FACTS For FACTS For FORCESFORCESDEFINITIONSDEFINITIONS

Review and learn the definitions of Review and learn the definitions of the special words used in the special words used in physics.physics.

By Rebecca K. Fraker, 2010

ForceForce

A push or a pull on an object.A push or a pull on an object.

MassMass

The amount of matter that The amount of matter that makes up an objectmakes up an object

Under normal circumstances the Under normal circumstances the amount of matter in an object does amount of matter in an object does not change.not change.

GravityGravity

The pull of matter on The pull of matter on matter.matter.

The greater the mass of an object The greater the mass of an object the greater its gravitational pull the greater its gravitational pull on another object.on another object.

• Small mass = small pullSmall mass = small pull

• Large mass = large pullLarge mass = large pull

Gravity continued…Gravity continued…• Small mass = small pullSmall mass = small pull

• Large mass = large pullLarge mass = large pull

You and everything that has mass You and everything that has mass (your dog, your pencil, your (your dog, your pencil, your lunch) has gravity.lunch) has gravity.

WeightWeight Weight is a measure of the force Weight is a measure of the force

of gravitational attraction.of gravitational attraction.

MASS does not change BUT…MASS does not change BUT…WEIGHT does. WEIGHT does.

Weight depends on the Weight depends on the gravitational attraction. gravitational attraction.

Weight continued…Weight continued…

Weight depends on the Weight depends on the masses of the objects,masses of the objects,

and the distance between and the distance between the centers of the masses.the centers of the masses.

Systems of Systems of MeasurementMeasurement

Mass and WeightMass and Weight

The Standard International The Standard International System, known as SI, is based System, known as SI, is based on the metric system.on the metric system.

The English System is based on The English System is based on pounds and ounces.pounds and ounces.

MeasuringMeasuring SystemsSystems

• SI SystemSI System: Mass is measured by : Mass is measured by kilograms, Weight by Newtons.kilograms, Weight by Newtons.

• English SystemEnglish System: Mass is measured : Mass is measured by slugs, Weight by pounds.by slugs, Weight by pounds.

Remember!Remember! Weight is the Weight is the measure of the gravitational measure of the gravitational pull on the mass of an object.pull on the mass of an object.

ShoutShout it out! it out!

Can you answer this Can you answer this question?question?

Here on Earth, your mass is 32 Here on Earth, your mass is 32 kilograms. What is your mass kilograms. What is your mass on the planet Quagga?on the planet Quagga?

ANSWER:ANSWER:

Your mass remains the Your mass remains the same on Quagga as on same on Quagga as on Earth, because mass is the Earth, because mass is the amount of matter in an amount of matter in an object and that does not object and that does not change when you travel.change when you travel.

Shout it out!Shout it out!

On the planet Zero On the planet Zero you weigh 100 you weigh 100 pounds. The gravity pounds. The gravity on planet Zero is on planet Zero is higher than that of higher than that of Earth. What Earth. What happens to your happens to your weight when you go weight when you go to Earth?to Earth?

You will weigh less on Earth You will weigh less on Earth because the pull of gravity is because the pull of gravity is less. But your mass remains the less. But your mass remains the same.same.

FrictionFriction

The rubbing of one object or The rubbing of one object or surface against another. surface against another.

A force on objects or A force on objects or substances in contact with substances in contact with each other that resists motion each other that resists motion of the objects or substances of the objects or substances relative to each other.. relative to each other..

Static FrictionStatic Friction

• Static frictionStatic friction arises between two arises between two objects that are objects that are not in motionnot in motion with with respect to each other: for example respect to each other: for example between a cement block and a wooden between a cement block and a wooden floor. floor.

• To counterbalance forces that would To counterbalance forces that would move the objects, it increases up to a move the objects, it increases up to a certain maximum level of force. At this certain maximum level of force. At this point the objects will begin moving. point the objects will begin moving.

• It is measured as the maximum force It is measured as the maximum force the bodies will sustain before motion the bodies will sustain before motion occurs.occurs.

Kinetic frictionKinetic friction

Kinetic frictionKinetic friction arises between arises between bodies that are bodies that are in motionin motion with with respect to each other: for example respect to each other: for example the force that works against sliding the force that works against sliding a brick along a wooden floor.a brick along a wooden floor.

Between two hard surfaces, the Between two hard surfaces, the kinetic friction is usually somewhat kinetic friction is usually somewhat lower than the static friction, lower than the static friction, meaning that more force is required meaning that more force is required to set the objects in motion than to to set the objects in motion than to keep them in motion. keep them in motion.

DisplacementDisplacement A rabbit is being chased by a fox. A rabbit is being chased by a fox.

It is running for its hole. But it It is running for its hole. But it does not run in a straight line. does not run in a straight line. Instead it zigzags.Instead it zigzags.

It runs ten feet to the right, then It runs ten feet to the right, then ten feet to the left and then 20 ten feet to the left and then 20 feet straight and another 5 to the feet straight and another 5 to the right. Altogether it ran 45 feet to right. Altogether it ran 45 feet to get to safety.get to safety.

DisplacementDisplacement

But if the rabbit had run straight,But if the rabbit had run straight, it would have only had to run 28 it would have only had to run 28

feet. feet.

That measurement is the That measurement is the displacement.displacement.

Momentum

The quantity of motion in a moving The quantity of motion in a moving body is its body is its momentum.momentum.

It is always proportioned to the It is always proportioned to the quantity of matter multiplied into quantity of matter multiplied into the velocity.the velocity.

Another word for it is impetus.Another word for it is impetus.

InertiaInertiaIt takes an extra strong push to It takes an extra strong push to start an object moving because of start an object moving because of inertiainertia. It wants to stay where it is.. It wants to stay where it is.

It takes an extra strong force to It takes an extra strong force to STOP something once it is moving STOP something once it is moving because of because of inertiainertia. It wants to . It wants to keep on moving.keep on moving.

InertiaInertia

This is stated in This is stated in Newton’s First Law Newton’s First Law of Motion: A body of Motion: A body at rest tends to stay at rest tends to stay at rest, and a body at rest, and a body in motion tends to in motion tends to stay in motion stay in motion unless acted upon unless acted upon by an outside forceby an outside force.

AccelerationAcceleration

You are driving in at 30 kilometers You are driving in at 30 kilometers per hour. Then you speed up and per hour. Then you speed up and go 50 kilometers per hour. go 50 kilometers per hour.

This upward change in speed is This upward change in speed is called called acceleration.acceleration.

Happy Learning!Happy Learning!