Motion and Force

• Chapter Twelve: Distance, Time, and Speed

• Chapter Thirteen: Forces

• Chapter Fourteen: Force and Motion

Chapter Thirteen: Forces

• 13.1 Forces

• 13.2 Addition of Forces

Investigation 13B

• Are there still forces even when things are not moving?

Equilibrium

13.2 Addition of vectors• Some things in science can be

completely described by a quantity and a unit.

• Temperature, time ,length, and speed are quantities that can be described this way.

13.2 Addition of vectors

• A vector is a measurement that includes both a quantity and a direction.

• Examples of vector quantities are velocity, acceleration, and force.

13.2 The force vector

• A force vector has units of newtons, just like all forces, but also includes enough information to tell the direction of the force.

• Positive and negative numbers indicate opposite directions.

13.2 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?

13.2 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. weight2. drag (air friction)3. the thrust of the engines, and 4. the lift force caused by the flow of air over

the wings.

13.2 The 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?

13.2 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.

13.2 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.

13.2 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.

13.2 Forces in springs and ropes

• You are already familiar with weight, normal force, friction, and contact forces.

• Some forces have special properties.

If the vectors are the same size is this block moving or at equilibrium?

13.2 Forces in springs and ropes

• Tension is a force that acts in a rope, string, or other object that is pulled.

• Tension always acts along the direction of the rope.

In what direction will the girls go?

13.2 Forces in springs and ropes• When you apply a

compression force to a spring, it collapses to become shorter.

• The force created by stretching or compressing a spring always acts to return the spring to its natural length.

13.2 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 direction with forces in the vertical direction.

What is the sum of the forces acting on this block?

Technology ConnectionDefy Gravity? It Can Be Done.

When the force of lift is stronger than the force of an airplane’s weight, it will leave the ground and fly. How do we create the force of lift?