What is Force? 2.2 p. 32-38 Mr. Richter
Agenda • Warm-Up • Intro to Acceleration (to the hallway!) • What is Acceleration? • Notes: • Definition of Acceleration • Units of Acceleration (What Acceleration really is) • Calculating Acceleration • Review HW Problems • Graphs of Acceleration (time Permitting)
Agenda Day 2 and 3 Tomorrow Tuesday Acceleration Lab • What is Force? • Newton’s Second Law (N2L) • Force and energy.
Objectives: We Will Be Able To… • Define and calculate acceleration. • Explain the relationship between force, mass, and acceleration. • Determine mass, acceleration or force given two of the quantities.
Warm-Up: • To the right is a Velocity vs. Time (VvT) graph of an object that starts at the origin. • Describe the motion of the object in one complete sentence. • Sketch the Position vs. Time graph you think describes the object Speed Time
Warm Up Velocity vs. Time Position vs. Time Speed Position Time Time
What is Acceleration? • What comes to mind when I say the word acceleration? • Discuss at your tables for 1 minute and then we will discuss as a class.
Definition of Acceleration • Your book: • Acceleration is the rate at which your speed increases. • Real definition: • Acceleration is the rate of change of an object’s motion. • What’s the difference? • Both have indicate that a change in speed is acceleration. • The real definition implies that a change of direction is also a form of acceleration, even if speed does not change.
Units of Acceleration • Acceleration is the rate of change of speed relative to time, so the calculation looks something like: • This means the units of acceleration are any unit of speed divided by the unit of time it takes to change that speed. • Can we think of some possible units of acceleration?
Units of Acceleration • The most common unit of acceleration we will use in this class is m/s2 (“meters per second squared”) • If acceleration is the rate of change in velocity [m/s] relative to time [s]… • The units for acceleration are: • But we really mean “meters per second per second”, or a change in meters per second (velocity) every second (time). • Therefore, 50 m/s2 really means a 50 m/s change in velocity every second.
Units of Acceleration: Practice 30 km/h/sec An increase of 15 m/s2 (= 15 m/s/s) An increase of 15 m/s after 1 second 30 m/s after 2 seconds 45 m/s after 3 seconds 150 m/s after 10 seconds Put it in a complete sentence. • 30 km/h after 1 second • 60 km/h after 2 seconds • 90 km/h after 3 seconds • 300 km/h after 10 seconds • Put it in a complete sentence.
Calculating Acceleration • Speed is how fast position changes, or the change in position divided by time. • Acceleration is how fast speed changes, therefore: • What if acceleration is negative? • Slowing down as opposed to speeding up. • Sometimes called deceleration.
Let’s Review Your Homework Problems • Remember to use the 5-step approach. • Looking for? • Given. • Formula • Solve • Check
Homework • p. 55 Section 2.2 • #3-6
Force • Acceleration is the change in an object’s motion. • To change an object’s motion, a force is needed. • A force is any push or pull, or any action that has the ability to change an object’s motion. • It can either change the speed of an object. • Or change an object’s direction.
Force, Mass and Acceleration • Reminder: mass is the amount of matter (atoms) in an object. • The amount of force required to accelerate an object depends on: • the amount of mass an object has • the rate of acceleration the object reaches • The is the basis of Newton’s Second Law
Newton’s Second Law (N2L) • Isaac Newton discovered that the amount of force required to move an object is proportional to the amount of mass the object has, and the amount of acceleration of the object. • In other words: • F = ma • force = mass*acceleration • Your book:
Newtons: Units of Force • Force = mass*acceleration • = [kg]*[m/s2] • We’re too lazy to write this, so we use Newtons to abbreviate. • 1 Newton [N]= 1 kg*m/s2 • Newtons are very small units of force. • Mr. Richter has a mass of about 95 kg. It takes 95 N of force for Mr. Richter to accelerate at 1 m/s2 • Mass resists acceleration • Force cause acceleration
Net Force • The net force is another way of saying the sum of all of the forces acting on an object. • If the forces are in the same direction, they are added together. If they are in opposite directions, they are subtracted.
Net Force and Acceleration • If there is a net force acting on an object ,there is also an acceleration. • Similarly, if there is no net force acting on an object, then there is no acceleration. • NOTE: this is not the same as no speed. • An object can still be moving even if it is not acceleration. • Constant speed means no acceleration which means no net force.
Calculations with N2L: Practice • A car has a mass of 1,000 kg. If a net force of 2,000 N is exerted on the car, what is its acceleration? • 2 m/s2
Force and Energy • Force is the action through which energy moves. Or: • Forces are created every time there is a difference in energy between a high-energy situation and a low-energy situation. • For an object to move from a high energy situation to a low energy situation, there must be a force acting on the object. • Think, what force moves a car from the top of a hill to the bottom?