The Nature The Nature of Forcesof Forces

What is Force?What is Force?

A force is a push or pull A force is a push or pull that gives energy to an that gives energy to an object, sometimes causing object, sometimes causing it to start moving, stop it to start moving, stop moving, or slow down.moving, or slow down.

Combining Forces Combining Forces

Forces occur in pairs and Forces occur in pairs and can be either can be either balancedbalanced or or unbalancedunbalanced..

Balanced forcesBalanced forces do notdo not cause a change in motion.cause a change in motion. They are equal in sizeThey are equal in size and can be and can be opposite in direction.opposite in direction.

Have you ever had an Have you ever had an arm wrestling arm wrestling competition with someone? If you compete competition with someone? If you compete against someone who is just about as strong as against someone who is just about as strong as you are, there will probably be a time when you are, there will probably be a time when both of you are pushing as hard as you can, but both of you are pushing as hard as you can, but your arms stay in the same place. This is an your arms stay in the same place. This is an example of balanced forces. The force exerted example of balanced forces. The force exerted by each person is equal, but they are pushing in by each person is equal, but they are pushing in opposite directions, in this case together. It opposite directions, in this case together. It would look something like this.would look something like this.

Because the force Because the force that each of you is that each of you is exerting exerting is equal, the two forces cancel is equal, the two forces cancel each other out and the resulting force is each other out and the resulting force is zero. Therefore, there is no change in zero. Therefore, there is no change in motion. motion.

Another great place to see balanced forces in Another great place to see balanced forces in action is in a action is in a tug of wartug of war. This is similar to an . This is similar to an arm wrestling match, only in this case the arm wrestling match, only in this case the forces are moving away from each other. Just forces are moving away from each other. Just like in arm wrestling, if the two teams have like in arm wrestling, if the two teams have equal strength, or force, the rope will stay equal strength, or force, the rope will stay pretty much in the same place. It would look pretty much in the same place. It would look like this. like this.

Again the resulting force is zero and there is no Again the resulting force is zero and there is no change in motion. change in motion.

Unbalanced forcesUnbalanced forces always causealways cause a a change in motionchange in motion. They are not equal and . They are not equal and opposite.opposite.

When two unbalanced forces are When two unbalanced forces are exerted in opposite directions, their exerted in opposite directions, their combined force is equal to the combined force is equal to the difference between the two forces difference between the two forces and is exerted in the direction of the and is exerted in the direction of the larger force.larger force.

Example One:Example One:

Think again about the arm wrestling Think again about the arm wrestling match. Only this time imagine that you are match. Only this time imagine that you are competing against a world famous body competing against a world famous body builder. Chances are that his builder. Chances are that his force will be force will be much greater than yoursmuch greater than yours. . Your arms Your arms will move in the direction he is will move in the direction he is pushingpushing with a force that is equal to his with a force that is equal to his force minus your force. It will look like this. force minus your force. It will look like this.

Example TwoExample Two

Or imagine that you and your friends Or imagine that you and your friends are having a tug of war with the U.S. are having a tug of war with the U.S. Olympic weight lifting team. Again Olympic weight lifting team. Again their force will probably be a little their force will probably be a little larger than yours. You will move in larger than yours. You will move in the direction they are pulling with a the direction they are pulling with a force that is equal to their force force that is equal to their force minus your force. It will look like this. minus your force. It will look like this.

Example Three Example Three Unbalanced forces can also be exerted in Unbalanced forces can also be exerted in

the same direction. For example, imagine the same direction. For example, imagine that your family's car breaks down on the that your family's car breaks down on the road and you have to push it into a road and you have to push it into a parking lot. If you and your brother or parking lot. If you and your brother or sister both push on the car, the resulting sister both push on the car, the resulting force on the car will be the sum of your force on the car will be the sum of your forces and of course be in the direction forces and of course be in the direction that you are applying the force. The figure that you are applying the force. The figure below shows how this would work. below shows how this would work.

Two important things to Two important things to remember when working with remember when working with balanced and unbalanced forces balanced and unbalanced forces are: are:

1) 1) forces in the forces in the samesame direction direction combine by combine by addadditionition, and , and

2) 2) forces in forces in oppositeopposite directions directions combine by combine by subtractsubtractionion..

Friction: A Force Opposing Motion Friction: A Force Opposing Motion

Friction is a force that acts in the Friction is a force that acts in the opposite direction of the motionopposite direction of the motion; it ; it will cause a moving object to slow will cause a moving object to slow down and finally stop. down and finally stop.

The amount of friction depends The amount of friction depends on factors such as the roughness on factors such as the roughness of the surfaces and the force of the surfaces and the force pushing the surfaces together.pushing the surfaces together.

The number one way to The number one way to reduce reduce

frictionfriction is to is to add a lubricantadd a lubricant..

Types of Friction Types of Friction

Three types of friction: Three types of friction:

1) sliding friction 1) sliding friction

2) rolling friction 2) rolling friction

3) fluid friction 3) fluid friction (water, oil, air…)(water, oil, air…)

InertiaInertia is the property of matter is the property of matter that tends to resist any change that tends to resist any change in motionin motion. .

The more massive an object is, The more massive an object is, the more difficult it is to change the more difficult it is to change its motion. (The more massive an its motion. (The more massive an object is, the more inertia it has.)object is, the more inertia it has.)

Newton’s First Law of MotionNewton’s First Law of Motion

An object at rest will remain at An object at rest will remain at rest and an object in motion will rest and an object in motion will remain in motion unless acted remain in motion unless acted upon by an unbalanced force.upon by an unbalanced force.

Newton’s Second Law of MotionNewton’s Second Law of Motion

The second law of motion The second law of motion describes how force, describes how force, acceleration and mass are acceleration and mass are related.related.

Force = Mass x AccelerationForce = Mass x Acceleration

Newton’s Third Law of MotionNewton’s Third Law of Motion

The third law of motion states The third law of motion states that for every action, there is an that for every action, there is an equal and opposite reaction.equal and opposite reaction.

Another way to state the third law is to Another way to state the third law is to say that every force must have an say that every force must have an equal and opposite force.equal and opposite force.

Newton’s Law of Newton’s Law of Universal Gravitation Universal Gravitation

This law states that all This law states that all objects in the universe objects in the universe attract each other by the attract each other by the force of gravity. force of gravity. (The size (The size of the force of gravity of the force of gravity depends on the masses of depends on the masses of the two objects and the the two objects and the distance between them.) distance between them.)

Gravity is a force of attraction that Gravity is a force of attraction that exists between all objects in the exists between all objects in the universe.universe.

Acceleration due to gravity Acceleration due to gravity at the at the surface of the Earth surface of the Earth is 9.8 is 9.8 m/sec/sec.m/sec/sec.

Weight and MassWeight and Mass

Weight is a measure of the pull of Weight is a measure of the pull of gravity on an object. gravity on an object.

Since weight is a force, Since weight is a force, its unit is the its unit is the newtonnewton (N). (N).

Mass is a measure of the amount of Mass is a measure of the amount of matter in an object. Mass is constant.matter in an object. Mass is constant.

Mass and weight are not the same. Mass and weight are not the same. Your mass will always stay the same Your mass will always stay the same unless you diet and exercise. unless you diet and exercise. Weight Weight can change with location. can change with location. (On top of (On top of a tall mountain, you weigh less, your a tall mountain, you weigh less, your mass however, remains the same.)mass however, remains the same.)