Section 1: The Nature of Force What is a force? A push or pull on an object Has both Size & Direction Size: Measured in SI units called newtons (N) Spring Scale 1 N = about 1/5th pound (lb)

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

What does balanced forces mean? Net force = 0 No motion 1.Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge? 2.Draw arrows showing the individual and combined forces of the tugboats in #1. 3.Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge? 4.Draw arrows showing the individual and combined forces of the tugboats in #3. 5. Two dogs pulling on a rope (tug of war) 10N in to the right the other dog pulls 12 N to the left. Draw a diagram showing their forces. What is the net force (size & direction)

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

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

What does Friction depend on? Friction depends on… Types of surfaces How hard surfaces push together The amount of friction depends on the force pushing the surfaces together. If this force increases, the hills and valleys of the surfaces can come into closer contact. The close contact increases the friction between the surfaces. Objects that weigh less exert less downward force than objects that weigh more, as shown on the next slide.

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

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

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

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

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

Only force acting on an object is gravity What is Free fall? Free Fall Only force acting on an object is gravity Objects in free fall accelerate as they fall All objects free fall at the same rate (9.8 m/s2) If you drop an elephant and a piece of paper at the same time law of gravity says they should hit the ground at the same time….why don’t they? They aren’t in free fall – other forces are acting on them (air friction) - But the elephant has more surface area therefore there will be more air friction acting on it….  yes, but this is offset by the much greater weight (more force of gravity acting on it) so it falls faster even if it experiences more air resistance! - DEMONSTRATE – drop 2 sheets of paper 1 flat, 1 on it’s side - Parachutes create lots of air resistance slowing object down.

Gravity & Freefall Recall Newton’s 2nd law of motion – what is the formula for calculating the relationship between acceleration, mass and force? Remember that gravity depends on mass (more mass = more gravity force)

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

What is Terminal Velocity? As an object falls it picks up speed Increased speed  increased air resistance Eventually force of air resistance = force of gravity  TERMINAL VELOCITY Object stops accelerating! If you drop a marble and a piece of paper at the same time law of gravity says they should hit the ground at the same time….why don’t they? They aren’t in free fall – other forces are acting on them (air friction) - Parachutes create lots of air resistance slowing object down.

Terminal Velocity

Section 3: Newton’s First and Second Laws What is Inertia? Tendency of object to resist a change in it’s motion When treadmill stops (you keep going), blood rushes to your head when in elevator

What does inertia depend on? Inertia depends on Mass “Amount” of inertia depends on objects’ mass When treadmill stops (you keep going), blood rushes to your head when in elevator

Newton’s 1st Law of Motion What is Newton’s 1st Law? Newton’s 1st Law of Motion Object at rest will remain at rest Object in motion will remain in motion unless acted on by an unbalanced force. ** Force is NOT needed to keep an object in motion (assuming no friction is resisting motion)

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: 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? 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.

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

What is Newton’s 2nd law of motion? Force, Mass & Acceleration are related Force = Mass X Acceleration OR Acceleration = Force ÷Mass FYI, 1 N = 1kg X 1 m/s2

Force, Mass & Acceleration

Force, Mass & Acceleration A 52 kg water skier is being pulled by a speedboat. The force causes her to accelerate @ 2 m/s2. Calculate the FORCE that causes this acceleration. F = 52 kg x 2 m/s2 = 104 kg x m/s2 = 104 kg*m/s2 = 104 N

Force, Mass & Acceleration What is the force on a 1000 kg elevator accelerating at 2 m/s2? 1000 kg X 2 m/s2 = 2000 N How much force is needed to accelerate a 55 kg cart at 15 m/s2? 55 kg X 15m/s2 = 825 N

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

How do forces always occur? Forces ALWAYS occur in pairs. Single forces NEVER happen 2 objects are involved in every force Action force: “A pushes B” Reaction force: “B pushes A” “For every action force, there is … a reaction force” means: Forces ALWAYS occur in pairs. Single forces NEVER happen Since a force is an interaction between objects, two objects are involved in every force. Call the objects A and B: Action force: “A pushes B” Reaction force: “B pushes A”

What do equal and opposite mean? In Newton’s Third Law, “equal” means: Equal in size Equal in time “Opposite” Means: Opposite in direction The action & reaction forces are EXACTLY the same size. The action & reaction forces occur at EXACTLY the same time. In Newton’s Third Law, “opposite” means: Opposite in direction The action and reaction forces are EXACTLY 180o apart in direction.

Don’t Action & Reaction forces cancel each other? Action & Reaction forces act on DIFFERENT objects In “Net force” problems, we are talking about opposing forces acting on the SAME object Two Logical Difficulties If Newton’s Third Law action & reaction forces are equal and opposite, how come they don’t always cancel, making net force and acceleration impossible? If the Newton’s Third La Only forces pushing or pulling on an object affect the object’s motion. Only forces that act on the same object can cancel. Newton’s Third Law action and reaction forces act on different objects, so they don’t cancel. w action and reaction forces are always equal and opposite, how do two objects of different sizes get different accelerations in the same interaction? (When a bug hits a windshield, different things happen to the bug and windshield.)

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.