Y2_T4_LSN_4.1

• Science
• Middelbare school
• havo, vwo
• Leerjaar 2

Get ready

• Your Macbook (join the LessonUp)
• Your notebook.
• Your textbook.
• Pen & highlighter

Topic 4

Moving in the world

Plan for today

• Planning topic 4
• Explanation T4.1 Forces
  
• Make homework
  

Planning Topic 4

• Quiz 4.1 - 4.3          in 2 weeks         (counts 2x)
• Test T4 (4.1-4.5)    in test week      (counts 4x) 
• Practica                   in 2/3 weeks     (no grade)

4.1 Forces

Learning goals

I can....

• explain what a force is an can identify different types of forces.
• calculate the gravitational force acting on an object.
• draw arrows on a force diagram with the correct point of application, magnitude, and direction.
• calculate the resultant force of forces going in the same direction or opposite direction of each other
• calculate the resultant force of two perpendicular forces

Key words

• Force
• Newton
• Gravity
• Applied force
• Muscle force
• Driving force
• Air resistance
• Drag
• Friction
• Tension force

• Spring force 
• Elastic force
• Normal force
• Support force
• Weight
• Point of application
• Direction
• Magnitude
• Resultant force

A world of forces

A force is defined as a push or pull. 

• unseen
• see the effects

The unit for forces is in Newtons (N).  

Types of forces

There are different types of forces: 

• Gravity = pulls objects towards the earth
• Applied force  = a general force applied to an object (muscle or motor)
• Air resistance  = force between air and an object
• Friction = force exerted by a surface sliding against an object
• Tension force = force spread through a rope or cable when pulled
• Spring force = force when a spring or elastic material is stretched
• Normal force = force supporting an object on a surface

Types of forces

Types of forces	Unit	Also known as
Gravity	F g
Applied force	F a	muscle force driving force
Air resistance	F air	drag
Friction	F f
Tension force	F t
Spring force	F s	elastic force
Normal force	F n	support force

Types of forces

Measuring gravity and weight

The force needed to lift 1 kg = 9.81 N on earth

9.81 is the gravitational constant on earth in N / kg or

On the moon the force needed is only 1.6 N

(So the mass is the same, but the

gravitational force is different)

Weight = the newtons for every kg of an objects mass

9.81 is the gravitational constant on earth in N / kg or  

m / s²

Calculating gravity force

Formula for calculating Gravity force:

F_g is the gravitational force in [N]

m is the mass in [kg]

F_g = m x 9.81

Calculating gravity force

Example: 

The mass of Peter is 70 kg. What is his gravitational force (F_g)?

F_g                        = m x 9.81 

F_g  = 70 [kg] x 9.81 [N/kg]

F_g                      = 868 [N]

Example: The gravitational force of my cat is 34.3N What is her mass ?

F​g​​

Example: John has a mass of 76 kg. What is his gravitational force, ?

F​g​​

Drawing force diagrams

Forces are represented by an arrow

It has three things:

• Point of application (origin of force)
• Direction
• Magnitude (the longer the arrow, the bigger the force)

For the magnitude you use a scale (in this case 1 cm = 10N)

A forces is drawn with a length of 4 cm
The scale is 1 cm = 30 N
What is the magnitude of the force drawn?

1. 60 N
2. 30 N
3. 120 N
4. 7,5 N

I want to draw a force with a magnitude of 300 N.
The scale must be 1 cm = 60 N
How large should the arrow be?

1. 2 cm
2. 5 cm
3. 10 cm
4. 12 cm

Calculating forces

If there is more than one force acting on an object then the forces can be combined to create one total force = resultant force 

Two forces in the same direction are added up

Two forces in opposite direction are subtracted from eachother

If forces are equal and in opposite direction, there is no motion!

F​res​​

Calculating forces

What is the resultant force?

1. 140 N to the left
2. 140 N to the right
3. 60 N to the left
4. 60 N to the right

What is the resultant force?

1. 225 N to the left
2. 225 N to the right
3. 25 N to the left
4. 25 N to the right

Perpendicular forces

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

        

F​res​​=√​30​2​​+40​2​​​​​

=50N

a​2​​+b​2​​=c​2​​

Homework

• Study the glossary of 4.1
  
• Complete the exercises of 4.1 (p. 158-161)