Chapter 2 Moving Objects Motion Speed Instantaneous Speed Constant Speed Average Speed Velocity Acceleration Force Net Force Balanced Forces Inertia Friction Gravity Weight Mass Apr-17 General Science Chapter 2

General Science Chapter 2 Section 1 Motion Motion Speed Instantaneous Speed Constant Speed Average Speed Velocity Acceleration Apr-17 General Science Chapter 2

General Science Chapter 2 Motion You don’t have to see something move to know that motion has taken place. You only have to know that its position has changed. Best described as a change in position. You need a reference point to know if position has changed. Depends on the observer’s frame of reference. Apr-17 General Science Chapter 2

General Science Chapter 2 Speed How fast something moves The rate of change in position The distance traveled in a given amount of time The rate of motion Rate means change over time Apr-17 General Science Chapter 2

General Science Chapter 2 Instantaneous speed The rate of motion at a given instant Speedometer Apr-17 General Science Chapter 2

General Science Chapter 2 Constant speed Speed that does not vary Cruise control Apr-17 General Science Chapter 2

General Science Chapter 2 Average speed Total distance traveled divided by total time of travel Used when the object in motions makes changes in speed, including starts and stops. Apr-17 General Science Chapter 2

General Science Chapter 2 Calculating speed If you know distance and time, you can calculate speed. Apr-17 General Science Chapter 2

General Science Chapter 2 Example You skate a distance of 100 m in 67 s. What is your average speed? Known: d = 100 m t = 67 s Unknown v = ? Apr-17 General Science Chapter 2

General Science Chapter 2 Example continued Apr-17 General Science Chapter 2

General Science Chapter 2 Graphing speed When distance is graphed on the y-axis and time is graphed on the x-axis, we call this a distance-time graph. The slope of a distance-time graph is the speed. What kind of graph would you use to graph distance vs time? Apr-17 General Science Chapter 2

Section 1: Discussion Questions What is motion? What two quantities do you need to know to calculate average speed? What does a flat line on a segment of a distance-time graph tell you about the average speed for that time period? Apr-17 General Science Chapter 2

General Science Chapter 2 Velocity vs. speed Velocity describes both speed and direction. Speed describes only rate of change in position. Apr-17 General Science Chapter 2

General Science Chapter 2 REVIEW #1 What is the difference between instantaneous and average speed? What is the difference between speed and velocity? What is the equation used to solve for average speed? What SI units are used for speed? What is motion? Apr-17 General Science Chapter 2

General Science Chapter 2 Acceleration The rate of change of velocity. Acceleration occurs if either the speed or direction changes, or if both change. “Slowing down” is a change in speed, so when something slows down, it is accelerating. When speed decreases, acceleration is opposite in direction to velocity. Apr-17 General Science Chapter 2

General Science Chapter 2 Acceleration The size of acceleration depends on both the change in velocity and the time interval over which the change takes place If the change in velocity is large, the acceleration will be large. If the time interval is short, then acceleration will be large. Apr-17 General Science Chapter 2

Calculating acceleration The triangle is the Greek letter delta, and stands for “change in” Apr-17 General Science Chapter 2

General Science Chapter 2 Units on acceleration Meters per second per second or Meters per second squared Written as m/s2 Apr-17 General Science Chapter 2

General Science Chapter 2 Example A car increases its velocity from 10 m/s to 32 m/s in 3 seconds. What is the acceleration of the car? Known: vi = 10 m/s vf = 32 m/s t = 3 s Unknown: a=? Apr-17 General Science Chapter 2

General Science Chapter 2 Example continued Apr-17 General Science Chapter 2

Discussion Questions, Part 2 What is acceleration? What are three ways an object can accelerate? What equation is used to determine acceleration? What is the SI unit of acceleration? Is the SI unit of acceleration a derived unit? Apr-17 General Science Chapter 2

General Science Chapter 2 Section 2 Forces Force Balanced Forces Net Force Inertia Friction Apr-17 General Science Chapter 2

General Science Chapter 2 Force A push or a pull one object exerts on another Apr-17 General Science Chapter 2

General Science Chapter 2 Balanced forces Forces that are equal in size and opposite in direction are called balanced forces. Tug-of-war Apr-17 General Science Chapter 2

General Science Chapter 2 Net Force When forces are unbalanced, there is a net force. A net force always causes a change in velocity. Could be a change in speed, a change in direction, or both. Apr-17 General Science Chapter 2

General Science Chapter 2 Inertia The tendency of an object to resist any change in its motion. If it is moving, it tends to keep moving at the same velocity unless a force acts on it. If it is not moving, it tends to remain at rest unless a force acts on it. Apr-17 General Science Chapter 2

General Science Chapter 2 Mass and inertia The more mass an object has, the greater its inertia is. Objects with greater inertia take more force to change their velocity. Apr-17 General Science Chapter 2

General Science Chapter 2 Newton’s first law An object moving at a constant velocity keeps moving at that velocity unless a net force acts on it. An object at rest stays at rest unless a net force acts on it. An object at rest/in motion will stay at rest/in motion unless a net force acts on it. Also called the law of inertia. Apr-17 General Science Chapter 2

General Science Chapter 2 Seatbelts When a car crashes, a force stops the car and any passengers in the car. A passenger who isn’t wearing her seatbelt isn’t acted on by the force of the seatbelt, so she keeps moving at the same velocity. Apr-17 General Science Chapter 2

General Science Chapter 2 Friction The force that opposes motion between two surfaces that are touching each other. Enables us to walk, hold things, etc. Apr-17 General Science Chapter 2

General Science Chapter 2 Friction The amount of friction depends on two things The kinds of surfaces The force pressing the surfaces together Apr-17 General Science Chapter 2

General Science Chapter 2 Discuss What is a force? What are balanced forces? What is inertia? What is friction? Apr-17 General Science Chapter 2

General Science Chapter 2 Section 3 Gravity Gravity Weight Mass Scales Apr-17 General Science Chapter 2

General Science Chapter 2 Gravity Every object in the universe exerts a force on every other object That force is gravity Often, the force is too small to notice Example – the force of gravity between two people Apr-17 General Science Chapter 2

General Science Chapter 2 Gravitational force The amount of gravitational force between two objects depends on two things Their masses and the distance between them. The mass of a person is small, so the gravitational force between two people is weak. The mass of the Earth is large, so the gravitational force between the Earth and a person is strong. Apr-17 General Science Chapter 2

General Science Chapter 2 Gravitational force Gets weaker as the distance increases Apr-17 General Science Chapter 2

General Science Chapter 2 Weight The measure of the force of gravity on an object. We usually use the term weight when we talk about the gravitational force between the Earth and an object near its surface. Apr-17 General Science Chapter 2

General Science Chapter 2 Weight Related to mass Objects with greater mass have greater weight. Weight is determined by the gravitational pull on the object. Apr-17 General Science Chapter 2

General Science Chapter 2 Weight and mass Weight is measured in Newtons (N). 1 kg of mass at the Earth’s surface has a weight of 9.8 N Apr-17 General Science Chapter 2

Weight on Earth vs. Weight on Moon Since the Earth has more mass than the moon, it exerts a greater gravitational force than the moon. So, things weigh less on the moon than they do on Earth. Do they have less mass on the moon? Apr-17 General Science Chapter 2

General Science Chapter 2 Scales Use the principal of balanced forces to measure how much something weighs. Scales use springs to balance the force of an object’s weight. Apr-17 General Science Chapter 2

General Science Chapter 2 Discuss What is the force of gravity? What is weight? Weight is measured in what SI units? What instrument is used to measure weight? Apr-17 General Science Chapter 2

General Science Chapter 2 Chapter Review Do you know the difference between: Speed and velocity Average speed and instantaneous speed Motion and speed Balanced forces and net forces Mass and Weight A balance and a scale Apr-17 General Science Chapter 2