Newton’s 2nd Law Force and Acceleration Ch

Let’s Review: Acceleration Who remembers what acceleration is? The rate at which velocity is changing Is this an increase or decrease in speed? –Both! What else can change acceleration? –Change in direction Acceleration = CHANGE in velocity/change in time

Objectives 1.Relationship between acceleration and net force 2.Relationship between acceleration and mass 3.Explain Newton’s 2nd Law of Motion

Acceleration Force causes acceleration Acceleration is proportional to net force a  F “  ” doesn NOT mean = What does “proportional” mean?

Acceleration Let’s take a look at the air track again…

Acceleration Formula One car traveling at constant speed of 185mph in a straight line. What is the acceleration? Blue-Footed Booby standing on a rock. What is the acceleration? In both cases a = 0

Zero Acceleration Zero acceleration applies both to the state of rest and constant velocity. No change in state of motion. Why? Forces that are acting on it are balanced –There’s exactly enough friction to counteract the force -->... Zero net force means zero acceleration! Zero acceleration is the evidence of zero net force.

Net Force Roll ball across floor –Assume that velocity is constant What is the net force here? 0 N! If no acceleration, then no net force. Suppose I pushed the ball with a force of 5 N. What would the friction be if the ball maintains a constant velocity? 5 N (in the opposite direction of ball) 5 N of force and 5 N of friction cancel each out and you have constant velocity.

Mass and Acceleration Mass resists acceleration Student/chair demo Toy car-rubber band demo More mass --> less acceleration The same force applied to twice as much mass results in only 1/2 the acceleration For 3 times the mass, only 1/3 the acceleration results Acceleration is inversely proportional to mass –Mass increases --> acceleration decreases Acceleration = 1/mass Values change in opposite directions (plug in #’s)

Check Question Suppose the pilot announces that the plane is flying at a constant 900 km/h and the thrust of the engines is a constant 80,000 N. What is the acceleration of the airplane? Zero, b/c velocity is constant What is the combined force of air resistance that acts on the plane’s outside surface? 80,000 N, to produce a zero net force. If resistance were less, the plane would speed up; if it were more, the plane would slow down.

thrust drag lift weight 80,000 N The acceleration is constant! LET’S DO WORKSHEET 5-1. FIRST SIDE ONLY.

Nonaccelerated Motion I’m going to pull a mass across table at a constant velocity (volunteer to read Newton scale) What is the acceleration? –0, no change in velocity once the mass is moving steadily What is the applied force? What is the value of the net force acting on the mass? –0, as evident by the non-changing velocity

Nonaccelerated Motion continued There are people out there who can look at this demo and tell me how much friction is acting on this object, as long as there is no acceleration. Are you sitting next to one of those people? This is zero net force on a moving system –Zero acceleration applies to state of rest and state of constant velocity The forces acting on the book balance to zero Zero acceleration is evidence of zero net force

Newton’s Discovery (This is going to take some brain power…) Galileo discovered that two objects will fall at the same time –Drop a book and a pen –But he didn’t know why… Newton put it all together One of the most powerful laws in physics! I’m going to try and explain the relationship between force, mass, and acceleration. (2 kids) More on this later with free fall…

Newton’s 2nd Law If you place force on an object, it will accelerate in the direction you push it –It accelerates in the direction you push it Acceleration is directly proportional to force –If you push it twice as hard, it accelerates twice as much Acceleration is inversely proportional to the mass of an object –If it gets twice the mass, it accelerates half as much acceleration = force/mass, –a = f/m Force = (mass)(acceleration) –F = ma

Your experience Take a minute to think of a situation you have had that illustrates Newton’s second law of motion and write it down. Then share it with your neighbor...

Newton’s 2nd Law a = f/m What do we notice about this equation? –Plug in the number 5kg for mass and 10N for force –Now plug in 20N for force. What happens to acceleration? Doubling the net force, doubles the acceleration (show) Doubling the mass, halves the acceleration Pushing an elephant –cd-rom (Hewitt)

Practice Problems For our purposes, let’s always use the equation, F = ma, and sub numbers into parts of the equation. Remember –F is always in Newtons –m is always in grams –a is always m/s 2 Let's practice