Physics 151 Week 3 Day 3

Topics– Strategic Problem Solving– Motion Diagrams & Motion Graphs

with acceleration– Adding and Subtracting Vectors Graphically

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Strategic Problem Solving Strategy (PSS)On problems, it is important to show how you reasoned from the information given in the problem and key physics ideas to your final answer. The correct final answer with units is only worth 1-3 points. The remainder of the points are given for the quality of your solution.†You are expected to include the following to receive full credit:

Prepare• Identify the Physics: State explicitly which physics principle apply to

the problem situation and that you will use to solve the problem• Drawing a Picture: Draw at least one picture to visualize the physics of

the problem and define your variables and constants. For motion problems this could be a motion diagram, motion graph, or pictorial diagram (Friday)

• Collecting Necessary Information: State all the information given in the problem with correct units. Include preliminary calculations such as unit conversions

Solve• Start with key equation(s) in symbol form• Solve for the unknown quantity in symbols explicitly before numeric

calculations• Then substitute numbers with units and calculate the numeric

answer

Assess• Check to see if your answer is reasonable

• Does it answer the question that was asked• Does it have the right units?

Be prepared to make reasonable estimations and state your assumptions when solving problems. Be aware of significant digits in your answers. (Keep lots of digits until the final calculation, then round to the appropriate precision. For intermediate steps, remember to keep one extra siginificant digit

Question

4. Velocity vectors point

A. in the same direction as displacement vectors.

B. in the opposite direction as displacement vectors.

C. perpendicular to displacement vectors.

D. in the same direction as acceleration vectors.

E. Velocity is not represented by a vector.

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4. Velocity vectors point

A. in the same direction as displacement vectors.

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Answer

Vectors

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Adding Vectors Graphically

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Vector Subtraction

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Adding Vectors

Find the following resultants graphically

Checking Understanding

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Which of the vectors below best represents the vector sum P + Q?

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Which of the vectors below best represents the vector sum P + Q?

Answer

Checking Understanding

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Which of the vectors below best represents the difference P – Q?

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Which of the vectors below best represents the difference P – Q?

Answer

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Acceleration Vectors

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The diagram below shows two successive positions of a particle; it’s a segment of a full motion diagram. Which of the acceleration vectors best represents the acceleration between vi and vf?

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The diagram below shows two successive positions of a particle; it’s a segment of a full motion diagram. Which of the acceleration vectors best represents the acceleration between vi and vf?

Answer

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From the Knight Workbook, Chapter 2, Problem 16A

The motion diagram on the rights shows an initial point 0 and a final point 1. A pictorial model would define five symbols, x0 and x1 (or y0 and y1), v0 and v1, and a. Determine whether each of these quantities is positive, negative, or zero. place either +, --, or 0 in each cell of the table below.

Clicker ChoicesA. + B. -- C. 0

A ball is released from rest at the point shown on the incline and speeds up. It then rolls onto a level section of track at constant speed, and then rolls onto a second incline with the same slope as the first where it slows down. The diagram below shows the location of the ball at several instants in time. NOTE: THIS IS NOT A STROBE PHOTOGRAPH OR MOTION DIAGRAM.

1. Draw a motion diagram for the ball’s mostion from t = 0.6 s to 5.1 s2. Draw delta v diagrams for each motion interval3. Use the delta v diagrams to find the ball’s acceleration

Ball on Ramps (From an Exam)

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.

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Motion Graph Demonstrations