 Work  Energy  Kinetic Energy  Potential Energy  Mechanical Energy  Conservation of Mechanical Energy

 The capacity to do work  The energy transferred to an object equals the work done on the object  E T = W equals the work done on the object  E T = W

Energy is present in many forms  Heat (thermal energy)  Kinetic Energy = “Motion Energy”  Potential Energy = “Stored Energy”  Mechanical energy – Sum of Potential and Kinetic  Nuclear  Sound  Electromagnetic energy  Chemical energy

The parallel component does work The (F cos  ) component does work

NO work done if there: 1.Is no movement 2.The force if perpendicular to the displacement

In which photo(s) is WORK being done? In which photo(s) is WORK being done? no yes

If a 10 Newton force displaces a 20 kg block 40 meters calculate the work done on the block by the force. The Normal force (F N ) and weight (mg) do no work in this problem, WHY? F = 10N

Work = Force  Distance The component of force parallel to the displacement DOES WORK The perpendicular component DOES NO WORK  - Between the force and displacement Scalar Measured in Joules (J)

Positive Work - Force and displacement in same direction. Negative Work - Force opposite the displacement. Zero Work - Force is perpendicular to the displacement. FdFd FdFd F d

If the Force is in the Direction of Motion: In this case,  = 0 o, so: If the Force is in the Opposite Direction of Motion: In this case,  = 180 o, so:

If the Force is Perpendicular to the Direction of Motion: In this case,  = 90 o, so: If the Object Being Pushed Doesn't Move: In this case, x = 0, so:

A 10 N force acts 30 0 above the horizontal and displaces an object 5 meters horizontally how much work is done? F= 10 N 30 0 W = F  d cos  = (10N)(5m) cos 30 0 = 43.3 J

How much work does gravity do on a 70 kg person who falls 100 meters in the free fall ride?

 Kinetic energy (KE) is the energy of a moving object.  Energy associated with motion and mass.

A 500 kg car is traveling at 10 m/s, 10 sec. later it is traveling 30 m/s. Calculate the following: 1. The initial kinetic energy 2. The final kinetic energy 3. The change in KE

Work Energy Theorem: The change in kinetic energy equals the work done.

The animation shows a block of mass and initial speed v sliding across a floor that is not frictionless. A kinetic friction force f k stops block during displacement d. Thus we can relate work done by friction to the change  E in the system's energy

For hammer: Moving hammer can do work on nail! For nail:

Matt’s little red wagon with a mass of 4.6 kg moves in a straight line on a frictionless horizontal surface. It has an initial speed of 10 m/s and is pulled by Matt 4.0 m with a force of 18N in the direction of the initial velocity. Use the work-energy relation (W Net =  KE) to calculate the wagons: a.Change in Kinetic Energy b.Final speed.

In a test of old sports car, it’s found that engines provided around 1,000 N of force. If the typical mass is 400 kg and they accelerate from rest, how fast will they be going 100 m down the road?

How much work is done holding a box in place on an incline? How much work is done pushing a 15 kg box up a 30° incline at a constant speed for 3 m

Force vs. Distance Area under curve equals the work done

Force vs. Distance Area under curve equals the work done

Area above the curve – work is positive Area below the curve – work is negative

Work done by a variable force equals Area under the curve

Gravitational Potential Energy (PE) : Gravitational Potential Energy (PE) : The energy an object has due to its height above a reference point. The potential energy change is independent of the path between the initial and final points. link

PE=mgh Can gains Potential Energy equal to mgh

One serving of Bagel Crisps contains 543 kJ. How many pull- ups are needed to burn it off? M = 60 kg∆h =.5 m ∆PE = mg ∆H J = (60kg)(9.81N/kg)(.5m)(n) 1845 = n But the human body is only abour 20% efficient so, n is only 369!

Who does more work in lifting the respective equal masses to the top of the incline at a constant speed?

1. His initial potential energy with respect to the ground 2. His potential energy 1 seconds after being released 3. His change in potential energy 4. Where did its energy go when it hits the ground? Rufus the 5 kg cat falls 10 meters from above the surface of the earth. Calculate:

Conservative Force : The work done is independent of the path taken. Only depends on the initial and final position. Ex: Gravity Non Conservative Force : The work done depends on the path taken. Ex: Friction

Don’t disregard the Conservation Laws