chapter: motion, forces, and simple machines table of contents section 3: work and simple...

Chapter: Motion, Forces, and Simple Machines

Table of ContentsTable of Contents

Section 3: Work and Simple Machines

Section 1: Motion

Section 2: Newton’s Laws of Motion

• To describe how fast something is traveling, you have to know two things about its motion.

• One is the distance it has traveled, or how far it has gone.

• The other is how much time it took to travel that distance.

Speed

MotionMotion

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• To calculate average speed, divide the distance traveled by the time it takes to travel that distance.

Average Speed

MotionMotion

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

Average Speed

MotionMotion

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• For example, the average speed of a bicycle is usually given in meters per second.

• Average speed is useful if you don't care about the details of the motion.

Instantaneous Speed

MotionMotion

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• When your motion is speeding up and slowing down, it might be useful to know how fast you are going at a certain time.

• The instantaneous speed is the speed of an object at any instant of time.

• Sometimes an object is moving such that its instantaneous speed doesn’t change.

Constant Speed

MotionMotion

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• When the instantaneous speed doesn't change, an object is moving with constant speed.

• The average speed and the instantaneous speed are the same.

• If an object is moving with constant speed, then the distance it travels over any period of time can be calculated using the equation for average speed.

Calculating Distance

MotionMotion

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• When both sides of this equation are multiplied by the time, you have the equation for distance.

• Notice that the units of time in the speed, s, and in the time, t, have to be the same.

Calculating Distance

MotionMotion

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• Otherwise, these units of time won't cancel.

• The velocity of an object is the speed of the object and its direction of motion.

Velocity

MotionMotion

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

Velocity

MotionMotion

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• If you are walking north at a constant speed then turn east, continuing at the same speed, you have changed yourvelocity.

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

Acceleration

MotionMotion

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• If the speed but not direction of an object is changing, the object is accelerating.

• The direction of the acceleration depends on whether the object is speeding up or slowing down.

• A marble rolling in a straight line down a hill speeds up.

Acceleration

MotionMotion

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• Its motion and acceleration are in the same direction.

• This marble is rolling in a straight line on a level surface with constant velocity.

Acceleration

MotionMotion

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• Its acceleration is zero.

• A marble rolling in a straight line up a hill slows down.

Acceleration

MotionMotion

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• Its motion and acceleration are in opposite directions.

• If an object changes speed but not direction then its acceleration can be calculated from the following formula.

Calculating Acceleration

MotionMotion

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• The acceleration of an object can be shown on a speed-time graph.

Graphing Speed

MotionMotion

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• The line on the graph rises when acceleration is in the direction of motion.

• A horizontal line shows that the acceleration is zero.

Graphing Speed

MotionMotion

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• Anytime you slow down, acceleration is opposite the direction of motion, and the line on a speed-time graph will slant downward.

11Section CheckSection Check

Question 1

It takes you 15 min to walk to school, which is 2 km away. What is your average speed?


Answer8 km/hr

distance: d = 2 kmtime: t =15 min = 0.25 hrspeed: s = ? m/s

Substitute the known values for distance and time into the speed equation and calculate the speed.


Question 2

Explain the difference between speed and velocity.

Answer

Speed is how fast an object is moving. Velocity describes both an object’s speed and its direction of motion.


Question 3In this speed-time graph, which segment shows the acceleration of the skater moving downhill?


Answer

Segment A shows that as the skater moves down the hill, the speed increases when the acceleration is in the direction of motion.

Force

• A force is a push or a pull.

• In SI units, force is measured in newtons (N).

• One newton is about the amount of force it takes to lift a quarter-pound hamburger.

Newton’s Laws of MotionNewton’s Laws of Motion

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Force and Acceleration• For an object's

motion to change, a force must be applied to the object.

• This force causes the object to accelerate.


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Force and Acceleration

• A force also can change the direction of an object's motion.

• An object has acceleration when its direction of motion changes.


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• Anytime an object's speed, or direction of motion, or both change, a force must have acted on the object.

Balanced and Unbalanced Forces

• More than one force can act on an object without causing its motion to change.

• Two or more forces are balanced forces if their effects cancel each other and they do not cause a change in an object's motion.


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• If the effects of the forces don't cancel each other, the forces are unbalanced forces.

Combining Forces• When more than one force acts on an object,

the forces combine. • The combination of all the force acting on an

object is the net force.


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Newton's Laws of Motion

• Sir Isaac Newton realized that he could explain the motion of objects using a set of principles, which in time came to be called Newton's laws of motion.


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Newton's First Law

• Newton's first law of motion states that an object will not change its motion unless an unbalanced force acts on it.


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• Therefore, an object that is not moving, like a book sitting on a table, remains at rest until something pushes or pulls it.

Newton's First Law• A moving object, like a ball moves in a

straight line with constant speed unless an unbalanced force acts on it.


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• After the ball has been hit, it will move along the ground in a straight line until it is acted on by another force.

Friction• A book slides across a table, slows down,

and comes to a stop.


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• Because its motion changes, a force must be acting on it and causing it to stop.

• This force is called friction. Click image to view movie.

Friction• Friction is a force between two surfaces in

contact that resists the motion of the surfaces past each other.


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• To keep an object moving when friction is acting on it, you have to keep pushing or pulling on the object to overcome the frictional force.

• In general, the rougher the surfaces are, the greater the friction will be.

Inertia and Mass

• The tendency to resist a change in motion is called inertia.


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• The more matter an object has, the harder it will be to move or stop.

• Mass measures the quantity of matter.

• The more mass an object has, the greater its inertia is.

Newton's Second Law• Newton's second law of motion states that if

an object is acted upon by a net force, the acceleration of the object will be in the direction of the net force, and the acceleration equals the net force divided by the mass.


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Mass and Acceleration


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• When pushing a refrigerator, which has a large mass, a large force is required to achieve a small acceleration.

• The more mass an object has, the more inertiait has, so the harder it is to accelerate.

Mass and Acceleration

• If you were to push an empty grocery cart with the same force, its acceleration would be larger.


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Newton's Third Law

• According to Newton's third law, when one object exerts a force on a second object, the seconds object exerts an equal force in the opposite direction on the first object.


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Newton's Third Law


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• When you walk, you push back on the sidewalk and the sidewalk pushes forward on you with an equal force.

Newton's Third Law

• The force exerted by the first object is the action force.


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• The force exerted by the second object is the reaction force.

• The action and reaction forces are equal, but in opposite directions.

Force Pairs Act on Different Objects

• If forces always occur in equal but opposite pairs, how can anything ever move? Won't the forces acting on an object always cancel each other?


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Force Pairs Act on Different Objects

• When you push on the book, your force is acting on the book.


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• When the book pushes back on you, its force is acting on you.

• Because the forces act on different objects, they don't cancel.

Section CheckSection Check

Question 1What is the net force acting on this box if Force 1 is 30 N and Force 2 is 16 N?

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A. 14 NB. 16 NC. 30 ND. 46 N


Answer

The correct answer is A. The net force is the difference of the two forces when two unequal forces act in opposite directions: Net force = Force 1 – Force 2 = 30 N – 16 N

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Question 2

Why is it easier to run on a sidewalk than to run across an ice rink without falling?

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Answer

A sidewalk has more friction. The rougher surface of the sidewalk has greater friction and resists the motion of your foot, so your shoe does not slip.


Question 3

Which object has more inertia: a 5 kg bowling ball or a 5 kg bundle of feathers?

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Answer

Neither. Both objects have the same inertia because they have the same mass.

Work

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

Work and Simple MachinesWork and Simple Machines

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Effort Doesn’t Always Equal Work

• For work to be done, two things must occur.


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• First you must apply a force to an object.

• Second, the object must move in the same direction as the force you apply.

Effort Doesn’t Always Equal Work


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• Work done can be calculated from the equation.

• Work is measured in joules (J).

What is a Machine?


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• A machine is a device that makes work easier. • A simple machine is a machine that uses only

one movement. • Simple machines

include the pulley, lever, wheel and axle, inclined plane, wedge, and screw.

What is a Machine?


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• A compound machine is a combination of simple machines.

• Machines can make work easier in two ways.

• They can change the size of the force you apply. They also can change the direction of the force.

Mechanical Advantage


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• The number of times the applied force is increased by a machine is called the mechanical advantage (MA) of the machine.

• The force you apply is called the input force (Fi).

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

Mechanical Advantage


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• The mechanical advantage is the ratio of the output force to the input force.

Work in and Work Out


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• An ideal machine is a machine in which there is no friction.

• Then the work done by the input force is equal to the work done by the output force.

• For the ideal machine, the work you do on the machine—work in—is equal to the work done by the machine—work out.

Increasing Force


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• A simple machine can change a small input force into a larger output force.

• Work equals force times distance. So, if the work in is equal to the work out, then smaller input force must be applied over a larger distance than the larger out put force.

Increasing Force


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• 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 a real machine, work out always will be less than work in.

The Pulley


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• A single pulley changes the direction of the input force.

• Its mechanical advantage is 1.

• A pulley is an object, like a wheel, that hasa groove with a rope or cable running thought it.

The Pulley


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• A combination of pulleys decreases the input force, so the mechanical advantage is greater than 1.

The Lever


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

• The pivot point is called the fulcrum.

• Levers can increase a force or increase the distance over which a force is applied.

The Lever


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• There are three types, or classes, of levers.• In a first-class lever, fulcrum is between the

input and output forces.

The Lever


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• If the output force is between the input force and the fulcrum, the lever is a second-class lever.

The Lever


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• In a third-class lever, the input force is located between the output force and the fulcrum.

• The mechanical advantage of a third-class lever always is less than 1.

The Wheel and Axle


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• A wheel and axle is made of two round objects that are attached and rotate together about the same axis.

• The larger object is called the wheel, and the smaller object is the axle.

The Wheel and Axle


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• The mechanical advantage of the wheel andaxle is greater than 1 because the radius of the wheel is greater than the radius of the axle.

The Inclined Plane


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

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

• The mechanical advantage of an inclined plane is the length of the inclined plane divided by its height.

Click image to view movie.

The Wedge


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• A wedge is a moving inclined plane with one or two sloping sides.

• Your front teeth are wedges.

• A wedge changes the direction of the input force.

The Screw


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• A screw is an inclined plane wrapped around a post.

• The inclined plane forms the screw threads.

• A screw changes the direction of the force you apply.

The Screw


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• 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.


Question 1

Although your arms may be straining as you carry a heavy box across the room, explain why no work is done by your arms.

Answer

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Work is done only when the box moves in the same direction as the force. Your arms exert a force upward to hold the box, but the box is moving forward as you walk.


Question 2

A horse did 3000 J of work pulling a cart for 500 m. How much force did the horse use?

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The answer is 6 N. (The explanation is on the next slide.)

Answer


33work: W = 3000 J = 3000 Nmdistance: d = 500 mforce: F = ? N

Begin with the work equation, W = Fd, then divide work by distance to find force. Then, substitute the known values for work and distance into the work equation and calculate force.


Question 3What is the mechanical advantage of the pulley system shown?

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The answer is 2. (The explanation is on the next slide.)

Answer


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Also, you can count the number of supporting ropes in a pulley system to find mechanical advantage. In this case, there are 2 supporting ropes, so the MA is 2.

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chapter: motion, forces, and simple machines table of contents section 3: work and simple...

Documents

speedtime graph

speed changes

object changes speed

time unit

instant of time

period of time

certain time

direction of motion