MOTION, FORCES, AND SIMPLE MACHINES!MOTION, FORCES, AND SIMPLE MACHINES!

Explore ActivityExplore Activity Release the ball from a point near the bottom of the

curve.Describe the motion.How high does it go?When is its speed the greatest?

Release the ball from a point near the top of the curve.Describe the motion.How high does it go?When is its speed the greatest?

Compare your results.

I. MotionI. Motion To describe how fast a

bike is traveling, you must know two things about its motion:

1. The distance it has traveled2. How much time it took to travel that distance.

A. Average SpeedA. Average Speed

Average speed = Total distance traveledtravel time

s = speed, d = distance, and t = time

s = d_ t

Speed cont.Speed cont.

Because average speed is always calculated by dividing distance by time, its units will always be a distance unit divided by a time unit.

EX: Speed of a car? Miles / hour

Calculating Average SpeedCalculating Average Speed Riding your bike, it takes you 30 minutes to get

to your friend’s house, which is 9 km away. What is your average speed?

SOLUTION: What you know: d = 9 km

t = 30 min = 0.5 h What you NEED to know: s Equation you will use: s = d/t Substitute in values: s = 9 km / 0.5 h

ANSWER s = 18 km / h

Calculating Average Speed PRACTICECalculating Average Speed PRACTICE

If an airplane travels 1,350 km in 3 h, what is its average speed?

SOLUTION: What you know: d = 1,350 km

t = 3 h What you NEED to know: s Equation you will use: s = d/t Substitute in values: s = 1,350 km / 3h ANSWER s = 450 km / h

B. Instantaneous SpeedB. Instantaneous Speed Average speed is useful when you don’t need to

know details of the motion. EX: a car trip…..traffic lights, traffic jam, high

speed highway…. What if you want to know your speed at a certain

time? Instantaneous speed: the speed of an object at

any instant of time.(in a car, how is your instantaneous speed given?)

B. Instantaneous Speed cont.B. Instantaneous Speed cont.

How is instantaneous speed different from average speed?

C. Constant SpeedC. Constant Speed

Constant Speed is when an object’s instantaneous speed does not change.

At a constant speed, average speed and instantaneous speed are the same!

D. DistanceD. Distance

If an object is moving with constant speed, you can use the equation you used for speed to calculate distance!

s = d speed = distance / time

t d = s x t distance = speed x time

Calculating DistanceCalculating Distance A marathon runner can maintain a constant speed of 16

km/h, how far can she run in 24 min (0.4 h)? SOLUTION: What you know: s = 16 km/h

t = 0.4 h What you NEED to know: d Equation: d = s x t Substitute values d = 16 km/h x 0.4 h SOLUTION d = 6.4 kmNOTICE: units of time in the speed MUST be the same as

the time or they won’t cancel! If they aren’t the same in your problem, you must convert!

Calculating DistanceCalculating Distance It takes your family 2 h to drive to Idlewild Park at an

average speed of 73 km/h. How far away is the park? SOLUTION: What you know: s = 73 km / h

t = 2 h What you NEED to know: d Equation: d = s x t Substitute values d = 73 km/h x 2 h SOLUTION d = 146 km

E. VelocityE. Velocity You are walking down the street at a constant

speed heading north. You turn when you reach an intersection and start walking with the same speed, but you are now heading east.

You motion has CHANGED even though your speed is the same.

To describe your motion, you need to tell not only how fast you were going but in what direction.

E. VelocityE. Velocity Velocity: the speed of an object and its direction

of motion.

Velocity changes when speed changes, the direction of motion changes or both change!

When you turned the corner, even though your speed didn’t change, your direction did, SO your velocity changed.

F. AccelerationF. Acceleration

You’re skateboarding the half-pipe. At the top, you are at rest, your speed is zero. When you start down, you smoothly speed up,

going faster and faster. If the angle of the half-pipe were steeper, you

would speed up at an even greater rate.

F. AccelerationF. Acceleration

Speed describes how distance traveled changes with time.

Acceleration describes how velocity changes with time.

Acceleration is the change in velocity divided by the time needed for the change to occur.

F. AccelerationF. AccelerationMarble

MarbleMarble

Ramp

Ramp

Ramp

Acceleration Acceleration

A marble rolling down a hill speeds up. Its motion and acceleration are in the same direction.

This marble is rolling on a level surface with constant velocity. Its acceleration is zero

A marble rolling up a hill slows down. Its motion and acceleration are in opposite directions.

Calculating AccelerationCalculating Acceleration If the direction of motion is not changing, motion

is in a straight line and you can use this formula to calculate acceleration:

Acceleration = change in speed / timea = acceleration, t = time

si = initial speed, sf = final speed

a = (sf – si) t

Calculating AccelerationCalculating Acceleration If an object at rest accelerates to a final speed of 10 m/s

in 2s, what is the acceleration? What you know: si (initial speed) = 0 m/s

sf (final speed) = 10 m/st = 2 s

What you want to know: a = ? FORMULA: a = (sf – si)

tSubstitute in Values: a = (10m/s – 0m/s)

2 sSOLVE: a = 5m/s2

Calculating AccelerationCalculating Acceleration You are sliding on a snow covered hill at a speed of

8m/s. There is a drop that increases your speed to 18 m/s in 5s. Find your acceleration.

What you know: si (initial speed) = 8 m/ssf (final speed) = 18 m/st = 5 s

What you want to know: a = ? FORMULA: a = (sf – si)

tSubstitute in Values: a = (18m/s – 8m/s)

5 sSOLVE: a = 2 m/s2

Calculating AccelerationCalculating Acceleration Practice Problem:

The roller coaster you are on is moving at 10 m/s. 5 s later, it toes a loop-the-loop and is now moving at 25 m/s. What is the roller coaster’s acceleration over this time?

Calculating AccelerationCalculating Acceleration What you know: si (initial speed) = 10 m/s

sf (final speed) = 25 m/s

t = 5 s What you want to know: a = ? FORMULA: a = (sf – si)

tSubstitute in Values: a = (25m/s – 10m/s)

5 sSOLVE: a = 3 m/s2

Graphing SpeedGraphing Speed The acceleration of an object can be shown on a

speed – time graph.

TIME

Speed As you skate down hill, the speed

increases when the acceleration is in the direction of the motion.

When the acceleration is zero, the speed

remains constant.

When the acceleration is in the opposite direction as the motion, the speed decreases.

PRACTICEPRACTICE1. During rush-hour traffic in a big city, it can take 1.5 h to

travel 45 km. What is the average speed in km / h for this trip?

2. A car traveling 20 m/s breaks and takes 3 seconds to stop. What is the acceleration in m/s?

3. A runner accelerates from 0 m/s to 3m / s in 23 s. What is the accelerations.

4. If an airplane is flying at a constant speed of 500 km /h. Can it e accelerating? Explain.

5. Describe the motion of a skateboard as it accelerates down one side of a half-pipe and up the other side. What would happen if the upside of the up side pipe were not as steep as the downside?

Section 2Section 2

Newton’s Laws of Motion

Objectives:Objectives:

Describe how forces affect motion Calculate acceleration using Newton’s second

law of motion Explain Newton’s third law of motion

ForceForce

A force is a push or a pull.

Forces are what cause objects to move. Force is measured in newtons (N)

One newton is about the amount of force it would take to lift a Quarter-Pounder.

Force and AccelerationForce and Acceleration Exerting a force on an object causes its motion

to change. THEREFORE: Forces cause objects to

accelerate. (an object has acceleration when it’s speed or

direction of motion changes.) Anytime a force acts on something, its speed

changes or its direction of motion changes, or BOTH change.

After a ball is thrown, it follows a curved path toward the ground. How does this curved path show that the ball is accelerating?

Balanced and Unbalanced ForcesBalanced and Unbalanced Forces If more than one force is acting on an object and

does not cause the objects motion to change, it is a balanced force.

Balanced forces cancel each other out. If more than one force is acting on an object and

it’s motion DOES change, the forces are unbalanced.

Unbalanced forces DO NOT cancel each other out.

Example – Balanced ForcesExample – Balanced Forces

Example – Unbalanced ForcesExample – Unbalanced Forces

Combining ForcesCombining Forces

When more than one force acts on an object, the forces combine

The combination of all of the forces acting on an object is called the net force.

Net ForcesNet Forces

If forces are in the same direction, they add together to form the net force.

If forces are in opposite directions, the net force is the difference between the two forces and is in the direction of the larger force.

Net ForcesNet Forces

Net Forces PracticeNet Forces Practice

GravityGravity If you hold a basketball shoulder height then let

it go, what force pulls it to the ground? Gravity of course! Gravity is defined as the pull that all objects

exert on each other. When you drop the basketball, the gravitational

force between the ball and Earth, pulled it toward the Earth.

GravityGravity Objects don’t have to be touching to exert

gravitational force on each other. The gravitational force between two objects gets

weaker as the objects get further apart. Also, the gravitational force is weaker between

objects of less mass – like you and your desk, compared to objects of greater mass like you and the Earth.

Gravity!Gravity!Less Gravitational Force between objects of smaller mass.

More gravitational Force between an object of small mass and one of large mass.

More Gravitational Force between objects that are close together.

Less Gravitational Force between objects that are further apart.

Newton’s First Law of MotionNewton’s First Law of Motion An object in motion stays in motion and an

object at rest stays at rest until a force acts on it.

This means, an object’s motion won’t change unless a force acts on it.

EX: Your paper stays on your desk, unless you pick it up or a breeze blows it.

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

Also, an object in motion continues to stay in a constant motion, unless a force acts on it.

WAIT! Say I throw a football….it eventually falls to the ground…

But I thought objects in motion stay in motion… What force acted on the football? GRAVITY!

FrictionFriction WAIT! Newton’s first law says that a moving object

should never slow down or change direction until a force acts on it.

BUT if you push a book across the table, it slows down to a stop! WHY?

FRICTION! This is the force that acts on it and causes it to stop.

Friction is a force that resists motion between two surfaces that are in contact. It always acts in the opposite direction to motion.

To keep an object moving when friction is acting on it, you have to keep applying force!

Friction cont.Friction cont.

Friction is caused by the roughness of the surfaces in contact.

A rougher surface will cause more friction and a smoother surface will cause less friction.

EX: if you push a hockey puck on ice it will travel further before it stops than if you push it on carpet.

Inertia and MassInertia and Mass

How easy would you say it is to move a refrigerator?

Pretty difficult huh? How about stopping a person on a skateboard

who is much bigger than you are? Ouch.

Inertia and MassInertia and Mass

When moving or stopping a big object, the object resists a change in motion.

Inertia, is the tendency of an object to resist motion – this is another way to state Newton’s first law of motion. The Law of Inertia

The more matter an object has the more inertia it has!

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

Newton’s first law says that a change in motion won’t occur unless there is a net force large enough to move the object or stop an object that is moving.

Newton’s SECOND law tells how a net force acting on an object changes the motion of an object.

Newton’s Second Law of MotionNewton’s Second Law of Motion Newton’s second law says:

A net force changes the velocity of the object and causes it to accelerate.

This includes two things:1. IF an object is acted on by a net force, the change in velocity will

be in the direction of the net force.2. Acceleration can be calculated from the following equation:

F = m x a

F (force) m (mass) a(acceleration)

in Newtons in kilograms in meters per

second squared.

Mass and AccelerationMass and Acceleration

When a net force acts on an object, it’s acceleration depends on the mass of the object!

The more mass an object has, the more force is necessary to cause it to accelerate!

EX: it’s much harder to move a refrigerator than it is to move an empty shopping cart,

BECAUSE – the refrigerator has more mass.

Mass and Acceleration cont.Mass and Acceleration cont. With the same force acting on a refrigerator and

an empty shopping cart, the refrigerator will have LESS acceleration than the empty cart.

MORE MASS = LESS ACCELERATION with the same amount of force.

Newton’s Third Law of MotionNewton’s Third Law of Motion Remember when Ms. Smyder pushed the wall? You might be surprised to know….THE WALL

PUSHED BACK! Well…the wall isn’t alive, but Netwon’s Third

Law says: When one object exerts a force on a second

object, the second object exerts an equal force in the opposite direction.

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

EXAMPLE: When you walk, you push down on the sidewalk and it pushes back up on you with an equal and opposite force.

The force exerted by the first object is the action force, the force exerted by the second object is the reaction force.

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

ACTION REACTION

Force Pairs Act on Different ObjectsForce Pairs Act on Different Objects

IF forces always act in equal but opposite pairs, you might ask – HOW CAN ANYTHING EVER MOVE??

Won’t forces acting on an object always cancel each other out?

REMEMBER, equal and opposite forces act on DIFFERENT objects.

When you push on a book, a force is acting on the book. When it pushes back on you, the force is acting on you!

Examples of Newton’s Third LawExamples of Newton’s Third Law Imagine you are jumping off of a small boat. You are pushing back on the boat with your feet

and the same force is pushing you forward. Since you have more mass than boat, it will

accelerate more and move farther than you do.

Examples of Newton’s Third LawExamples of Newton’s Third Law This would be the opposite if you jumped off of a

bigger boat. Since the boat has so much more mass, the

force that you exert on the boat only gives a tiny acceleration – in fact, you probably won’t notice the boat moving at all. BUT the force the boat exerts on you might easily propel you to the dock near you.

Section 2 - PracticeSection 2 - Practice 1. Make a table listing Newton’s laws of motion. For

each law, include the definition and give at least one example from your everyday life. Do not use examples from the notes!!!!

2. Does a force act on a car if it moves at a constant speed while turning? Explain.

3. You throw a ball to your friend. If the ball has a mass of 0.15 kg and it accelerates at 20 m/s2, what force did you exert on the ball?

4. Give at least two examples of using inertia to your advantage.

5. Newton’s third law is a good example of cause and effect. Explain why, using a ball bouncing off of a wall as an example.

Section 3

Work and Simple MachinesDefine WorkDistinguish the different types of simple machinesExplain how machines make work easier

Section 3

Work and Simple MachinesDefine WorkDistinguish the different types of simple machinesExplain how machines make work easier

WorkWork

Newton’s Laws explain how forces change the motion of an object.

When you apply an upward force on a box, it moves UP!

What is work??

Work cont…Work cont…

In science, WORK is done when a force causes an object to move in the same direction that the force is applied.

Effort doesn’t always Equal WorkEffort doesn’t always Equal Work

Remember when Ms. Smyder pushed the wall? She tried really hard! But the wall didn’t move.

In order for WORK to be done, two things MUST happen:

1. A force must be applied on an object2. The object must move in the direction of the

force.If the object doesn’t move, no work was done.So, did Ms. Smyder do work when she pushed the

wall?NO!

Imagine you are lifting a box. You can feel your arms exerting a force as you lift upward. The box moves upward, in the direction of your force.YOU HAVE DONE WORK!

Imagine you carry the box forward.You can still feel your arms applying an upward force on the box.BUT the box is moving forward….not up.NO WORK IS BEING DONE BY YOUR ARMS!Because the motion is not in the same direction as the force.

Calculating WorkCalculating Work

The greater the force applied that makes an object move, the more work is done.

Which of these tasks would involve more work? Lifting a shoe from the floor to your waist

OR Lifting a pile of books the same distance

Calculating Work cont…Calculating Work cont…

Even though you lifted them the same distance, more work is done when you lift the books because it takes more force to lift them!

You can calculate work:work = force x distance W = F x d

Calculating Work cont…Calculating Work cont…

Force is measured in newtons (N)

Distance is measured in meters (m)

Work is measured in joules (J)Energy is also measured in joules. Named for

James Prescott Joule, a 19th century physicist who showed that work and energy are related.

Calculate WorkCalculate Work A weight lifter lifts a 500 N weight a distance of 2 m

from the floor to a position over his head. How much work does he do?

Know: Force = F = 500 Ndistance = d = 2 m

Want to know: work = W Equation: W = F x d Substitute: W = 500 N x 2 m Solve: W = 1000 J

PRACTICEPRACTICE Using a force of 50 N, you push a computer cart

10 m across a classroom floor. How much work did you do?

Know: F = 50 Nd = 10 m

Want to Know: WEquation: W = F x dSubstitute: W = 50 N x 10 mSolve: W = 500 J

What is a Machine?What is a Machine?

A machine is a device that you use that makes work easier.

A can opener changes a small force applied by your hand into a larger force and makes it easier to open the can.

Simple MachinesSimple Machines A simple machine is a machine that uses only

one movement. The following are Simple Machines:

1. Inclined Plane2. Wedge3. Screw4. Lever5. Wheel and axle6. Pulley

Simple MachinesSimple Machines

EX: A screwdriver is a simple machine, it only

requires one movement – to turn it!

Compound MachinesCompound Machines A compound machine is a combination of simple

machines. A can opener is an example of a compound

machine – it combines several simple machines.

Machines make work easier in two ways:1. They can change the size of a force applied2. They can also change the direction of the force.

Mechanical AdvantageMechanical Advantage The number of times the applied force is

increased by a machine is called the mechanical advantage.

When you push on the handles of the can opener, the force that you apply is called the input force (Fi).

The can opener changes your input force to the force that is exerted by the metal cutting blade on the can.

The force exerted by a machine is called the output force (Fo).

Mechanical AdvantageMechanical Advantage Mechanical advantage is the ratio of the output

force to the input force:

Mechanical advantage = Output force input force.

MA = Fo / Fi

Work in and Work outWork in and Work out

In a simple machine, the input and the output force do work:

You push on the handles of a can opener and the handles move – work is done (input)The blade of the can opener moves down and punctures the can – work is done (output)

In an ideal machine, there is no friction.

Increasing ForceIncreasing Force A simple machine can make a small input force

into a larger output force. W = F x d SO if work in is equal to work out, a smaller input

force must be applied over a larger distance than the larger output force.

Think about that can opener, it increases the force that you apply to the handle SO the distance you move the handle is large compared to the distance the blade of the can opener moves as it pierces the can.

Increasing Force cont…Increasing Force cont…

In all real machines, friction always occurs as one part moves past another.

Friction causes some of the input work to be changed into heat, which can’t be used to do work.

SO for real machines, work out will always be less than work in.

The PulleyThe Pulley Think about a window blind. To raise it,

you pull down on the cord. A pulley is an object with a groove, like

a wheel, with a roper or chain running through the groove.

It changes the direction of the input force.

A rope thrown over a railing can be used as a pulley.

A simple pulley changes only the direction of the force, so its mechanical advantage is 1.

The PulleyThe Pulley It is possible to have a large

mechanical advantage if more than one pulley is used.

A double pulley system has a mechanical advantage of 2.

Each supporting rope holds half of the weight, so you need to supply only HALF the input force to lift it!

The LeverThe Lever

Probably the first simple machine used by humans was the lever

A lever is a rod or plank that pivots about a fixed point.

The pivot point is called the fulcrum. Levers can increase force or increase the

distance over which a force is applied.

The LeverThe Lever There are 3 classes of

Levers

A first – class lever:The Fulcrum is located between the input force and output force.It is usually used to increase force.

The LeverThe Lever A second – class lever:

When output force is between the input force and the fulcrum, like a wheelbarrow.The output force always is greater than the input force for this type of lever.

The LeverThe Lever A third – class lever:

The input force is located between the output force and the fulcrum.The mechanical advantage of a third – class lever is always less than one.It increases the distance over which the input force is applied.An example is a hockey stick.

The Wheel and AxleThe Wheel and Axle

Think about opening a doorknob. Now think about trying to open it by holding the

narrow base of the knob. Which is easier? Holding the knob of course! This is an example of a wheel and axle

The Wheel and Axle cont…The Wheel and Axle cont… A wheel and axle is made of two round

objects that are attached and rotate together about the same axis.

Usually, the larger object is called the wheel and the smaller object is called the axle.

Mechanical advantage of a wheel and axle can be calculated by dividing the radius of the wheel by the radius of the axle.

The Inclined PlaneThe Inclined Plane

An inclined plane is a sloped surface, sometimes called a ramp.

It allows you to lift a heavy load using less force over a greater distance.

The Inclined Plane cont…The Inclined Plane cont… Imagine having to lift a couch 1 m off the ground onto a truck,

if you used an inclined plane, you would have to move the couch farther than if you lifted it straight up. Either way, the same amount of work needed to move the couch.

Because the couch moves a longer distance using the inclined plane, doing the same amount of work takes less force.

The WedgeThe Wedge When you take a bite out of an apple, you are

using wedges, your teeth. A wedge is a moving inclined plane with one or

two sloping sides. As you push your front teeth into the apple, the

downward input force pushes the skin of the apple apart.

Knives and axes are also wedges that are used to cut.

The ScrewThe Screw Roads going up a mountain usually wrap around

the mountain instead of going straight up. Since it’s less steep, it’s easier to climb even

though you have to travel a greater distance. This is similar to a screw.

The Screw cont…The Screw cont… A screw is an inclined plane wrapped

around a post. The inclined plane forms the screw’s

threads. A screw changes the direction of the

force that you apply. When you turn a screw, the input force is

changed by the threads to an output force that pulls the screw into the material.

Friction between the threads and the material holds the screw tightly in place.

Practice!Practice!1. How much work would it take to lift a 1,000 kg

limestone block 146 m to the top of the Great Pyramid?

2. Explain how you can tell if work is being done on an object.

3. Using a pulley system with a mechanical advantage of 10, how large an input force would be needed to lift a stone slab weighing 2,500 N?

4. Compare a wheel and axle to a lever.5. Identify two levers in your body. What class of lever

is each?