FORCES AND

FRICTION

Bellringer #6

1. What percentage of the Earth’s water is drinkable? a. 3% c. .003%

b. 13% d. 30% 2. In a convection current in the mantle, hot magma ________, while cooler magma ________. a. sinks, rises b. rises, sinks c. rises, stays the same d. syays the same, sinks 3. The earth has day and night because a. the earth is revolving around the sun b. the moon blocks the light from the sun c. the earth is a sphere and only ½ can be facing the sun at a time d. the earth is a sphere and where it is at in its revolution around the sun determines

whether it is day or night

Bellringer #7

1. A force is:a. Anything that is in motion.b. Any push or pull on an object.c. Anything that reacts with another object.

2. Motion is a result of:a. Any force, balanced or unbalanced.b. Only balanced forces.c. Only unbalanced forces.

3.Friction is a result of:a. Two objects rubbing against each other.b. Tiny hills and valleys on the surface of object rubbing

together.c. Both A and B

4.Forces always act in pairs:a. Trueb. False

BELLRINGER

READ THE FIRST PARAGRAPH ON PAGE 11 OF YOUR BOOK. THINK ABOUT OTHER WAYS YOU HAVE HEARD THE WORD FORCE USED.

WHAT IS A FORCE?A force is simply a push or a pull.

THERE IS A BIG RELATIONSHIP BETWEEN

FORCE AND MOTION

WITHOUT A FORCE THERE WOULD BE NO MOTION

Force gives an object the energy to move, stop moving, or change direction.

FORCES ALWAYS OCCUR IN PAIRS

Examples of forces acting on objects would be:

A flag being blown by the force of the wind.

Iron being pulled toward a magnet.

A jet engine propelling an airplane forward.

HOW DOES FORCE CAUSE MOTION?

Motion is a result of UNBALANCED FORCES.

UNBALANCED FORCE is the total force on an object in one direction .

HOW DO UNBALANCED FORCES CAUSE MOVEMENT?

REMEMBER: Forces always come in pairs.

2 Unbalanced forces acting in the same direction get added together and cause motion.

EXAMPLES OF UNBALANCED FORCES

PUSHING A SHOPPING CART

SKIING DOWNHILL

WHAT IS A BALANCED FORCE?

Balanced Forces cause no motion. The paired forces occur in opposite directions and are of equal force.

EXAMPLES OF BALANCE FORCE

Two dogs of equal strength playing tug of war

Skier on a flat surface

LET’S PRACTICE

WHICH PICTURES SHOW BALANCED FORCE?WHICH PICTURES SHOW UNBALANCED FORCE?

FRICTION : FORCE THAT OPPOSES MOTION

Friction - The force that opposes motion between two surfaces.

Friction occurs because the surface of any object is rough. All surfaces are covered with microscopic hills and valleys. When surfaces touch, the hills and valleys of one surface stick to the hills and valleys of the other surface. This causes friction even if the surfaces look smooth.

Would you rather catch a ball diving in the grass or on concrete?Why do ice skates only have a small blade?Why put oil in your car?Why can’t you open a jar with lotion on your hands?

All of these questions deal with friction !!!!!

FACTORS THAT AFFECT THE AMOUNT OF FRICTION

1. Roughness - The more rough the surface; more friction created.2. Force - The more force pushing the object; the more friction between valleys.3. Attraction between objects(static) - The greater the attraction; the more friction created.

Humans have tiny ridges in their hands and feet which increase the amount of friction. This allows

us to clasp objects and avoid slipping.

Tires are designed to increase friction on the road. HOW DO THEY DO THIS?

The rougher the surface the friction, ____________ because ______________________.If the force is increased, the hills and valleys of the surfaces can come closer contact so friction is _______________.Less massive objects exert less force on surfaces than more massive objects so friction is less.

3. Fluid Friction

TYPES OF FRICTION

1. Sliding Friction

2. Rolling Friction

SLIDING FRICTIONDefinition - Friction caused by objects sliding across one another.

Examples - Pushing an object (dresser), sledding, brakes, chalk

ROLLING FRICTIONDefinition - Friction produced by objects such as wheels or ball bearings.

Examples - Wheels are placed under objects to reduce sliding friction. Cars, moving large objects with dollies,

FLUID FRICTIONDefinition - Force exerted by a fluid(gas and liquids).

Examples - Water, oil, and air. Sliding friction is usually changed to fluid friction by adding lubricants.

Why is it on a slick floor?

STATIC FRICTIONDefinition - When a force is applied, but the object does not move.

Examples - Block and a table. Block will move when the force is large enough to overcome the friction.

WAY FRICTION IS HELPFUL

1. Push against car to make it move.2. Brakes to stop objects.3. Between pencil and paper.4. Friction between floor and feet.

WAYS FRICTION CAN BE HARMFUL

1. Friction in the engine causes temperature to rise.2. Wind and water causes erosion.3. Holes in you jeans.4. Hard to move heavy objects.

WAYS TO REDUCE FRICTION

1. Lubricant(liquid or gas)2. Wheels3. Ball bearings4. Make surfaces that rub against each other smoother.

WAYS TO INCREASE FRICTION

1. Make surface rougher(sand on ice or batters glove).2. Increase the force pushing the surfaces together.

Name the different types of friction. Give examples of each.

Name two ways friction is helpful and harmful.

Name two ways to overcome friction.

Gravity, Free Fall, and Air Resistance

Gravity - the force that pulls objects toward one another.

Free Fall – when only gravity is acting on an object. This is rare because of air resistance.

All objects free fall at a rate of 9.8 m/s. Agolf ball and basketball will hit the groundat the same time.

Air Resistance – type of fluid friction which increases with the surface area of the object.