1-6 Drawing forces

• Natuurkunde
• Middelbare school
• vwo
• Leerjaar 3

Lesson goals:

• Draw or sketch a force diagram when forces work in different directions. 

Vector

A vector has magnitude, 

direction and a point of application.

We draw a vector with an arrow:

• The length of the arrow indicates the size of the force.
• The direction of the arrow indicates the direction of the force.
• The place where the arrow begint indicated the point of application of the force.

Equilibrium

When two equal forces act in opposite directions. For example, a book that lies on a table.

Normal 

When forces are in equilibrium we say the resultant force 

equals 0N

(F​r​​)

Resultant force

The sum of all te forces.

Also called: nett force or sum force.

(F​r​​)

Forces acting in the same direction...

are added together.

Forces acting in opposite direction...

are subtracted.

do the exercises 13 to 21

Combining forces

• Sometimes you have forces acting 

      in different diractions.

• In this case you get the resultant 

      force by combining the forces. 

• Both the size and magnitude of the resultant force is important in your answer.
  

Steps for drawing a vector diagram

Step 1 - Choose a scale

                         (e.g. 1 cm = 5N, so 15 N = 3 cm)

Step2 – Think about the point of application and the direction.
Stap 3 – Draw the force 

                  Example: Draw a force of 60N to the right. 

                  Scale 1cm = 10 N

Parallelogram method

1. Choose an appropriate scale.
   
2. Draw the forces on scale using the given angle (like two adjacent sides of a parallelogram).
   
3. Complete the parallelogram.
   
4. Draw an arrow from the starting point to the opposite corner of the parallelogram.  
5. Measure the length of the arrow, give magnitude and direction.

Example from the book (p. 17)

Do exercise 22 (p. 19)

Do exercise 23 (p. 19)

Do exercise 24 (p. 19)

Perpendicular forces

When two forces are acting perpendicular to each other, you can calculate the resultant force with pythagoras.

F​res​​=√​30​2​​+40​2​​​​​

=50N

Perpendicular forces

You can then calcualtet he angle of the resultant force using

tan(α)=​F​2​​​​F​1​​​​

Homework

Test yourself 1.2

What did you
learn today?

Conclusion:

When forces act in different directions 

you have to combine the forces.

You can use the parallelogram method.

When the forces at at an angle of 90 degrees then you can calculate the unknown side using pythagors.