1. University Physics: Mechanics
Lecture 3
Ch2. STRAIGHT LINE MOTION
University Physics: Mechanics
Dr.-Ing. Erwin Sitompul

2. Announcement 11.16.10 17.30–18.30 : Class session 19.00–20.15 : Quiz 1
–18.30 : Class session & Make-up Quiz 1
19.00–20.15 : Discussion of Quiz 1
–20.30 : Mid-term Examination

3. Solution of Homework 2: Aprilia vs. Kawasaki
An Aprilia and a Kawasaki are separated by 200 m when they start to move towards each other at t = 0. The Aprilia moves with initial velocity 5 m/s and acceleration 4 m/s2. The Kawasaki runs with initial velocity 10 m/s and acceleration 6 m/s2.
200 m
(a) Determine the point where the two motorcycles meet each other.

4. Solution of Homework 2: Aprilia vs. Kawasaki
Possible answer
Both motorcycles meet after 5 seconds
Where?
Thus, both motorcycles meet at a point 75 m from the original position of Aprilia or 125 m from the original position of Kawasaki.

5. Solution of Homework 2: Aprilia vs. Kawasaki
(b) Determine the velocity of Aprilia and Kawasaki by the time they meet each other.
What is the meaning of negative velocity?

6. Free Fall Acceleration
A free falling object accelerates downward constantly (air friction is neglected).
The acceleration is a = –g = –9.8 m/s2, which is due to the gravitational force near the earth surface.
This value is the same for all masses, densities and shapes.
With a = –g = –9.8 m/s2,
Accelerating feathers and apples in a vacuum

7. Example: Niagara Free Fall
In 1993, Dave Munday went over the Niagara Falls in a steel ball equipped with an air hole and then fell 48 m to the water. Assume his initial velocity was zero, and neglect the effect of the air on the ball during the fall.
(a) How long did Munday fall to reach the water surface?
Niagara Falls

8. Example: Niagara Free Fall
In 1993, Dave Munday went over the Niagara Falls in a steel ball equipped with an air hole and then fell 48 m to the water. Assume his initial velocity was zero, and neglect the effect of the air on the ball during the fall.
(b) What was Munday’s velocity as he reached the water surface? or
Positive (+) or negative (–) ?

9. Example: Niagara Free Fall
In 1993, Dave Munday went over the Niagara Falls in a steel ball equipped with an air hole and then fell 48 m to the water. Assume his initial velocity was zero, and neglect the effect of the air on the ball during the fall.
(c) Determine Munday’s position and velocity at each full second.

10. Example: Baseball Pitcher
A pitcher tosses a baseball up along a y axis, with an initial speed of 12 m/s.
(a) How long does the ball take to reach its maximum height?

11. Example: Baseball Pitcher
A pitcher tosses a baseball up along a y axis, with an initial speed of 12 m/s.
(b) What is the ball’s maximum height above its release point?

12. Example: Baseball Pitcher
A pitcher tosses a baseball up along a y axis, with an initial speed of 12 m/s.
(c) How long does the ball take to reach a point 5.0 m above its release point?
Both t1 and t2 are correct
t1 when the ball goes up, t2 when the ball goes down

13. University Physics: Mechanics
Lecture 3
Ch3. VECTOR QUANTITIES
University Physics: Mechanics
Dr.-Ing. Erwin Sitompul

14. Vectors and Scalars
A vector quantity has magnitude as well as direction.
Some physical quantities that are vector quantities are displacement, velocity, and acceleration.
A scalar quantity has only magnitude.
Some physical quantities that are scalar quantities are temperature, pressure, energy, mass, and time.
A displacement vector represents the change of position.
Both magnitude and direction are needed to fully specify a displacement vector.
The displacement vector, however, tells nothing about the actual path.

15. Adding Vectors Geometrically
Suppose that a particle moves from A to B and then later from B to C.
The overall displacement (no matter what its actual path) can be represented with two successive displacement vectors, AB and BC.
The net displacement of these two displacements is a single displacement from A to C.
AC is called the vector sum (or resultant) of the vectors AB and BC.
The vectors are now redrawn and relabeled, with an italic symbol and an arrow over it.
For handwriting, a plain symbol and an arrow is enough.

16. Adding Vectors Geometrically
Two vectors can be added in either order.
Three vectors can be grouped in any way as they are added.

17. Adding Vectors Geometrically
The vector is a vector with the same magnitude as but the opposite direction. Adding the two vectors would yield
Subtracting can be done by adding

18. Components of Vectors Adding vectors geometrically can be tedious.
An easier technique involves algebra and requires that the vectors be placed on a rectangular coordinate system.
For two-dimensional vectors, the x and y axes are usually drawn in the drawing plane.
A component of a vector is the projection of the vector on an axis.

19. Components of Vectors
Note, that the angle θ is the angle that the vector makes with the positive direction of the x axis.
The calculation of θ is done in counterclockwise directions.
What is the value of θ on the figure above?
θ = ° or –35.54°

20. Components of Vectors
Vectors with various sign of components are shown below:

21. Components of Vectors
Which of the indicated methods for combining the x and y components of vector are proper to determine that vector?

22. Components of Vectors Example: Solution:
A small airplane leaves an airport on an overcast day and it is later sighted 215 km away, in a direction making an angle of 22° east of due north. How far east and north is the airplane from the airport when sighted?
Solution:
Thus, the airplane is km east and km north of the airport.

23. Trigonometric Functions
Sine
Cosine
Tangent

24. Unit Vectors
A unit vector is a vector that has a magnitude of exactly 1 and points in a particular direction.
The unit vectors in the positive directions of the x, y, and z axes
are labeled
Unit vectors are very useful for expressing other vectors:

25. Adding Vectors Geometrically
Vectors can be added geometrically by subsequently linking the head of one vector with the tail of the other vector.
The addition result is the vector that goes from the tail of the first vector to the head of the last vector.

26. Adding Vectors by Components
Another way to add vectors is to combine their components axis by axis.

27. Adding Vectors by Components
Component Notation
Magnitude-angle Notation
where
In the figure below, find out the components of

28. Adding Vectors by Components
Desert Ant

29. Trivia
A bear walks 5 km to the south. Resting a while, it continues walking 5 km east. Then, it walks again 5 km to the north and the bear reaches its initial position.
What is the color of the bear?
5 km, south
5 km, north
5 km east
Polar Bear with White Fur

30. Homework 3: The Beetles
Two beetles run across flat sand, starting at the same point. The red beetle runs 0.5 m due east, then 0.8 m at 30° north of due east.
The green beetle also makes two runs; the first is 1.6 m at 40° east of due north.
What must be (a) the magnitude and (b) the direction of its second run if it is to end up at the new location of red beetle?
2nd run?
1st run
2nd run
1st run