Transcript Chapter 2
Chapter 2 Newton’s First Law: The Law of Inertia 1. Aristotle: Two Classifications • Natural: motion directed up or down; natural for heavy objects to fall faster than lighter objects • Ex: Boulder falls toward earth, Smoke rises • Unnatural: motion that requires forces like people or animals use • Ex: Getting a cart to move, pushing a box, pulling a handle 2. Galileo: force needed to start movement, all things fall at same rate 3. Basis for idea of INERTIA • Things in motion stay in motion, things at rest stay at rest • Speed = distance/time – *A car goes 100 miles in 4 hours. What is the average speed the car traveled? • 25 miles per hour (mph) • Velocity = Speed + Direction – *Using the example above, make the speed a velocity 100 miles 4 hours Constant Speed vs. Constant Velocity • Constant Speed = no change in speed, object does not speed up or slow down • Constant Velocity = no change in speed or direction • SO… *Can you have constant speed traveling in a circle? What about constant velocity? Motion • What is motion? How do you know that something is moving? • Are you moving right now? • Looking at Motion & Inertia: – Place the index card flat on your cup – Place the penny on top of the card – Now, pull the card out from under the penny without moving the penny – Can you do it? Why or Why not? Newton’s First Law: Law of Inertia • Every object continues in a state of rest, or in a state of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it • In other words: With no force exerted on it, an object in motion remains in motion in a straight line, an object at rest stays at rest • Example: penny on a card, dishes on a table, toilet paper/paper towels, car accident Inertia • *If there are no other forces acting on the object, does it ever stop moving? – No • *In real life, what force often causes objects to slow down and eventually stop? – Friction Example • When the space shuttle moves in a nearly circular orbit around the earth, is a force needed to maintain its high speed? – No, the shuttle continues in orbit because no forces act on it…gravity keeps it in orbit • If the force of gravity were suddenly cut off, what type of path would the shuttle follow? – Straight Equilibrium • Equilibrium = state of no change – ∑F = 0 – The sum of the forces equals zero • Moving Equilibrium – How can an object be at equilibrium and be moving at the same time? – No CHANGE in speed! • Can an object with only one force acting on it be in equilibrium? Why or why not? – No, because forces must be balanced, opposite and equal, for an object to be in equilibrium Equilibrium Example • Consider the boy pulling the wagon at left. Assume he is pulling only horizontally( ) on the wagon. If the speed he pulls the wagon at is constant, what does that mean about the forces? – They must be equal! • What are the forces acting on the wagon? Friction Pulling Force Equilibrium Examples • If a gymnast hangs with all her weight on one ring, how much force must be on the ring? – Her weight! • If the gymnast weighs 200 Newtons, how much force is on the rope? – 200 Newtons Equilibrium Examples • If the gymnast now hangs with her weight divided evenly between the two rings, how would the amount of force from each ring compare with her weight? – They would add up to be equal • Fred and Barney are painting the side of the building supported by a scaffold. Draw arrows showing the forces at work to keep the scaffold from falling. • The tension in the ropes is equal. How do we know that?...How would the picture look different if the tension was not equal? Example: Moving Equilibrium • Let’s say this girl is trying to move her playhouse. If there are 75N of friction, how much energy must she exert to move the house at a constant speed? Practice • Equilibrium WS – Due Wednesday • Book Tour is also due tomorrow! Forces • The books on a desk are at rest, which means they are in equilibrium. What forces are acting on the books? Support Force • The upward force acting on an object, supporting it • Also called the normal force • The support force must equal the weight of the object • Overall, what is the NET force acting on the books? – If they are at rest, they are in equilibrium, so the NET force is zero! Net Force • Force = Net Force = All the forces acting on an object, measured in Newtons (N) • If one person pulls on the box with 6N and the other person pushes in the same direction with 6N, what is the NET force? – 12N in the direction they are pushing/pulling – Arrows are used to show force because it is a vector. What does that mean? – Fill in the table in your notes showing Net Force Draw the following example • An object has a 12N force pulling up on it and a 4N force pulling down on it. Draw the forces and the net force. Which direction would the object move? The Earth Moves Around the Sun • Before the 16th century, people believed the sun orbited the earth. They used the example of a bird sitting on a tree as proof. If the bird wants the worm below, and the earth were moving, the worm would move away once the bird dropped out of the tree and no longer be below the bird….Why is this idea wrong? • How does inertia apply? Appendix C: Vectors What is the difference between speed and velocity? Vectors vs. Scalars • Vector – has magnitude and direction • Scalar – has magnitude only • Vectors must be represented by arrows – Length tells you magnitude – Arrow tells you direction Adding Vectors • To add two vectors 1. Make a paralellogram 2. Draw a diagonal from the origin to the other side of the paralellogram 3. The diagonal is the sum of the vectors 1 2 Resultant Finding the Parts of a Vector 1. Draw a dotted line through the tail end of the vector 2. Draw another dotted line at a right angle to the first 3. Make a rectangle…the diagonal should hit the corner of the rectangle 4. Draw in the two components Examples #2: Is it easier to push or pull a wheelbarrow over a step? We can find out using a vector diagram… Pushing: Part of the force is downward…Does that help you move over the step? Pulling: Part of the force is upward…Does that help move over the step? Sail Boat » This boat is trying to get to a point at the other side, but the current is taking it downstream. Where will the boat end up? Practice • WS Due Tomorrow!! Ch. 2 Quiz tomorrow, too! • Use the examples we’ve done and the book to help you with the worksheet • You may work with those sitting around you, but still do your own work!