Rewriting formulas

• Natuurkunde / Scheikunde
• Middelbare school
• havo, vwo
• Leerjaar 2

Rewriting formulas

Schedule for this lesson

Explanation about rewriting formulas

Practicing rewriting formulas

Explanation about completing formulas

Pacticing completing formulas

Learning objectives

You are able to rewrite different formulas used in physics

You are able to complete a formula

Formulas

In physics we use a lot of different formulas

For density we use ρ = m / V

For distance we use s = v • t

For gravity we use Fz = m • g  

For electric power we use P = U • I

Formulas

Sometimes a different unit is asked, but we are able to convert the formulas

So for density we use ρ = m / V

But we can also calculate the mass if we know ρ and V

Or we can calculate V if we know ρ and M

Formulas

Simply by rewriting the formule!

If ρ = m / V, we could also write that down as 3 = 6 / 2

If we need to know V we can put a vinger on 2 (which simulates for V)

To see that 2 = 6/3

So V = m/ρ

Formulas

Another example

For distance we use s = v • t  (where v = velocity and t = time)

Again rewriting this with number, 6 = 3 • 2

If we want to know v, we put our finger over 3

So that makes...

• v=s/t

Practicing converting formulas

Density ρ = m / V

Distance s = v • t

How do we get m if we know ρ and V?

• m = ρ • V

And how do we get t is we know v and t?

• t = s/v

Practicing converting formulas

Gravity Fz = m • g

Electric power P = U • I

Rewrite both formulas so we know how to get m and g, and U and I

• m= Fz/g
• g= Fz/m
• U = P/I
• I=P/U

Formulas

When given a question with a lot of information where you need to use a formula, the question can be a bit hard to follow

Sometimes you need to rewirte the formule

Sometimes you need to convert the units (for example from km to m)

Formulas

To make these questions easier, we can follow a few steps (5 in total)

1 What is the given information?

2 What is asked?

3 Which formula can i use?

4 What is the calculation?

5 What is the answer?

Formulas

For example: A car drives and has a velocity 10m/s. The car drives this speed for 0,5 minutes, what is the distance the car has driven?

1 given ?

2 asked?

3 formula ?

4 calculation?

5 answer?

Formulas

For example: A car drives and has a velocity 10m/s. The car drives this speed for 0,5 minutes, what is the distance the car has driven?

1 given ? We know v and t, t needs to be converted to seconds first!

2 asked? We are asked what the driven distance (s) is

3 formula = s = v • t ? 

4 calculation?

5 answer?

Formulas

For example: A car drives and has a velocity 10m/s. The car drives this speed for 0,5 minutes, what is the distance (in meters) the car has driven?

1 given ? We know v and t, t needs to be converted to seconds first!

2 asked? We are asked what the driven distance (s) is

3 formula ? s = v • t 

4 calculation? =

5 answer?

• s = 10 • 30 = 300

Formulas

For example: A car drives and has a velocity 10m/s. The car drives this speed for 0,5 minutes, what is the distance the car has driven?

1 given ? We know v and t, t needs to be converted to seconds first!

2 asked? We are asked what the driven distance (s) is

3 formula ? s = v • t 

4 calculation? = s = 10 • 30 = 300

5 answer? s = 300 m

Formulas practicing

A formula is given with this one! (P = U • I) (6 = 3 • 2)

A lamp uses 5 watts (W) of power(P). The electric current (I) that goes through this lamp is 500 mili-amperes (mA). What is the voltage (U) the lamp uses?

Use the 5 steps to answer this question!

Formulas practicing

A lamp uses 5 watts (W) of power(P). The electric current (I) that goes through this lamp is 500 mili-amperes (mA). What is the voltage (U) the lamp uses?

1 given ? P = 5W I= 500mA which converts to 0,5A

2 asked? U

3 formula ? if P = U • I then U = P/I

Formulas practicing

A lamp uses 5 watts (W) of power(P). The electric current (I) that goes through this lamp is 500 mili-amperes (mA). What is the voltage (U) the lamp uses?

1 given ? P = 5W I= 500mA which converts to 0,5A

2 asked? U

3 formula ? if P = U • I then U = P/I

4 calculation? = U = 5 / 0,5 = 10

5 answer? U = 10V

Formulas practicing

For gravity we use Fz = m • g

On earth g is always 9,8.

A weight has a gravity (F) of 100 Newton (N) on earth

What is the mass of this weight

Formulas practicing

A weight has a gravity (Fz) of 100 Newton (N).

What is the mass of this weight

1 given ? Fz = 100N g = 9,8 

2 asked? m

3 formula ? if Fz = m • g then m= Fz/g

Formulas practicing

A weight has a gravity (Fz) of 100 Newton (N).

What is the mass of this weight on earth, and what is the mass of this weight on the moon?

1 given ? Fz = 100N g = 9,8 

2 asked? m

3 formula ? if Fz = m • g then m= Fz/g

4 calculation?  m = 100 / 9,8 = 10,2 m

5 answer? m = 10,2kg 

Formulas practicing

For gravity we use Fz = m • g

An apple has a mass of 85 g. The g on earth is 9,8, while the g on the moon is 1,6

What is the difference in Fz on the earth and on the moon?

Formulas practicing

An apple has a mass of 85 g. The g on earth is 9,8, while the g on the moon is 1,6

What is the difference in Fz on the earth and on the moon?

1 given ? m 85g = 0,085kg g earth = 9,8 and g moon = 1,6

2 asked? Difference between Fz (apple) on earth and on the moon

3 formula ? Fz = m • g

Formulas practicing

An apple has a mass of 85 g. The g on earth is 9,8, while the g on the moon is 1,6

What is the difference in Fz on the earth and on the moon?

4 calculation Fz earth = 0,085 • 9,8 = 0,8N Fz moon = 0,085 • 1,6 = 0,1N

5 answer The Fz difference between 0,8N on earth and 0,1N on the moon is 0,7N

End of the lesson