Monday, November 14, 2016

Activity 31: Introduction to Forces in Equilibrium

Activity 43.20 - Forces in Equilibrium

Newton's first law of motion is often stated as
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.  
So an object that is not moving, or an object that is moving at a constant velocity will not change its motion unless an unbalanced force is applied to it.  The term unbalanced force is simply a way of stating that if I add up all the forces on an object then that total force  is not equal to zero.  If however the forces on an object are in balance then the net force is zero and the object's motion or lack of motion will not change.


An object is said to be in linear equilibrium if the net force on the object is equal to zero.  Another way to state this is,  the net force on an object which is not accelerating must be equal to zero.

Forcenet = 0 if an object is in equilibrium ( it is not accelerating).

Newton's 2nd law is often stated as Fnet = Mass X Acceleration.

As you can see from this equation if an object's acceleration is equal to zero (it is in equilibrium) then its Fnet = 0.

If the net force on an object is zero it then follows that:

  • All of the forces in the horizontal direction must add up to zero.
  • All of the forces in the vertical direction must add up to zero.
You may wonder why we care about the summation of an object's horizontal and vertical vectors adding up to zero.  The can be summarized as:

  • The main technique to adding vectors is to add their horizontal and vertical components together.  If we know an object is in equilibrium, then we know the summation of its horizontal and vertical components must also equal zero.  Otherwise it would be accelerating either horizontally or vertically or both.

Watch this video to help you understand how you can use the concept of forces in equilibrium to calculate the forces on an object.

Activity 43.30 Vector Addition Review

Answer to following problems 5,6,7,9 &10:

Activity 43.40 Vector Components

Pick up a paper copy of this worksheet.  Review the drawings on the first page and answer the questions on page 168.   Check your answers on the last page of the document.

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