Cornell Notes 3.2 Addition of Forces November 22, 2011 Pages 85 & 87

The force vector A force vector has units of newtons (its magnitude), but also includes enough information to tell the direction of the force. Positive and negative numbers indicate opposite directions.

Addition of vectors It is sometimes helpful to show the strength and direction of a force vector as an arrow. When drawing a force vector, you must choose a scale. If 1 cm = 1 N, how many newtons of force does this line represent?

Adding force vectors To figure out if or how an object will move, we look at ALL of the forces acting on it. Four forces act on a plane: 1.weight 2.drag (air friction) 3.the thrust of the engines, and 4.the lift force caused by the flow of air over the wings.

Net Force The sum of all the forces on an object is called the net force. The word net means total but also means the direction of the forces has been taken into account. In what direction will this plane go?

Forces in the Same Direction When two forces act on an object in the same direction, the two forces add to each other. 60 N 20 N40 N

When two forces act on an object in opposite directions, the two forces cancel each other out. Forces in the Opposite Direction 20 N - 40 N -20 N

Equilibrium and Normal Forces When the forces are balanced, the net force is zero. When the net force on an object is zero, we say the object is in equilibrium.

Equilibrium and Normal Forces A normal force is created whenever an object is in contact with a surface. The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight.

Addition of Vectors How do you keep track of many forces with different directions? Draw a free-body diagram that contains the objects, like a book on a table.

Draw This Picture!

Solving Equilibrium Problems For an object to be in equilibrium, all the forces acting on the object must add to zero. Do not mix forces in the horizontal (left-right) direction with forces in the vertical (up down) direction. What is the sum of the forces acting on this block?