Energy Physics 11

Think about… 5 min 1) Why is energy important? 2) Where does energy come from? Where does it go? 3) How do we capture energy? 4)How does energy impact kinematics (motion) and dynamics (forces)? 5)Name as many different types of energy as you can. Think of an example to go with each.

What is energy? (copy definition)  Energy: The measure of a system’s ability to do work.  UpiV2I_IRI UpiV2I_IRI  Minutes 13 onward  19:30 onward

Types of Energy (Summary)  There are 2 main classifications of energy:  1) Potential Energy – The energy stored in a body or system as a consequence of its position, shape or form. Example: An object being held up has potential energy because of its position (gravitational potential energy). Example: A compressed spring has potential energy (elastic potential energy to spring open).  2) Kinetic Energy – The energy of motion Example: When you walk across the classroom you have kinetic energy.

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Work and Energy Relationship (Copy)  There is not much difference between work and energy.  In order to do work, an object must have energy.  In order to have energy, an object must have work done on it.

Energy Formulae - Copy  Kinetic Energy = E k = KE = ½ mv 2  Gravitational Potential Energy = E g = PE= mgh  Units: Joules (J) – same as work!  NOTE: h = height of the object measured from the reference level (measured in metres). It can be d from our kinematics equations as well as long as it is vertical distance.

What is the effect of doing work on an object? (Copy)  You can give an object more energy (any type of energy) by doing more work on it.  W = ΔE = change in energy (kinetic, gravitational, elastic)  W = E f – E i

Work- Energy Theorem  W = ΔE  The work done is equal to the change in total energy of an object.

Example 1  A 145g tennis ball is thrown horizontally at a speed of 25m/s.  A) What is the ball’s kinetic energy?  B) How much work was done to reach this speed assuming the ball started from rest.

Answers  A) 45 J  B) W = ΔKE = 45 J – 0J  ** It was thrown from rest so kinetic energy initial is zero!!!

Ex 2: Work on a moving object A 2kg object is moving at 10 m/s when a force is applied to it accelerating it to 20 m/s over a distance of 5m horizontally. What is the work done on the object?

Answer

Worksheet – Kinetic Energy

Check your understanding  A kg car is travelling at a speed of 50.0km/h when the driver applies the brakes. The car comes to a stop after travelling 15 m.  a) How much work was done by friction in stopping the car?  b) What was the magnitude of the average frictional force applied to the car?  c) If the same car were travelling at 100. km/h when the driver applied the brakes and the car experienced the same frictional force, how far would the car travel before coming to a stop?

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Ex 3: Potential Energy and Work What is the work done on a 12kg object to raise it from the ground to a height of 1.5m?

Potential Energy and Work What is the work done on a 12kg object to raise it from the ground to a height of 1.5m?

Ex 4:  A 1000kg car moves from point A to point B and then point C. The vertical distance between A and B is 10.m and between A and C is 15 m.  A) What is the PE at B and C relative to A?  B) What is the ΔPE (ΔPE = PE f – PE i ) when it goes from B to C?  C) Repeat a) and b) but take the reference level at C (switch all letters).

Answers  A) B: J  98000J, C: J  J  B) a decrease of J  J  C) A: J B: J  J Difference from A to B: increase of J

Worksheet – potential energy

Comprehension Check  A truck pushes a car by exerting a horizontal force of 500. N on it. A frictional force of 300. N opposes the car’s motion as it moves 4.0m.  A) Calculate the work done on the car by the truck.  B) Calculate the work done on the car by friction.  C) Calculate the work done on the car overall (net work).

Answers  A) W = Fd = 500 x 4 = 2000 N = 2.00 x 10 3 J  B) W = Fd = -300 x 4 = J  C) W net = 2000 – 1200 = 800J

Comprehension Check  Calculate the work done by a horse that exerts an applied force of 100. N on a sleigh, if the harness makes an angle of 30’ with the ground and the sleigh moves 30.m across a flat, level ice surface (ie, no friction).

Answer  W = Fd cosΘ = (100)(30)cos(30) = 2.6 x 10 3 J

Comprehension Check  A 50. kg crate is pulled 40. m along a horizontal floor by a constant force exerted by a person (100. N) which acts at an angle of 37’. The floor is rough and exerts a force of friction of 50.N. Determine the work done by EACH FORCE acting on the crate, and the net work done on the crate.  DRAW A DIAGRAM!!!

 W Fg = FdcosΘ  Work is 0J as the force is perpendicular to gravitational force.  W FN = 0J (same reason as above)  W Fapp = Fdcos Θ =(100)(40)cos37’ = 3195J  S.F.  3200 J  W Ff = Fd = -50(40) = J  -2.0 x 10 3 J  Wnet = 3200 – 2000 = 1200 J

Comprehension Check  Mrs. Evans is holding a 2.4kg textbook at a height of 3.4m above the floor.  a) What is the type of energy (potential or kinetic)? How do you know?  b) How much energy is there (use your equation)?  c) What is the velocity of the book at this point (ie, velocity initial)?  d) If Mrs. Evans drops the book, what is the final velocity assuming she doesn’t throw it (use your kinematics equations!)?  e) If Mrs. Evans drops the book as in d), what is the type of energy when the book hits the floor?  f) How much of this energy is there when it touches the floor?  g) Is there any time when there are both kinds of energy? If so, when? Explain.

Answers  A) Potential: It is not moving, it has the potential to move (fall).  B) PE = mgh = 2.4x9.81x3.4 = 80.J  C) v = 0 (at rest, not thrown)  D) vf 2 = vi 2 + 2ad = 2(9.81)(3.4) = Vf = 8.2m/s [down] E)KE = ½ mv 2 = ½ (2.4)(8.2) 2 = 80.J F)80.J G)When the object is falling, there is both PE and KE. When it falls 1.7m, there is equal PE and KE. Before this point (higher than 1.7m) there is more PE. After this (lower than 1.7m) there is more KE.

Work-Energy Theorem (copy)  If the object is experiencing KE:  if Wnet is +ve, KE increases (moves in direction of force or speeds up)  if Wnet is -ve, KE decreases (moves in direction of friction or slows down)  If the object is experiencing PE:  if Wnet is +ve, PE increases (is lifted)  if Wnet is -ve, PE decreases (is lowered)