Ch 4 notes

Work Press down on your desk with your hand. Are you doing any work? work – force applied through a distance The transfer of energy when a force makes an object move. if an object does not move, then you have not done any work

Work 2 conditions for work to be done 1. the object has to move 2. the motion of the object must be in the same direction as the applied force insert Fig. 1 – pg. 106

SI Unit for work is the joule (J) (N × m) when you pick up a cell phone from the floor you do about 1 Joule of work Work = force x distance W=f x d

Work Example: You push a refrigerator with a horizontal force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work do you do? :

Example: You push a refrigerator with a horizontal force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work do you do? known: force (F) = 100 N, distance (d) = 5 m solve problem:

Work Example: You move a 75–kg refrigerator 35 m. This requires a force of 90 N. How much work, in joules, was done while moving the fridge?

Work think about when you are carrying books to class Are you doing any work on the books? – No, since the books are moving horizontally, & your arms are only putting a force upward on the books (so they don’t fall) – your legs are doing the work

Work since there is a 90 angle between this force & the motion of the object, there is no work done if the force is in the same direction (parallel) as the motion, then the work = the force times the distance traveled – insert Fig. 2 – pg. 108

Machines machine – a device that makes doing work easier by increasing the force applied to an object, changing the direction of an applied force, or increasing the distance over which a force can be applied

Machines simple machine – a machine that does work with only one movement of the machine (six types) 1. lever 2. pulley 3. wheel & axle 4. inclined plane 5. screw 6. wedge insert Fig. 4 – pg. 109

Machines compound machine – a combination of two or more simple machines – Example: a pair of scissors (wedge & 2 levers), a bike look at a can opener  the handles act as a lever and increase the force applied on a wedge, that cuts into the can  as you turn the handle (a wheel & axle) the can is opened

Two ways for Machines to make work easier machines can increase force or speed, but not at the same time when using a machine you always put more work into a machine than you get out of the machine

SKIP

Machines efficiency – the ratio of output work to input work (%) – tells how much of the work put into a machine is changed into useful work

Machines W out is never greater than W in  not all of the energy the machine receives is transferred to the object  some lost as heat due to friction efficiency can be increased by reducing friction (adding oil or grease)

Machines Example: You do 20 J of work pushing a crate up a ramp. If the output work from the inclined plane is 11 J, then what is the efficiency of the inclined plane? known: work in (W in ) = 20 J, work out (W out ) = 11 J solve problem:

How Machines Help can increase speed  increase speed by decreasing force can change direction of force  insert Fig. 5a & b – pg. 111

Machines can increase force  increase force by decreasing speed  insert Fig. 5c – pg. 111 to describe the effectiveness of a machine at increasing force use its mechanical advantage mechanical advantage – the ratio of output force to input force

Machines Mechanical Advantage Equation  input force is the force that a person or device applies to a machine  output force is the force that the machine applies to another object

Machines Example: A crate weighs 950 N. If you can use a pulley system to lift the crate with a force of only 250 N, then what is the mechanical advantage? known: output force (F out ) = 950 N, input force (F in ) = 250 N solve problem:

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Energy energy – the ability to cause change think of a tennis racket hitting a ball  the racket caused the ball to change  so the racket must have some energy to cause this change  insert Fig. 6 – pg. 114

Energy the racket also does work on the ball by applying a force to the ball through a distance racket transfers energy to the ball  energy also described as the ability to do work  energy measured in same units as work (joules)

Energy if the tennis racket does 250 J of work on the ball, then 250 J of energy is transferred to the ball from the racket the tennis racket & tennis ball are systems – anything around which you can imagine a boundary  can be a single object or a group of objects – insert Fig. 6 – pg. 114

Energy energy can come in many forms:  mechanical  electrical  chemical  radiant  insert Fig. 7 – pg. 115

Kinetic Energy when you think of energy, you often think of objects moving kinetic energy – the energy due to motion  all moving things have kinetic energy amount of kinetic energy depends on  object’s mass  object’s velocity

Kinetic Energy SI Unit for kinetic energy is the joule (J) if the velocity is doubled the kinetic energy will….quadruple

Kinetic Energy the more mass or velocity a moving object has, the more kinetic energy it has Which objects have the greater kinetic energy?

Example: A jogger with a mass of 60.0 kg is moving forward at a speed of 3.0 m/s. What is the jogger’s kinetic energy from this forward motion?

Kinetic Energy Example: A jogger with a mass of 60.0 kg is moving forward at a speed of 3.0 m/s. What is the jogger’s kinetic energy from this forward motion? known: mass (m) = 60.0 kg, speed (ν) = 3.0 m/s solve problem:

Potential Energy energy does not always have to have motion motionless objects have energy too potential energy – energy that is stored due to the interactions between objects  think of an apple hanging on a tree  if it falls from the tree, a change occurs  since it has the ability to cause change (gravity), it has energy

Potential Energy think of a rubber band if you stretch the rubber band & let go, what happens? as it flies across the room, it has kinetic energy due to its motion Where did the energy come from?

Potential Energy just as the apple had potential energy, the rubber band has stored energy stored as elastic potential energy – energy that is stored by compressing or stretching an object – rubber bands or springs

Potential Energy foods you eat also have stored energy (so does gasoline for cars) energy is stored in the chemical bonds between the atoms chemical potential energy – energy that is stored due to the chemical bonds

Potential Energy look at the burning of natural gas (CH 4 ) energy is stored in the bonds that hold the carbon & hydrogen atoms together energy gets released when the gas is burned Figure 8 – Pg. 117

Potential Energy looking at the blue vase in Figure 9 – it has potential energy with the Earth gravitational potential energy (GPE) – energy that is due to the gravitational forces between objects amount of GPE depends on 3 things 1. the mass of the object 2. the acceleration due to gravity (9.8 m/s 2 ) 3. the height the object is above the ground

Potential Energy look at Fig. 9 – which objects have the most gravitational potential energy? objects higher off the ground will have more GPE  if objects are at the same height, objects with more mass have more GPE  insert Fig. 9 – pg. 118

Example: A 4.0 kg ceiling fan is placed 2.5 m above the floor. What is the gravitational potential energy of the Earth–ceiling fan system relative to the floor?

Potential Energy Example: A 4.0 kg ceiling fan is placed 2.5 m above the floor. What is the gravitational potential energy of the Earth– ceiling fan system relative to the floor? known: mass (m) = 4.0 kg, gravity (g) = 9.8 m/s 2, height (h) = 2.5 m solve problem:

Energy think of a swing, as you swing higher, your GPE changes then you start to fall & pick up speed, your KE changes  at the top of the swing the GPE is large & KE is small  at the bottom of the swing the GPE is small & KE is large

Energy when the swing moves back & forth, kinetic & potential energy change as one increases the other decreases energy changes forms, but is not destroyed Law of Conservation of Energy – energy can not be created nor destroyed – it can only be converted from one form to another

Energy Transformations mechanical energy – the sum of the kinetic energy & potential energy of the objects in a system mechanical energy remains nearly constant

Energy Transformations think of the apple that fell & hit Newton on the head  it had gravitational potential energy while it was in the tree due to gravity  the instant the apple fell it accelerated due to gravity  as it fell, its height decreased so its GPE decreased  this potential energy is not lost – its converted into kinetic

Energy Transformations  from the equation for mechanical energy, if the potential energy is converted to kinetic energy, then the mechanical energy of the apple does not change  the potential energy that the apple lost is gained back as kinetic energy  the form of energy changes, but the total amount of energy remains the same

Energy Transformations energy transformations also occur during projectile motion Figure 12 – Pg. 122

Energy Transformations or a swing – insert Fig. 13 – pg. 123

Energy Transformations think of a swing where no one pushes or you stop pumping your legs  eventually the swing will stop the mechanical energy of the swing decreases, but is energy destroyed if the energy of the swing is decreasing then another energy must be increasing

Energy Transformations every time the swing moves, the swing’s ropes or chains rub on the hooks holding the swing in place air also pushes on the rider friction & air resistance cause some of the mechanical energy to change to thermal energy (temperature of hooks increases) – insert Fig. 14 – pg. 124

Energy Transformations energy transformations can also involve electrical energy  some devices convert electrical energy into thermal energy (toaster)  some convert to radiant energy / sound energy (television)  some into mechanical energy (washing machine) – insert Fig. 15 – pg. 125

Energy Transformations chemical energy can also be transformed gasoline stores energy in the form of chemical potential energy the engine transforms the chemical potential energy into kinetic energy of a moving car plants convert radiant energy (the sun) to chemical potential energy

Power power – the rate at which energy is converted  also tells you how much work can be done in a certain amount of time SI Unit for power is the watt (W) (1 joule per second)

Example: You perform 950 J of Work to push a sofa. If it took you 5.0 s to move the sofa, what was your power?

Power Example: You transform 950 J of chemical energy into mechanical energy to push a sofa. If it took you 5.0 s to move the sofa, what was your power? known: energy transformed (E) = 950 J, time (t) = 5.0 s solve problem:

Energy Transformations all the processes in your body follow the law of conservation of energy your body stores energy in the form of fat & other chemical compounds this energy is then used to make your body move

Energy Transformations the amount of energy you get from food is measured in Calories (C) 1 C = 4,184 J 1 gram of fat consumed = 10 C of energy 1 gram of carbohydrate / protein = 5 C of energy

Energy Transformations your body uses different amount of energy for different activities insert Table 1 – pg. 127