts in the opposite direction of the motion

-Air resistance is the force air exerts on a moving object Forces that oppose motion -Air resistance is the force air exerts on a moving object

Friction What Causes Friction? Microscopic, electrostatic interactions between contacting surfaces Friction

3 types of Friction 1. 2. 3.

Types of Friction 1. Static – force required to overcome inertia of a stationary object

Static friction- occurs between the surfaces of 2 objects that touch but do not move against each other. Static friction must be overcome for one of the objects to move.

Static Friction (FS)

Not enough static friction

Types of Friction (cont’.) 2. Sliding – force required to keep an object sliding at a constant speed 3. Rolling – force required to keep an object rolling at a constant speed To decrease friction, surfaces can be lubricated with a liquid such as oil.

CLIP The amount of force required to overcome static friction is always greater than the force required to overcome sliding or rolling friction.

Hammer vs Feather - Physics on the Moon Gravity in Space

Pendulum Lab Find the pendulum’s period…time for one swing…for 3 different masses. Questions: How does the mass change the period of a pendulum? Does mass affect the acceleration of a falling object?

What causes object to fall towards the Earth? Does the mass of the object determine how fast it falls?

Misconceptions about falling objects video clip *All objects fall toward the earth at the same rate of acceleration, regardless of their masses! The acceleration which is gained by an object as it falls because of gravitational force SI unit is m/s2 gEarth= 9.8 m/s2 g on different planets Misconceptions about falling objects video clip

GRAVITY!! 1- Masses (greater mass means more gravity) The force of gravity is an attractive force between objects. The amount of GRAVITATIONAL FORCE between objects depends upon their: 1- Masses (greater mass means more gravity) 2- Distance apart (greater distance means less gravity)

The law of Gravitation 26 …states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

Free fall &Terminal Velocity The force of air resistance increases with speed. Terminal Velocity is the speed an object reaches when falling when the force of gravity is balanced by the force of air resistance. The object is no longer accelerating. Objects accelerate towards the Earth until the force of gravity is cancelled or balanced by this air resistance.

Free Fall Practice Ms. Gore drops a rock off a cliff. How fast is it going after 5 sec? (in the absence of air) How long does it take the rock to reach a speed of 78.4 m/s?

Terminal Velocity is the speed an object reaches when falling when the force of gravity is balanced by the force of air resistance. The object is no longer accelerating. A projectile is anything that is shot or thrown through the air, and is only under the force of gravity. Law of Gravatition: states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. Acceleration due to gravity: The acceleration which is gained by an object as it falls because of gravitational force. SI unit is m/s2

Why does the person weight different amounts?

Why is the mass the same but the weight different? Weight is measured in Newton

W =m x g Weight and Gravity Weight is the force of gravity between the earth and a body on its surface. Weight ≠ mass! Weight is measured in Newtons (N). W =m x g BECAUSE IT’S A FORCE!

W= m∙g Gmoon= 1.6 m/s2 60

Could your weight change by simply going to the top of a mountain?

Acceleration due to Gravity (g) g vs. altitude on Earth

Weight W = (m)(g) Ex. Mass = 100kg Acc.= 9.81 m/s2 Weight = 981 N When the mass of an object and the acceleration due to gravity are known, the weight of an object can be calculated. W = (m)(g) Ex. Mass = 100kg Acc.= 9.81 m/s2 Weight = 981 N

Example Problem What is your weight if your mass 4.52 kilograms? Given: Unknown: m = 4.52 kg W = ? g = a = 9.8 m/sec² Formula: W = mg W = (4.52kg)(9.8 m/sec²) W = 44.296 kg•m/sec² = 44.296 N

Finding “g” What is the acceleration due to gravity on Planet X?

Projectile Motion Projectiles follow a curved path because of the Earth’s gravitational pull. They have 2 types of motion that are independent of each other: 1. Horizontal motion 2. Vertical motion A projectile is anything that is shot or thrown through the air, and is only under the force of gravity.

Projectile Motion Which will hit the ground first? A ball that is dropped straight down or a ball that is “shot” out horizontally? * An object launched horizontally will land on the ground at the same time as an object simply dropped from the same height!!!!

Another Example of Projectile Motion

only centripetal is the real force Circular Motion Roller Coaster Clip An object moving in a circular motion at the same speed is accelerating toward the center because its direction is constantly changing. Centripetal acceleration is acceleration toward the center of a curved path. 'Centripetal' comes from the Latin word for 'center seeking' Centrifugal 'force' is really a function of the inertia of the object being pushed into a circle. It is not really a force at all, it is simply the tendency of an object to go in a straight line. only centripetal is the real force

Circular Motion cont.’ Centripetal force is the force that causes a moving object to move in a curved path Without a centripetal force, an object in motion continues along a straight-line path. With a centripetal force, an object in motion will be accelerated and change its direction.

Amusement Park Forces

Review of Newton’s 3 laws http://www.teachertube.com/video/three-laws-of-motion-539