Physical Science Chapter 12 Force

Ball demos Ball on table Rolling

Net Force More than one force can act on an object Sum of all forces = net force Do not write: Examples of 2 forces in 1D (same direction, direction, net=0) Chapter 12: Force Lesson1: What is force and Newton’s 1 st and 2 nd laws What is Force? A push or a pull Is force a vector? Balanced and Unbalanced Force Balanced forces add up to zero Unbalanced do not (push desk w/ student)

Newton’s 1 st Law of Motion: Inertia Simple: Objects at rest tend to stay at rest and objects in motion tend to stay in motion More complex: Objects at rest tend to stay at rest and objects in motion tend to stay in motion unless acted on by a force Even more complex: Objects continue to stay at rest or in motion in a straight line with constant speed unless net force is NOT zero

What is a force (Take 2)? Something that causes an object to change its motion (or velocity) – Causes something: Speed up Slow down Change direction Inertia How hard it is to change an objects motion (velocity)

I’ve never seen anything move w/o slowing down Pioneer and voyager – 1970s Question: Do objects always move in the same direction as force?

Newton’s 2 nd Law Force, Mass and Acceleration In Newton’s First Law, we mention, changing motion or velocity. What do we call changing velocity? More mass  __________ to accelerate Acceleration is “inversely proportional” to mass – Mass goes up, acceleration goes ______ – Mass goes down, acceleration goes ___ Force and Acceleration Push harder  _______ acceleration Acceleration is “directly proportional” to force – Force goes up, acceleration goes __ – Force goes down, acceleration goes _____

Newton’s 2 nd Law Force, Mass and Acceleration In Newton’s First Law, we mention, changing motion or velocity. What do we call changing velocity? More mass  harder to accelerate Acceleration is “inversely proportional” to mass – Mass goes up, acceleration goes down – Mass goes down, acceleration goes up Force and Acceleration Push harder  more acceleration Acceleration is “directly proportional” to force – Force goes up, acceleration goes up – Force goes down, acceleration goes down

Newton’s 2 nd Law

Do we need Newton’s 1 st Law?

How does this explain free fall?

Units of Force

Example Problems A 5Kg block is accelerating at 10m/s 2. – What is the force on the block? A force of 20 Newtons is applied to a 10Kg block – What is its acceleration? A force of 10 N is applied to a block and it accelerates at 5 m/s 2. – What is the block’s mass?

Define Mass with respect to Newton’s laws and intertia

Do now A 20 Kg object has a sliding force of friction of 100 N. How hard would you have to push it to accelerate at a rate of 10 m/s 2 ? An object has a sliding force of friction of 300 N. When pushed with a force of 800N it accelerates at a rate of 10m/s 2. What is the mass of the object?

Lesson 3 Newton’s 3 rd Law

Objectives At the end of this lesson you will be able to: – Explain Newton’s 3 rd law: equal and opposite forces

Newton’s 3 rd Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. Sometimes stated: every action has an equal and opposite reaction

In other words Nothing can ever push something else – things only push against each other in pairs

Questions A 2000Kg truck hits a 500Kg car. The truck exerts a 1000N force on the car. What is the force on the truck?

Question So, if 2 things push off each other, do they always accelerate at the same rate?

Answer No F=m*a The more mass, the less acceleration for the same force

Question If one thing moves, does something else always have to move in the opposite direction?

Answer No One thing may already be moving and it will just slow down

Don’t get confused Some people have asked: if there are equal and opposite forces, why does the ball move at all? There are not equal and opposite forces ON the ball. There is only one force acting on the ball. The “action” acts on the ball. The “reaction” acts on the bat.

Question Whenever one thing moves (or accelerates), does something else also have to move (or accelerate) in the opposite direction?

Answer Yes Many things that move are pushing off of the earth When we run or drive a car we exert a force on the earth in the opposite direction The Earth has so much mass (F=m*a) so Earth’s acceleration in almost nothing

Question What about when we hit a baseball? The ball exerts a force on the opposite direction on Earth through the batter

Question If you were stopped in outer space, how could you move if all you had was a baseball?

Question How do rockets move?

Weight  Mass Weight is force due to gravity Force = mass * acceleration Acceleration from gravity = 9.8m/s 2 Weight [N] = mass [Kg] * 9.8m/s 2 Units of weight will be Newtons [N] Units of mass will be kilograms [Kg]

Example Problems How much does 5Kg of cheese weigh? What is the mass of 200 N of potatoes?