Some Upfront Definitions…

1. Some Upfront Definitions… • Scalars – a quantity that has only a magnitude (with unit). • Ex: mass, temperature, speed • Vectors – a quantity that has both magnitude (with unit) and direction. • Ex: force, velocity, acceleration. • Inertia - the property of an object to resist changes in motion • Friction – a force that exists when objects touch. It usually works to slow down moving objects • Mass – a measure of how much stuff is in an object; a quantification of inertia. • Mass is not weight, but is related to weight. Unit: kg • Weight – a measure of how heavy an object is in the presence of gravity. • Weight is not mass, but is related to mass • Position – the location of an object. Defined by a coordinate system like x, y, z. Unit: m • Displacement – a measurement of how much an objects position changes. Unit: m • Speed – a measure of how fast an object changes position. • Speed is a scaler quantity. Unit: m/s • Velocity – a vector quantity that measures the change in an objects position. Unit: m/s • Acceleration – a vector quantity that measures the change in an objects velocity. Unit: m/s2 • Force – a push or a pull. Forces produce accelerations. Unit: Newton, N • Net Force – the grand total of all pushes and pulls.

2. Tyler Junior CollegePhysics 1405Elementary Physics NEWTON’S 1st LAW--INERTIA

3. Newton’s 1st Law of Motion • aka – the law of inertia • “Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.”

4. 1st Law Implies 3 States of Motion • At rest • Constant speed in a straight line • Accelerated motion caused by forces

5. ConcepTest If an object is moving, then it must be experiencing a net force. • True • False

6. ConcepTest Which of the following is NOT in one of Newton’s three states of motion? • fish on a merry-go-round • truck driving in a straight line but speeding up • astronaut weightless in space • all the above • none of the above

7. Implications of Newton’s 1st Law • Objects at rest experience no net force. • Objects moving in a straight line at constant speed experience no net force. • Objects that move in any other way are experiencing a net force. • When a net force stops acting, the object must begin to move in either a straight line at constant speed or it must come to rest.

8. ConcepTest Which of the following is always true? • If an object is moving, it must be experiencing a net force. • If an object is moving, it must have constant velocity • If an object is at rest, then it must be experiencing no net force. • All the above • None of the above

9. block force So What’s a Force, eh? • A Force is a push or a pull • Forces have both magnitude & direction • Something with both magnitude & direction is called a ______ • All Forces are Vectors • Vectors are usually symbolized as arrows.

10. Vectors • Vectors are usually drawn as arrows • Vectors can be added • The sum of vectors is called the resultant + =

11. 5N 10N 5N 5N 5N 10N 5N 5N Net Forces Are What Really Matter • Net Force is the resultant of all forces.

12. What is the net force acting on this object? a. 0 N b. 10 N, right c. 5N, down d. 10 N, left e. 5N, up 10N 15N 25N 5N 15N 5N 5N 5N ConcepTest

13. Vector Addition • Vectors in non-parallel directions can be added by the parallelogram method. resultant

14. Vector Decomposition • Vectors can also be broken down into perpendicular component parts. resultant

15. ConcepTest resultant 2 vectors Where is the resultant? parallelogram

16. ConcepTest What is the net force on a cart pulled to the left with a 100 Newton force & to the right with a 40 Newton force? a. 60 N to the right b. 60 N to the left c. 40 N to the right d. 0 N e. None of the above

17. ConcepTest What will be the direction of the net force? Assume all vectors are equal magnitude. a. straight down b. down-right c. down-left d. right e. up-right

18. Tyler Junior CollegePhysics 1405Elementary Physics Linear Motion

19. More about speed • Instantaneous speed – how fast something moves at any particular instant in time. • Average speed – how fast something moves on average.

20. ConcepTest Driving to Dallas (about 100 miles) requires about 2 hours. What is the average speed? What is the instantaneous speed?

21. Now back to Velocity • Velocity is the rate of change of position. • Velocity is a vector quantity! • Velocity is speed in a given direction. • Unit: m/s, but must include direction info.

22. Velocity can be constant • Constant speed does not necessarily mean constant velocity. • Constant direction does not necessarily mean constant velocity. • However, constant velocity does mean constant speed in a constant direction. • Also, velocity can have positive or negative values.

23. A truck drives in a steady circle with an unchanging speed. The truck has… a. constant velocity b. constant direction c. constant position d. constant speed e. None of the above ConcepTest

24. If an object has constant velocity then… a. no net force is acting on the object b. the object must be accelerating c. the object is experiencing a net force d. All the above e. None of the above ConcepTest

25. Acceleration • …is the rate of change, or change per second, of velocity. • Acceleration occurs whenever there is an increase or a decrease or a direction changein velocity. • Acceleration has odd looking units

26. Accelerating Confusion • Note: acceleration is initially a confusing subject. • It is complex because it is a rate of a rate. • Acceleration is not velocity, nor is it even a change in velocity. Acceleration is the rate at which velocity itself changes.

27. Acceleration Can Be Constant • Since acceleration is the rate at which velocity changes, • then constant acceleration means the velocity changes at a constant rate. • A constant rate of change can be represented as a line on a graph.

28. In this graph a. velocity is constant b. acceleration is zero c. velocity is not changing d. acceleration is constant e. all the above velocity time ConcepTest

29. In this graph a. velocity is constant b. acceleration is zero c. velocity is not changing d. acceleration is constant e. all the above velocity time ConcepTest

30. Graphing Constant velocity At rest (special case of constant velocity) Constant acceleration

31. velocity position time time Constant Velocity 1

32. Constant Velocity 2 acceleration time

33. At Rest 1 velocity position time time

34. acceleration time At Rest 2

35. Constant Acceleration 1 velocity position time time

36. acceleration time Constant Acceleration 2

37. This graph best represents which of the following? a. Object at rest b. Object moving with constant velocity c. Object moving with constant acceleration d. All the above position time ConcepTest

38. Draw the position vs time & velocity vs time graphs for an object in free fall. ConcepTest velocity position time time

39. A truck drives in a steady circle with an unchanging speed. Does the truck experience an acceleration? a. yes b. no c. Not enough information to tell ConcepTest

40. A truck drives in a straight line while slowing to a stop. Does the truck experience an acceleration? a. yes b. no c. Not enough information to tell ConcepTest

41. True or False? • If position is constant, then velocity is zero. • If position changes then velocity is nonzero. • If speed or direction changes then velocity changes. • If speed and direction are constant, then velocity is constant. • If velocity is zero or constant, then acceleration is zero. • If velocity changes then acceleration is nonzero. • If velocity decreases then acceleration is negative.

42. Galileo’s work • Does a falling object accelerate? • Galileo used inclined planes to slow falling objects. • Galileo found that when air resistance is small enough to neglect, all objects fall with the same unchanging acceleration. • During each second of fall, objects gain a speed of 10 m/s. • In other words, the acceleration due to gravity for a freely falling object is about 10 m/s2

43. Galileo’s Free Fall Table

44. Galilean Motion • Galileo discovered that all falling objects experience an acceleration, g, of about 10 m/s2 • So…you can figure out how fast any object falls (ignoring air resistance)… • And…you can figure out how far an object falls (ignoring air resistance)…

45. It takes about 10 seconds for a rock dropped from a cliff to hit the ground. How high is the cliff? a. 50 m b. 100 m c. 250 m d. 500 m ConcepTest

46. A kicker can kick a ball about 125 m straight up. How long will the ball be in the air? a. 5 sec b. 10 sec c. 15 sec d. 20 sec e. 25 sec f. None of the above ConcepTest

47. A kicker can kick a ball about 125 m straight up. What is the velocity of the ball when it returns to the ground? a. 50 m/s b. 500 m/s c. 100 m/s d. 10 m/s e. None of the above ConcepTest

48. A kicker can kick a ball about 125 m straight up. What is the velocity of the ball at the top? a. 50 m/s b. 500 m/s c. 100 m/s d. 10 m/s e. None of the above ConcepTest

49. Mechanical Equilibrium • Occurs when the sum of all forces equals zero. • If ∑F≠0, then the object is “accelerating” and NOT in a state of mechanical equilibrium.

50. Weight of an object • On earth all objects experience a force due to Earth’s gravity. • The acceleration due to this force is g. • The force itself is called weight. • Weight = mass x g • Weight is a Force