Physics 31

Friday August 29

Our Class

Major 17 Bio, 1 Eng, 1 PsychologyHS Physics 11/19 yesCalculus I or higher, taken or taking 14/1922 Lab 14 yesMCAT or PCAT 5 yesEXCEL 16 yes

What is the difference between speed and velocity?

1. Speed is an average quantity, while velocity is not.2. Velocity contains information about the direction of motion

while speed does not. 3. Speed is measured in mph, while velocity is measured in

m/s.4. The concept of speed applies only to objects that are

neither speeding up nor slowing down, while velocity applies to every kind of motion.

5. Speed is used to measure how fast an object is moving in a straight line, while velocity is used for objects moving along curved paths.

The quantity 2.67 x 103 m/s has how many significant figures?

1 2 3 4 5

0% 0% 0%0%0%

1. 12. 23. 34. 45. 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

If Sam walks 100 m to the right, then 200 m to the left, his net displacement vector points

To the rig

ht

To the le

ft

Has z

ero le

ngth

Cannot tell w

ithout f

urt...

0% 0%0%0%

1. To the right2. To the left3. Has zero length4. Cannot tell without

further information

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

Vectors and Motion

A quantity that requires both a magnitude (or size) and a direction can be represented by a vector. Graphically, we represent a vector by an arrow.

The velocity of this car is 100 m/s (magnitude) to the left (direction).

This boy pushes on his friend with a force of 25 N to the right.

Slide 1-28

Displacement Vectors

A displacement vector starts at an object’s initial position and ends at its final position. It doesn’t matter what the object did in between these two positions.

In motion diagrams, the displacement vectors span successive particle positions.

Slide 1-29

QUIZ #1

• Alice is sliding along a smooth, icy road on her sled when she suddenly runs headfirst into a large, very soft snow bank that gradually brings her to a halt. Draw a motion diagram for Alice. Show and label all displacement vectors.

• Make a plot of her velocity vs. time. Plot same times as in part 1.

• Name on paper

Adding Displacement Vectors

Slide 1-31

Example: Adding Displacement Vectors

• Jenny runs 1 mi to the northeast, then 1 mi south. Graphically find her net displacement.

N

E

S

W

Adding Vectors Graphically

Slide 1-33

Velocity Vectors

Slide 1-34

Example: Velocity Vectors

Jake throws a ball at a 60° angle, measured from the horizontal. The ball is caught by Jim. Draw a motion diagram of the ball with velocity vectors.

Slide 1-35

Adding velocity vectors

• add red vector to initial velocity to get next velocity vector and so on…..

Homework for today

• Problems Ch. 1: MC24, MC28, 9, 10, 25, 36, 37, 38, WB 1.6(27,28)

Homework for Monday

• NO ClassLABOR DAY

Homework for Wednesday

• Reading: 2: 1-2-Motion in One Dimension-Constant Velocity

• Problems: 1: 28, 30, 32, 35, 43, 47, 51, 56