Section 1: The Nature of Force What is a force? Force Force A push or pull on an object A push or pull on an object Has both Size & Direction Has both Size & Direction Size: Size: Measured in SI units called newtons (N) Measured in SI units called newtons (N) Spring Scale Spring Scale

How do you combine forces? Direction: Direction: Same direction: Add (+) Same direction: Add (+) Diff. direction: Subtract (-) Diff. direction: Subtract (-) Net force = combination of all forces acting on object Net force = combination of all forces acting on object Unbalanced Forces Unbalanced Forces Result in motion Result in motion

What does balanced forces mean? Balanced Forces Balanced Forces Net force = 0 Net force = 0 No motion No motion

Lab: The Nail Challenge! Objective: Objective: Balance nails on single nail head Balance nails on single nail head Work in pairs Work in pairs

Section 2: Friction and Gravity What is friction? Friction Friction Resistance to motion Resistance to motion Opposite direction of travel Opposite direction of travel Caused when 2 surfaces rub together Caused when 2 surfaces rub together resistive force (slows down objects) resistive force (slows down objects)

What does Friction depend on? Friction depends on… Friction depends on… Types of surfaces Types of surfaces How hard surfaces push together How hard surfaces push together

What are sliding and rolling friction? Types of Friction Types of Friction Sliding Friction: solid surfaces slide over each other Sliding Friction: solid surfaces slide over each other Rolling Friction: object rolls over surface Rolling Friction: object rolls over surface

What are fluid and static friction? Types of Friction Types of Friction Fluid Friction: object moves through fluid (or air) Fluid Friction: object moves through fluid (or air) Static Friction: objects not moving Static Friction: objects not moving

What are some uses for friction? Is Friction harmful or helpful? Is Friction harmful or helpful? Ways to reduce friction Ways to reduce friction Ways to increase friction Ways to increase friction

What is a gravitational force? Gravitational Force Gravitational Force Force of attraction between 2 objects Force of attraction between 2 objects Pulls things toward each other Pulls things toward each other Depends on: Depends on: Mass Mass Distance Distance

What is the difference between mass and weight? Mass Mass Amount of matter Amount of matter Same no matter where you are Same no matter where you are SI units = kilograms (kg) SI units = kilograms (kg) 1 kg = 1000 grams (g) 1 kg = 1000 grams (g) Weight Weight Force of gravity Force of gravity SI units = newtons (N) SI units = newtons (N) Depends on where you are Depends on where you are

What is Free fall? Free Fall Free Fall Only force acting on an object is gravity Only force acting on an object is gravity Objects in free fall accelerate as they fall Objects in free fall accelerate as they fall All objects free fall at the same rate (9.8 m/s 2 ) All objects free fall at the same rate (9.8 m/s 2 )

Gravity & Freefall

What is Air resistance? Air resistance Air resistance Type of fluid friction Type of fluid friction Opposes motion of objects through air Opposes motion of objects through air Depends on: Depends on: Size, Shape, Speed Size, Shape, Speed

What is Terminal Velocity? Terminal Velocity Terminal Velocity As an object falls it picks up speed As an object falls it picks up speed Increased speed  increased air resistance Increased speed  increased air resistance Eventually force of air resistance = force of gravity  TERMINAL VELOCITY Eventually force of air resistance = force of gravity  TERMINAL VELOCITY Object stops accelerating! Object stops accelerating!

Terminal Velocity Terminal Velocity

Section 3: Newton’s First and Second Laws What is Inertia? Inertia Inertia Tendency of object to resist a change in it’s motion Tendency of object to resist a change in it’s motion

What does inertia depend on? Inertia depends on Mass Inertia depends on Mass “Amount” of inertia depends on objects’ mass “Amount” of inertia depends on objects’ mass

What is Newton’s 1 st Law? Newton’s 1 st Law of Motion Newton’s 1 st Law of Motion Object at rest will remain at rest Object at rest will remain at rest Object in motion will remain in motion unless acted on by an unbalanced force. Object in motion will remain in motion unless acted on by an unbalanced force.

Practice Problem 1 Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: a) gradually stop. b) continue in motion in the same direction at constant speed.

Practice Problem 2 An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction? An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction? 0 N (no force) 0 N (no force)

Practice Problem 3 Ben Tooclose is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion. Ben Tooclose is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

Newton’s 1 st Law Review Unbalanced force from another car changes your CAR’s motion Unbalanced force from another car changes your CAR’s motion You continue as before until your seatbelt changes YOUR motion You continue as before until your seatbelt changes YOUR motion

What is Newton’s 2 nd law of motion? Newton’s 2 nd law of Motion Newton’s 2 nd law of Motion Force, Mass & Acceleration are related Force, Mass & Acceleration are related Force = Mass X Acceleration OR Acceleration = Force ÷Mass Force = Mass X Acceleration OR Acceleration = Force ÷Mass FYI, 1 N = 1kg X 1 m/s 2 FYI, 1 N = 1kg X 1 m/s 2

Force, Mass & Acceleration

A 52 kg water skier is being pulled by a speedboat. The force causes her to 2 m/s 2. Calculate the FORCE that causes this acceleration. A 52 kg water skier is being pulled by a speedboat. The force causes her to 2 m/s 2. Calculate the FORCE that causes this acceleration. F = 52 kg x 2 m/s 2 F = 52 kg x 2 m/s 2 = 104 kg x m/s 2 = 104 kg*m/s 2 = 104 kg x m/s 2 = 104 kg*m/s 2 = 104 N = 104 N

Force, Mass & Acceleration Force, Mass & Acceleration What is the force on a 1000 kg elevator accelerating at 2 m/s 2 ? What is the force on a 1000 kg elevator accelerating at 2 m/s 2 ? 1000 kg X 2 m/s 2 = 2000 N 1000 kg X 2 m/s 2 = 2000 N How much force is needed to accelerate a 55 kg cart at 15 m/s 2 ? How much force is needed to accelerate a 55 kg cart at 15 m/s 2 ? 55 kg X 15m/s 2 = 825 N 55 kg X 15m/s 2 = 825 N

Section 4: Newton’s Third Law What is Newton’s 3 rd Law? Newton’s 3 rd law of Motion Newton’s 3 rd law of Motion For every, there is an and reaction For every action, there is an equal and opposite reaction “Action” & “Reaction” are names of “Action” & “Reaction” are names of forces

How do forces always occur? Forces occur in pairs. Forces ALWAYS occur in pairs. Single forces happen Single forces NEVER happen objects are involved in every force 2 objects are involved in every force Action force: “A pushes ” Action force: “A pushes B” Reaction force: “ pushes A” Reaction force: “B pushes A”

What do equal and opposite mean? In Newton’s Third Law, “equal” means: In Newton’s Third Law, “equal” means: Equal in Equal in size Equal in Equal in time “Opposite” Means: “Opposite” Means: Opposite in Opposite in direction

Don’t Action & Reaction forces cancel each other? Action & Reaction forces act on objects Action & Reaction forces act on DIFFERENT objects In “ force” problems, we are talking about forces acting on the object In “Net force” problems, we are talking about opposing forces acting on the SAME object

Try These!! If forces are equal and opposite why don't they cancel each other out? They occur on two different objects. Forces can only cancel out when the forces are acting on the same object.

If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction? (Remember F=ma) Different accelerations are obtained when the objects have different masses.

When a small bug is splattered across a fast moving windshield what experiences more force- the bug or the windshield? Some what of a tricky question, they both experience the SAME force.

Why does the force have a greater effect on the bug? Because the bug's mass is much much smaller than the car's, it will experience a much greater change in acceleration than the car. This change in acceleration over a very small fraction of time is why the bug experiences a greater effect.