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Chapter 12 Warm Up 11/20 1. Give an example of a unit for time 2. Give an example of a unit for speed 3. Give an example of a unit for velocity 4. Give an example of a unit for acceleration. Forces Part 1 A force is a PUSH or a PULL on an object Causes things to: Move Accelerate Change direction 2 Unit: Newton or kg*m/s Balanced Forces When opposing forces are EQUAL, they are BALANCED and nothing happens Ex: Tug of war with equal teams Arm wrestling with 2 people of equal strength Unbalanced Forces When opposing forces are UNEQUAL, the LARGER one wins Ex: Tug of war with twice as many people on one side Miss J arm wrestling with Thor Friction When objects touch and try to MOVE past each other, the force of FRICTION opposes that -Friction DECREASES as something begins to move FASTER -can happen with any of the states of matter 4 Types of Friction: Static Sliding Rolling Fluid Static Friction • Acts on objects that aren’t moving Ex: Trying to make a wall move Sliding Friction Friction that opposes the motion of the object -smaller than static, use less force to keep an object moving Ex: Pushing a box Sliding a paper Rolling Friction • Acts on rolling objects • A lot less than sliding and static friction Ex: Pushing a cart Rollerblading Fluid Friction • Opposes motion through fluid Fluids= liquids as well as gas mixtures Ex: Running in Water Airplane flying Sailing Which type of does each object have working on it? How can you reduce friction? • Lubricate with something like OIL • ROLLING vs. SLIDING • SMOOTHING surfaces • Decrease FORCES pressing surfaces together Mythbusters: Separating Phone Books • https://youtu.be/AX_lCOjLCTo • https://youtu.be/QMW_uYWwHWQ Warm Up 1. What are the 4 types of friction? 2. Which type of friction happens only with balanced forces? 3. What is the equation to find velocity? Gravity Force between any TWO masses pulling them together • Objects do not need to be in CONTACT and it does not act over LARGE distances • Gravity pulls down toward the center of the OBJECT but there is an equal FORCE pushing back • The amount of force depends on the MASS and DISTANCE between two objects Falling Motion • Things fall at the same ACCELERATION and move CONTINUOSLY unless something slows them down • As things fall through the air, AIR RESISTANCE slows them down • Gravity acts DOWNWARD, air resistance acts UPWARDS and slows acceleration EX: • When objects fall, they ACCELERATE • The faster they fall, the MORE the air resistance • Eventually, they fall no faster (TERMINAL VELOCITY) • Net force = ZERO, the object moves at a constant velocity Equations • Our equations for falling motion are different than horizontal motion. Constant acceleration for gravity = 9.8 m/s2 To calculate velocity, we multiply the acceleration due to GRAVITY and the time of the fall Velocity equation: v = at = (9.8 m/s2)( t ) To calculate distance for falling objects, you will multiply ½, acceleration due to gravity, and time squared. Distance equation: d=½ 2 at = (.5)(9.8 m/s2 )( t2) CALCULATING THE ACCELERATION OF A FALLING OBJECT • Constant acceleration for gravity = 9.8 m/s2 v = at = (9.8 m/s2)( t ) d=½ 2 at = (.5)(9.8 m/s2 )( t2) • When you throw a ball, it follows a CURVED path. When something falls after it is given a forward velocity, it is called a PROJECTILE The only forces acting on projectiles are AIR RESISTANCE and GRAVITY • Once you release a projectile, you no longer have any influence on it. • Has CONSTANT HORIZONTAL VELOCITY(we ignore air resistance) • DOWNWARD velocity is accelerated due to GRAVITY • MYTHBUSTERS: BULLET FIRED VS. SHOT • https://youtu.be/2tiOmp1BE8Y?list=PLqCHwhfQ685Qm4WXfkWKUpe_j75wIgFv Warm Up 11/24 What variable are we solving for in the following questions? 1. How fast? 2. How far? 3. How high? 4. How many seconds? 5. Average velocity? An object starts at rest and falls freely for 8 seconds. a. What is its velocity? b. What is its average velocity? c. What is the distance it falls? A car slowed down from a velocity of 20 m/sec and stopped in 2 seconds. a. What was its deceleration? b. What was its average velocity? c. How far did it go while slowing down? A cliff diver jumped from a cliff and waited 2 seconds before opening his parachute. a. How fast was he going when he released it? b. How far did he fall during the 2 seconds? Warm Up 12.1 Read Section 12.2 on Newton’s 1st and 2nd Laws On a separate sheet of paper: 1. Define Each law 2. Give an example of each law 12.2 Newton’s Laws of Motion Sir Isaac Newton Newton’s st 1 Law Unless a force acts on it, an object will stay at rest and object in motion stays in motion with same speed and direction AKA: Law of Inertia Inertia=tendency of object to resist change in its motion • An object at rest stays at rest • An object in motion stays in motion with the same direction and speed You experience inertia when you ride in a car. When someone slams on the brakes, you lean forward – continue in path of initial force Acceleration of ball is PROPORTIONAL to force acting on it. -If you DOUBLE ACCELERATION, you will double FORCE Swinging ball on string TableCloth Hammer and Mass https://youtu.be/SgAoyHfRd4s Quarter Stack Egg Drop https://youtu.be/6gzCeXDhUAA Raw egg vs. Boiled egg Warm Up 12/1 1. Explain how throwing a baseball follows Newton’s 1st law. 2. How can you increase the acceleration of a baseball in the air? Some things to keep in mind: • How do unbalanced forces affect an object’s velocity and acceleration? • Changes them • If you throw a ball, it accelerates. What happens if you use more force and throw harder? • It accelerates more. • Acceleration of ball is directly proportional to force acting on it. • If you double the force, you will double acceleration. Newton’s nd 2 Law Acceleration of object is equal to force acting on it divided by mass • Doubling the mass cuts the acceleration in half The larger the force acting on the object, the greater the acceleration of the object. The larger an object’s mass, the greater the force needed to give it the same acceleration. • Also works if force acts in direction opposite object’s motion like in seatbelts. • Seatbelt applies force that opposes or restricts motion and decelerates (negative acceleration) passenger. • Newton’s 2nd Law: a = _F_ OR F = ma m a = acceleration F = force m = mass N = kg*m s2 Weight and Mass • Would your weight be the same on the moon? • No. • Would your mass be the same on the moon? • Yes. • Mass: amount of matter in an object; units: g or kg, etc. Weight and Mass – mass does NOT equal weight! -Weight: force of GRAVITY acting on an object -Mass and weight are PROPORTIONAL (Doubling the mass doubles the weight.) Formula: 𝑤 = 𝑚 𝑋 𝑔 w = weight m = mass g = acceleration due to gravity (9.8 m/s2) Math Practice • A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0N? • a = F = 60 N = 1.5 m/s2 m 40 kg Math Practice • What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000N acts on it in an upward direction? • a = F = 10,000 N = 2 m/s2 m 5000 kg Math Practice • An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the formula for force.) • F = ma = 1200 kg * 3.0m/s2 = 3600 N Math Practice • A 25 N force accelerates a boy in a wheelchair at .5 m/s2. What is the mass of the boy and the wheelchair? (Hint: Solve formula for mass.) • m = F = 25 N a .5m/s 2 = 50 kg Warm Up 12/2 A 25 N force accelerates a boy in a wheelchair at .5 m/s2. What is the mass of the boy and the wheelchair? Newton’s rd 3 Law Forces exist in pairs; whenever 1 object exerts force on 2nd object, 2nd object exerts = and opposite force on 1st object • Bumper cars are examples of Newton’s 3rd Law in action. • Also known as Action and Reaction Forces • Ex.: Bumper Cars: Action Force: force of your bumper Reaction Force: force of other car on me • They are equal in size and opposite in direction. • Describe hammer and nail: Action: When hammer contacts nail • Reaction: From nail - Equal and opposite to hammer and ends hammer’s motion • Newton’s 3rd Law • Action-Reaction Forces and Motion • Swimming Action Forces: arms push against water • Reaction Forces: by water on swimmer and pushes her forward • • Not all action-reaction forces lead to motion. Newton’s 3rd Law • Action-Reaction Forces do NOT cancel for the swimmer • Why? • • don’t act on same object (water, person) When would they cancel each other? • If equal and opposite on SAME object • 2 people pushing on a cart with the same force but in opposite directions Newton’s 3rd Law • Ex: Rocket Propulsion How? • 1st: Fuel burns making hot gases. • 2nd: Hot gases expand and push against inside of the rocket and escape to bottom of rocket (action force). • 3rd: Rocket accelerates in opposite direction (reaction force). • • https://youtu.be/gDIL9SdN-ls http://science.nasa.gov/headlines/y2003/28jan_envirorocket.htm Momentum • Definition: product of object’s mass and velocity • Difficult to stop object with large momentum. • • Ex.: Can you stop a speeding bullet? Ex.: Can you stop a speeding bus? • Momentum is large if product of mass and velocity is large. Momentum • At rest, momentum = 0!!! It doesn’t matter how large the object is. •momentum = m * v •p = m * v • • p = momentum; m = mass; v = velocity unit = kg * m/s Momentum • The momentum of a ball dropped from a bridge is 1.5 kg * m/s. Its mass is .25 kg. Find velocity. • p = m * v v = p/m • v = 1.5 kg * m/s = 6 m/s .25 kg Conservation of Momentum • means momentum doesn’t increase or decrease. • Law of Conservation of Momentum: if no net force acts on system then total momentum of system doesn’t change Momentum is neither created nor destroyed. It is transferred between objects. • Momentum can change only if mass, velocity, or both change. • • Key Concept: Loss of momentum of one = gain in momentum of another object. • Momentum is conserved. Metal Balls • https://youtu.be/r8E5dUnLmh4 -car crash • https://youtu.be/OuA-znVMY3I -massive • https://youtu.be/cP0Bb3WXJ_k?list=PLlwib77IOrGGqlNBnl7FstlxnF7QUkYO -explanation • https://youtu.be/sDfsdtRo2PE?list=PLlwib77IOrGGqlNBnl7FstlxnF7QUkYO -pool shots Centripetal Force • center-directed force that continuously changes direction of object to make it move in circle • • • • • Means “toward the center” Ex: Sunglasses slide across dashboard away from driver as make sharp left. Ex: Car rounding curve at proper speed – follows curve easily Ex: Car rounding curve at high speed – Inertia greater and car leaves road at curve Ex: Highways and racetracks are banked. • Satellites in Orbit – need centripetal force provided by gravity and its inertia to stay in orbit https://youtu.be/yyDRI6iQ9Fw Warm Up 12/4 1. What is the equation for momentum? 2. What variable has the unit m/s 3. What variable has the unit kg*m/s 4. What unit does mass need to be in to calculate momentum? Review Problems What is the force required to push a wheelbarrow with a mass 25 kg, and holding 10kg of dirt, at an acceleration of 1.5 m/sec2 ? What is the acceleration of a bowling ball, with a mass of 11,000 grams, if a force of 50 Newton’s is applied? What is the weight on Earth of a girl with a mass of 30kg? A 5 g marble travels with a velocity of 5 m/s, what is the momentum of the marble? The mass of a gerbil is 430 grams and its momentum is 5 kg *m/s forward, what is the velocity of the gerbil?