5-2 Resultant force (5.2)

• Natuurkunde
• Middelbare school
• havo
• Leerjaar 3

Resultant force (5.2)

Lesson goals:

• Understand gravitational acceleration.
• Understand what a driving force is and 

       what a resisting force is.

• Make calculations with a resultant force. 

Gravitational acceleration

- In a lift moving with the same velocity as you

   > it feels as if gravity is absent

- In a lift that is accelerating 

   > it feels as if there is a gravitational force

Therefore you can conclude that you can not seperate acceleration and gravitational force from each other.

Driving force and resisting force

The two most important resisting forces:

- Air friction 

- Rolling friction

and driving forces are

- gravity

- force of a motor

- pushing force

Air friction

Air friction is due to the collision with 

air particles

You can decrease air friction by:

- improving your streamline

- making your frontal cross-section 

   smaller

Forces on a car...

             = pushing foce

             = frictional force

The pushin force and frictional force act in the opposite directions.                  is greater than             

therefore                            

is to the right.

F​p​​

F​r​​

Fp

F​r​​

F​res​​

Resultant force causes acceleration

If the pushing force is equal to the frictional force then the resultant force is equal to 0 N

Then there is no acceleration THEREFORE

• the car stand still OR
  
• the car has a constant velocity
  

F​p​​=F​r​​

F​res​​=0N

Resultant force changes velocity

If the pushing force is greater than the frictional force the resultant force is greater than 0 N

               > 

The acceleration will be in the 

direction of the resultant force.

This is also known as Newtons 

first law of motion

F​p​​

F​r​​

Resultant force changes velocity

If there is no pushing force only the frictional force is left and the resultant force acts opposite 

to the the direction of motion.  

The car will decelerate

(or rather have a negative 

acceleration)

Example

The resultant force can do three things

• Accelerate an object (if the resultant force is in the direction fo motion),
  
• Decelerate an object (if the resultant force is in the opposite direction of motion)
  
• Change direction of an object 

      (when a force comes from the side)

Now try the following question in your digital method.

17, 18 and 19

How did the lesson go? Any questions?