# Motion and Force.

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Motion and Force

Chapter Four: Forces 4.1 May the Force Be With You

4.1 The cause of forces A force is a push or pull, or any action that has the ability to create change. The key word here is action, force is an action.

4.1 How do you measure force?
The conventional unit for measuring force is in pounds For example: A lion bites down with 1,235 pounds of bite force A boxer can deliver 2,350 pounds of force with a single punch

4.1 Metric Units of Force The newton (N) is a metric unit of force.
One pound of force equals 4.48 newtons. Examples: A lion bites down with 5,533 N of force A boxer can punch with 10,528 N of force

4.1 Gravity and weight The force of gravity on an object is called weight. Mass and weight are not the same thing!

Remember….g = 9.8 An elephant has a mass of 6,800 kg. What is his weight in Newtons?

4.1 Gravity and weight A 10-kilogram rock has a mass of 10 kilograms no matter where it is in the universe. A 10-kilogram rock’s weight however, can vary greatly depending on where it is.

g (moon) = 1.68 So….has the MASS of the elephant changed, just because he is on the moon? What is his new WEIGHT due to the difference in gravity?

4.1 Addition of vectors It is sometimes helpful to show the strength and direction of a force using an arrow. This arrow is known as a FORCE VECTOR If 1 cm = 1 N, how many newtons of force does this line represent?

4.1 The force vector A force vector has units of newtons, just like all forces. Forces can be added together to determine what motion an object will have. Positive and negative numbers indicate opposite directions.

4.3 Adding forces 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?

4.3 Adding forces 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: weight drag (air friction) the thrust of the engines, and the lift force caused by the flow of air over the wings.

4.3 Balanced & Unbalanced Forces
When several forces act on the same object: The net force is zero, or The net force is NOT zero. Which of these situations will result in the box moving?

Balanced 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. What force is acting to push the book down?

4.3 Equilibrium and normal forces
A normal force is created whenever an object is in contact with a surface. It is also known as support force The normal/support has equal strength to the object’s weight.

4.3 Solving equilibrium problems
For an object to be in equilibrium, all the forces acting on the object must add up to zero. Is this object in equilibrium?

4.1 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 each boy go?

4.1 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.

Review Questions Page 86 – 2,3,4,5 and 8 Page ,2,3,4,6 and 7