How to do Physics Problems

1
How to do Physics Problems
P

• Pioneer High School Physics

2
Steps to Problem Solving

• Read the whole problem
• Draw a picture (optional)
• Record known variables
• Record unknown variable
• Select an equation
• Solve for unknown variable
• Plug in values and solve
• Check

3
Read the whole problem

• This gives you the general idea.
• Watch out for
• Directions i.e. North, left, up etc
• If there are many steps
• Conversions

4
Draw a picture

• If it helps
• Only draw to help clarify the problem
• Does not need to be good
• Label the picture

5
Record known variables

• Use units of number in problem to assign variable

6
Record unknown variable

• Use phrases to assign unknown variable
• How far Dx displacement
• How long Dt time
• How fast v velocity

7
Select an equation

• Pick an equation that has the unknown variable
• Most of the time the equation will have all of
  the known variables too
• Beware of Red Herrings numbers that are not
  used in the problem

8
Solve for unknown variable

• The equation needs to be solved for the variable
  before plugging in numbers
• For example
• Dx ? v 10.0m/s Dt 2.0s
• v Dx/Dt solve for Dx ? Dx v Dt

9
Plug in values and solve

• Before you plug in, convert if needed
• Remember all the units have to match

10
Check

• Check for sig figs
• Check for units
• Check for realism

11
Around the World

• Get out a piece of paper
• Write your name on it
• Copy the problem off the screen
• Do the first step of problem solving and pass the
  paper back one person
• The last person in the row passes forward to the
  front person
• Now do the 2nd step on the paper you received
• Pass that paper, get another, do step 3
• Continue until you have done step 8

12
Around the World

• Sample problem
• A nutria is being hunted by a hungry Cajun. Mmm,
  I reckon dat der nutrias crawlin at about 4.5m/s
  to the right! says the bristly hunter. If hes
  three hundred meters away, the bullet will get to
  the nutria in 1.0s. How far ahead of the nutria
  should the hunter aim his rifle so that the
  bullet will hit the nutria?

13
Read

• The nutria is walking to the right
• The bullet has to be shot ahead of the nutria so
  it hits the nutria where the nutria will be 1.0s
  later
• All the units for displacement are in m
• All the units for time are in s

14
Picture
Takes 1.0s to hit nutria
Moving 4.5m/s right
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Known Variables

• 4.5m/s to the right
• v 4.5m/s
• the bullet will get to the nutria in 1.0s
• Dt 1.0s

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Unknown Variables

• How far ahead of the nutria
• Dx ?

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Select Equation

• v Dx/Dt

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Solve for unknown variable

• v Dx/Dt
• Solve for Dx
• Dx vDt

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Plug in Values

• Dx vDt
• v 4.5m/s
• Dt 1.0s
• Dx (4.5m/s) (1.0s)
• Dx 4.5m

20
Check
2 sig figs in the problem 2 sig figs in the answer

• Dx vDt
• v 4.5m/s
• Dt 1.0s
• Dx (4.5m/s) (1.0s)
• Dx 4.5m

m/s times s cancels seconds Dx is in the right
units
Box your answer