Chapter 8 Notes Applied Physics Energy Unit

Introduction ______________ is the most underlying concept for all sciences. It was not recognized as a factor until around ______________, even though it had been there all along. Though it is not usually directly measured, the ______________ are detectible. The unit for energy is the ______________ (J), named after a British scientist, James Prescott Joule ( ). Energy 1850 effects Joule

Work (8.1) Push a cart over a long distance and you get tired. This is because you exerted ______________ in the process. This particular spending of ______________ can be called doing ______________. In order to do this, you must exert a ______________ over a ______________. Thus, the equation for work is as follows: force energy work force distance Work (J) Force (N) Distance (m)

Work Example Example 1: How much work is required to push a box a distance of 7m across the floor if you need to exert 14N of force?

Power (8.2) A log can be sawed in two ways, by using a hand saw or by using a chainsaw. In each case the ______________ amount of work has been done. However, the chainsaw used more ______________. ______________ is the rate at which work is done. In other words, it takes ______________ power to do a job faster. Power is measured in ______________ (W), named after another British Scientist. The equation for power is as follows: same power Power more Watts Power(W) Work(J) Time(s)

Sample Power Problem Example 2: How much power does it take for a truck expending 660J of work to drag a stump in 6s?

Sample Power Problem Example 3: How much power does it take a bulldozer to push a concrete slab if 5000N of force is exerted over a distance of 20m in 40s?

Mechanical Energy (8.3) When ______________ has been acquired enabling a system to do ______________, this is called ______________ ______________. For our purposes, mechanical energy will be broken into two categories: ______________ energy and ______________ energy. These two important energies relate to ______________ and ______________ respectively. energy work energymechanical potential kinetic motion position

Potential Energy (8.4) ______________ energy is the type of mechanical energy that relates to position, more particularly, how ______________ an object is (above the ground). An object on the ground will have a potential energy of ______________. An object’s potential energy in ______________ (J) can be calculated using the equation shown below. Potential high zero Joules Potential Energy (J) Mass (kg) Gravity (10m/s 2 ) Height (m)

Sample Potential Energy Problem Example 4: What is the potential energy of a medicine ball (m = 4kg) placed on a high shelf that is 2m high.

Kinetic Energy (8.5) ______________ energy is the type of mechanical energy that relates to motion. An object at rest will have a kinetic energy of ______________. The kinetic energy of an object in ______________ (J) can be calculated using the equation shown below. Kinetic zero Joules Kinetic Energy (J) Mass (kg) Velocity (m/s)

Kinetic Energy Sample Problems Example 5: What is the kinetic energy of a 1kg rabbit running at ? Example 6: What is the kinetic energy of the rabbit if it doubles its speed to ? *Notice that a doubling of speed ___________ kinetic energy. quadruples

Conservation of Energy (8.6) Just, as momentum is conserved in a closed isolated system, so is ______________. Thus, the total energy remains ______________. However, energy may change ______________ between kinetic and ______________. The energy conservation equation is shown below. The W o term refers to any ______________ done on the system(+) or by the system(-). An example of positive work would be that done by an ______________ or ______________. Here energy is ______________ to the system. An example of negative work would be that done by ______________. Here energy is ______________ from the system. energy constant form potential work enginemuscle added friction removed

Sample Energy Conservation Problem Example 7 (No Work Present): A ball (m = 0.2kg) is dropped from a height of 5m and allowed to fall the full distance. Assume no friction losses. a. What is the kinetic energy of the ball the instant it was released? b. What is the potential energy of the ball the instant it was released? c. What is the potential energy of the ball the instant before contacting the ground? d. What is the kinetic energy of the ball the instant before contacting the ground? 000

Sample Energy Conservation Problem Example 8 (Work Present): A dog drags a sled (m = 10kg) from level ground (h 1 = 0) to the top of a small hill (h 2 = 13m). The dog was moving at a velocity of at the bottom of the hill, but slowed down to a velocity of upon reaching the top. a. What is the kinetic energy of the sled at the bottom of the hill? b. What is the potential energy of the at the bottom of the hill? c. What is the kinetic energy of the sled at the top of the hill? d. What is the potential energy of the sled at the top of the hill?

Sample Energy Conservation Problem (cont) Example 8 (Work Present): A dog drags a sled (m = 10kg) from level ground (h 1 = 0) to the top of a small hill (h 2 = 13m). The dog was moving at a velocity of at the bottom of the hill, but slowed down to a velocity of upon reaching the top. e. What is the work done by the dog to get the sled to the top of the hill? 0

Machines (8.7) A ______________ can be defined as any device that either ______________ forces, changes the ______________ of forces, or both. Though machines may do these things, they do not ______________ energy because there is always a ______________ between ______________ and ______________. (Remember W = Fd) The amount of times that a machine multiplies force is called the ______________ ______________. machine multiples direction create tradeoff forcedistance mechanicaladvantage Mechanical Advantage Output Force (N) Input Force (N)

Mechanical Advantage Example Example 9: A wedge (another machine) provides a force of 400N when only 50N is applied. What is its mechanical advantage?

Levers The simplest machine we discuss is the ______________. It always consists of three parts: the ______________ (pivot point), the ______________ (input force), and the ______________ (output force). Because there are three parts to a lever, there are ______________ configurations. Type I (1st Class): Fulcrum in Center Examples: Type II (2nd Class): Load in Center Examples: Type III (3rd Class): Effort in Center Examples: F L E F L E E F L lever fulcrumeffort load three see saw pliers scissors wheel barrownutcracker baseball bat

Pullies Machines also include the ______________, a grooved wheel connected to a mounting. Pulleys can be used to ______________ or ______________ depending on how they are placed. Mechanical advantage can be found by simply counting the number of supporting ______________. See the examples below. pulley multiplyredirect strings 3221

Efficiency (8.8) Due to the presence of ______________, machines to not convert all of the input work (W I ) into useful output work (W O ). Because of this, it is important to find out how efficient a machine is. The ______________ of a machine is the ______________ of the work output (W O ) to the work input (W I ). This is then converted into a percentage for convenience. friction efficiency ratio Work Output (J) Work Input (J) Efficiency

Efficiency Example 1 Example 10 (Work Given): A pulley system is set up to lift a box. The work done by the pulley on the box (W O ) was 120J. The work done on the pulley system by the person (W I ) was 130J. What is the efficiency of the pulley system?

Efficiency Example 2 Example 11 (Work To Be Found): A pulley system connected to a tree is set up to pull a truck out of the mud. The pullers pull on the rope with a force of 340N a distance of 15m. The pulley system pulls on the truck with a force of 900N and moves 5m.