FORCES AND

MOTION

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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.

ForcesWhat types of forces can you think of?

How is the object affected by those forces?

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.

Examples of forces acting on objects would be:

Opening and closing a door.

A space shuttle lifting off

wagons

What type of force would these objects need to move?

kites

sleds

Sports equipment

Forces give an object the energy to move, stop moving, or change direction.

FORCES ALWAYS OCCUR IN PAIRS

Definition

Characteristics

Examples

Picture

Force

1.Force2.Net Force3.Unbalanced

Force4.Balanced

Force5.Friction

6. Motion7. Reference Point8. Speed9. Average Speed10. Velocity11. Acceleration

Total amount of force acting on an object.Will also determine motion and direction of

motion.

Total amount of force acting on an object.Will also determine motion and direction of

motion.

The total amount of forces acting on an object.

When the forces are acting in the opposite direction you subtract the forces.

Total amount of force acting on an object.When forces are acting in the same direction

you add the forces together to get the net force.

HOW DOES FORCE CAUSE MOTION?

Motion is a result of UNBALANCED FORCES.

UNBALANCED FORCE is when the net force on an object is greater than 0.

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. = when the net force on an object is zero.The paired forces occur in opposite directions and are of equal force.

EXAMPLES OF BALANCE FORCE

There is an equal amount of people playing tug of war.

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

FRICTION : FORCE THAT OPPOSES MOTIONFriction - The force that opposes motion between

two surfaces. Friction occurs because all surfaces are

covered with microscopic hills and valleys, which make the surface rough.

When surfaces touch, the hills and valleys stick to each other and this causes friction even if the surfaces look smooth.

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?

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.

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 !!!!!

The rougher the surface, the ________ the friction.

If the force is increased, the hills and valleys of the surfaces can come in closer contact so friction is _______________.

Less massive objects exert less force on surfaces than more massive objects so friction is _______.

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.

Motion is a change in an objects position over time when compared with a reference point.

Reference point or frame of reference is the background or object that appears to stay in place when compared to an object being observed in motion.***Most common reference point is the Earth’s surface.***Other reference points are buildings, trees, mountains etc.

What reference point do the balloonists use?

Reference Point

Motion is not absolute, it depends on the observer. For example, if Jack and Jill are sitting in a bus, Jack sees Jill at rest and Jill sees Jack at rest. BUT, if we zoom out, we see that Jack and Jill are sitting on a bus moving at a constant speed of 25mph. From the reference INSIDE the bus, Jack sees Jill at rest and Jill sees Jack at rest.

Jill’s mother, waiting for the bus, sees Jill’s bus go by at 25mph. She sees Jill traveling 25mph. Jack on the other hand, still sees Jill as going 0mph. What they observe seems to depend on their reference point.

While riding the bus, Jill begins throwing cookies to Jack. Jill can throw cookies at a speed of 30 mph. From their reference point, Jack and Jill both see the cookies going 30mph.

As Jill’s mom observes the cookie tossing from where she is standing on the side of the road, she sees the cookies going 30mph on a bus moving 25mph in the same direction. SO, she observes the

cookies going 55mph!!!!!!

As Jill continues to throw 30mph cookies at Jack, the bus drives by Jack’s mom on a bicycle. She is riding 10mph in the same direction as the bus is traveling 25mph. From her point of reference, Jack seems to only be going 15mph (25mph – 10mph) and she see the tossed cookies going 45mph (15mph + 30mph).

As the cookie tossing between Jack and Jill continues, a car drives by going 40mph in the other direction. The observer in the car sees Jack going 65mph (40mph + 25mph). The observer then notices the flying cookies to be traveling a whopping 95mph!!!!!! (40mph + 25mph + 30mph).

Projectile Motion: the curved path an object follows when thrown near the surface of the Earth.

Centripetal Motion: when an object moves in a circular path.Ex: Moon in centripetal motion as it revolves

around the Earth.

Speed is the rate at which an object moves. This is determined by the distance traveled and the time it takes to travel.

SO… SPEED = DISTANCE / TIMEUNITS FOR SPEED: meters per sec (m/s)miles per hour (mi/hr), feet per sec (ft/s)

While on vacation, Lisa traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed?

While on vacation, Lisa traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed?

SPEED – 55 MILES PER HOUR

SPEED = DISTANCE/TIME OR SPEED = D/T

DISTANCE – 440 MILES

TIME – 8 HRS

Average Speed = Total distance traveled divided by Total time of the trip

Constant speed – speed that does not change. Every plot point on the graph has the same speed

When you see a graph like this, you will know that the object is traveling at a constant speed. Double check yourself by calculating the speed at each point.

Velocity is the speed of an object in a specific direction.

Speed and Velocity do not mean the same thing

Velocity changes as speed or direction changes.

Which of the following are examples of velocity?

1. 25 m/s forward2. 1/500 km/h3. 55 m/h south4. 10, 000 m/s

X

X

2 velocities in the SAME DIRECTION – add them together to find the resultant velocity.

2 velocities in DIFFERENT DIRECTIONS – subtract the smaller velocity from the larger velocity to find the resultant velocity.

Acceleration is the rate at which velocity changes. SO…..if an object speeds up, slows down or changes direction, it has ACCELERATED.

SO… what about this? What type of motion does it have? Is the object accelerating?

Is the object changing speed?

Is the object changing direction?YES!!!! An object traveling in a circle is constantly changing direction. SO… is the object accelerating?

NO!

Objects that are continuously accelerating is called CIRCULAR MOTION. This means that the direction is constantly changing.

LET’S PRACTICE

1. Consider a car moving with a constant velocity of 10m/s for 5 seconds. Look at the data table and draw a graph of the results into your notes.

Does your graph look like this? What kind of graph is this? What is the speed?

The object is traveling at a constant speed of 10 m/s for 5 seconds.

2. Consider a car moving with a constant velocity of 5 m/s for 5 seconds, stopping abruptly, and then remaining at rest for 5 seconds. Graph the information into your notes.

Does your graph look like this? Whenever there is a flatline, this means motion has stopped.

3. Look at the picture and graph the information.

Does your graph look like this? What is the speed? Graphs that look

like this always show …….

ACCELERATIONSpeed is NOT the same at every point along the line

CAR 1 SPEED GRAPH

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Drag Car 1 traveled 240m in 12 sec. Its average speed was 20m/s. I know it traveled at a constant speed because of the diagonal line on the graph and the fact that the speed was the same at every point along the graph.

CAR 2 SPEED

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Drag Car 2 traveled 150m in 12 sec. Its average speed was 12.5m/s. I know the car accelerated for the first 9 sections of the race and then came to an abrupt stop at 10 seconds and remained stopped for the remainder of the race.

CAR 3 SPEED

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Drag Car 3 traveled 112m in 12 sec. Its average speed was 9.3m/s. This car accelerated throughout the entire race going faster at the beginning but very slowly at the end.

CAR 4 SPEED

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Drag Car 4 traveled 315m in 12 secs. Its average speed was 26.3m/s. Car 4 accelerated for the first two seconds and then came to an abrupt stop. It stayed still for 3 seconds. It started moving again and accelerated very rapidly until the end of the race.

THINK ABOUT RIDES IN AN AMUSEMENT PARK

1. WHICH RIDES HAVE CONSTANT SPEED?

2. WHICH RIDES HAVE CIRCULAR MOTION?

3. WHICH RIDES ACCELERATE?

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.