Forces

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) with a Measured in SI units called newtons (N) with a spring scale spring scale

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

Section 1: The Nature of Force Balanced Forces Balanced Forces Net force = 0 Net force = 0 No motion No motion Practice Calculating Net Force Practice Calculating Net Force

2-3: 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)

2-3: Friction Friction depends on… Friction depends on… Types of surfaces Types of surfaces How hard surfaces push together How hard surfaces push together

2-3: Types of 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

2-3: Types of 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

2-3: Uses of 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

Force Diagrams a graphical illustration used to visualize the applied forces, movements, and resulting reactions on a body. They depict a body or connected bodies with all of the applied forces and moments, as well as reactions, that act on that/those body.

Description of Force F app – Applied force- Applied force to an object by a person or another object. F app – Applied force- Applied force to an object by a person or another object. F grav- Force of gravity-Force which a massively large object attracts another object towards itself. By definition, this is the weight of the object F grav- Force of gravity-Force which a massively large object attracts another object towards itself. By definition, this is the weight of the object

F norm- normal force is the support force exerted upon an object that is in contact with another stable object F frict- Friction force is exerted as an object moves across it. Friction force opposes motion. F air- Air resistance is a special type of frictional force that acts upon objects as they travel through the air.

2.3: 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

Mass vs. Weight Mass vs. 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

2.3: Gravity & Freefall Acceleration 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 )

2.3: Gravity & Freefall

2.3: Free Fall Acceleration For every second an object falls its downward velocity increases by 9.8 m/s For every second an object falls its downward velocity increases by 9.8 m/s

2.3: 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

2.3: 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!

2.3: Terminal Velocity

Section 1: Newton’s 1 st Law Inertia Inertia Tendency of object to resist a change in it’s motion Tendency of object to resist a change in it’s motion

Section 1: Newton’s 1 st Law Inertia depends on Mass Inertia depends on Mass “Amount” of inertia depends on objects’ mass “Amount” of inertia depends on objects’ mass

Section 1: 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. a) gradually stop. b) continue in motion in the same direction at constant speed. b) continue in motion in the same direction at constant speed.

Practice Problem 2 A 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? A 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.

2-1: 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

2-2: Force, Mass & Acceleration 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 Acceleration Force = Mass Acceleration OR Acceleration = Force OR Acceleration = Force Mass Mass FYI, 1 N = 1kg 1 m/s 2 FYI, 1 N = 1kg 1 m/s 2

2-3: Force, Mass & Acceleration

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

2-3: 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 2 m/s 2 = 2000 N 1000 kg 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 15m/s 2 = 825 N 55 kg 15m/s 2 = 825 N

Calculating Force IF a 5 kg ball is accelerating 1.2 m/s 2, what is the force acting on it? 6 kg m/s 2 6N

Calculating Force A person on a skateboard is accelerating 2 m/s 2. If the person has a mass of 50 kg, how much force is acting on that person? A person on a skateboard is accelerating 2 m/s 2. If the person has a mass of 50 kg, how much force is acting on that person? 100 kg m/s kg m/s N 100 N

Calculating Acceleration Half the people on the team decide not to pull the airplane. The combined force of those left is 4500 N, while the airplane’s mass is still 30,000 kg. What will be the acceleration? Half the people on the team decide not to pull the airplane. The combined force of those left is 4500 N, while the airplane’s mass is still 30,000 kg. What will be the acceleration?.15 m/s 2.15 m/s 2

Calculating Acceleration A girl pulls a wheeled backpack with a force of 3 N. If the backpack has a mass of 6 kg, what is its acceleration? A girl pulls a wheeled backpack with a force of 3 N. If the backpack has a mass of 6 kg, what is its acceleration? 0.5 m/s m/s 2

Calculating Mass A model rocket is accelerating at a rate of 3 m/s 2 with a force of 1 N. What is the mass of the rocket? A model rocket is accelerating at a rate of 3 m/s 2 with a force of 1 N. What is the mass of the rocket?.3 kg.3 kg A boy pushes a shopping cart with a force of 10 N, and the cart accelerates 2 m/s 2. What is the mass of the cart? A boy pushes a shopping cart with a force of 10 N, and the cart accelerates 2 m/s 2. What is the mass of the cart? 5 kg 5 kg

Centripetal Force Any force that keeps an object moving in a circle. Any force that keeps an object moving in a circle. Without centripetal force, an object would go flying off in a straight line. Without centripetal force, an object would go flying off in a straight line.

If you were twirling a ball on a string, the centripetal force keeps it going in a circle If you were twirling a ball on a string, the centripetal force keeps it going in a circle IF you were to let go of the string, the ball would go flying off in the direction it was headed when you let go. IF you were to let go of the string, the ball would go flying off in the direction it was headed when you let go.

2-4: Newton’s 3 rd Law Newton’s 3 rd law of Motion Newton’s 3 rd law of Motion For every action, there is an equal and opposite reaction For every action, there is an equal and opposite reaction “Action” & “Reaction” are names of forces “Action” & “Reaction” are names of forces

2-4: Action & Reaction Forces Forces ALWAYS occur in pairs. Forces ALWAYS occur in pairs. Single forces NEVER happen Single forces NEVER happen 2 objects are involved in every force 2 objects are involved in every force Action force: “A pushes B” Action force: “A pushes B” Reaction force: “B pushes A” Reaction force: “B pushes A”

2-4: “Equal” Forces In Newton’s Third Law, “equal” means: In Newton’s Third Law, “equal” means: Equal in size Equal in size Equal in time Equal in time “Opposite” Means: “Opposite” Means: Opposite in direction Opposite in direction

2-4: Possible Problems Don’t Action & Reaction forces cancel each other? Don’t Action & Reaction forces cancel each other? Action & Reaction forces act on DIFFERENT objects Action & Reaction forces act on DIFFERENT objects In “Net force” problems, we are talking about opposing forces acting on the SAME object In “Net force” problems, we are talking about opposing forces acting on the SAME object

2-4: Try These!! If forces are equal and opposite why don't they cancel each other out? 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. 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? If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction? Different accelerations are obtained when the objects have different masses. 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? 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. Some what of a tricky question, they both experience the SAME force.

Why does the force have a greater effect on the bug? 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. 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.

Momentum A measure of mass in motion A measure of mass in motion The greater the mass the greater the momentum The greater the mass the greater the momentum Momentum is similar to inertia in that it depends on its mass Momentum is similar to inertia in that it depends on its mass Unlike inertia, momentum also considers how fast the object is moving. Unlike inertia, momentum also considers how fast the object is moving.

To Calculate Momentum Momentum = mass X velocity Momentum = mass X velocity p = mv p = mv Momentum is a vector and has both size and direction Momentum is a vector and has both size and direction

Practice Problems Which has more momentum, a 1000 kg car moving 1 m/s or a 70 kg person sprinting at 8 m/s? Which has more momentum, a 1000 kg car moving 1 m/s or a 70 kg person sprinting at 8 m/s? A ball is moving at 7.0 m/s and has a momentum of 100 kgm/s. What is the ball’s mass? A ball is moving at 7.0 m/s and has a momentum of 100 kgm/s. What is the ball’s mass?

Practice A 1.5 kg ball is thrown at 10 m/s. What is the ball’s momentum? A 1.5 kg ball is thrown at 10 m/s. What is the ball’s momentum? Your brother’s mass is 40 kg and he has a 1.30 kg skateboard.What is the combined momentum of your brother and his skateboard they are going 8.5 m/s. Your brother’s mass is 40 kg and he has a 1.30 kg skateboard.What is the combined momentum of your brother and his skateboard they are going 8.5 m/s.

Which has more momentum: a 250 kg dolphin swimming at 4 m/s or a 350 kg manatee swimming at 2 m/s? Which has more momentum: a 250 kg dolphin swimming at 4 m/s or a 350 kg manatee swimming at 2 m/s?

The Shinkansen, Japan’s high-speed “bullet train,” consists of several different versions of trains that have been in service since The 100- series trains consists of 16 steel cars that have a combined mass of 9.25 x 10 5 kg. The top speed of a regular 100-series train is 220 km/h. What would be the momentum of one of these trains

Lab: The Nail Challenge! Objective: Objective: Balance nails on single nail head Balance nails on single nail head 1 st & 2 nd highest # for each period gets bonus coupon 1 st & 2 nd highest # for each period gets bonus coupon Work in pairs Work in pairs