WORK AND ENERGY. QUICK REVIEW KinematicsDynamics UCM and Newton’s Law of Gravitation Studied...
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Transcript of WORK AND ENERGY. QUICK REVIEW KinematicsDynamics UCM and Newton’s Law of Gravitation Studied...
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WORK AND ENERGY
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QUICK REVIEW
Kinematics DynamicsUCM and
Newton’s Law of Gravitation
• Studied motion but not cause of motion.
• Relationships between position, displacement, velocity, acceleration and time.
• Mathematical relationships
• Applied kinematics• Studied the cause
of changes in motion.
• Newton’s Laws of Motion
• Physical and mathematical relationships
• Dynamics applied further
• Constant speed, change in direction
• Universal Law of Gravity
• Forces and motion of planetary systems
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ENERGY•Not easily defined
–Usually described by type of energy
• Classical Physics –Energy is the ability to do work.
–More energy = more ability to do work
• Conservation of Energy–The total amount of energy in the entire Universe is
conserved.
–Cannot be created or destroyed
–Can transfer to different types or be stored
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WORK
•Work is the product of the force and displacement.
• In this equation θ is the angle between F and Δx.
• The unit would be the Newton-meter (Nm) or the Joule (J)
𝑊=𝐹 ∆𝑥𝑐𝑜𝑠 𝜃
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MORE WORK
•Work is scalar – no direction in its value
• Even though your are multiplying two vectors, you get a scalar
• In maths this is called a dot product. It its one of two ways of multiplying vectors.
• A dot product aims to multiply the components of vectors that are parallel to one another!
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WORK EXAMPLES
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EXAMPLE• Tarzan is trying to impress Jane with his new cart. The elephant is gone for the day so while Tarzan steers, Jane pushes the cart 19 m using a force of 210 N. If Jane pushes in the direction the cart is heading, how much work will she do?
• If Tarzan weighs 600 N, how much work does he do by pushing down on the cart?
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EXAMPLE• Farmer Mellyn hitches his tractor to a sled loaded with firewood and pulls it 20 m. The tractor exerts a constant force of 5000 N at an angle of 30o as shown. There is a constant force of friction of 3500 N. Find:
– the work done by friction
– the work done by the tractor
– the total work done
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EXAMPLE• A block of mass 3 kg slides up an inclined plane tilted at 37o to the horizontal. There is a frictional force of 8 N. When the block has slide 3 m up the ramp, how much work had been done by:
–Gravity
–Friction
–Normal
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EXAMPLE• A girl pulls a crate 10 m by applying a force of 150 N at 37o above the horizontal. The crate has a mass of 5 kg and there is a coefficient of friction of 0.3 between the crate and the floor. Determine the work done by:
– the girl.
– friction.
–weight.
–Determine the total work done.
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POWER
• Power is the rate at which work is done.
• Measured in J/s or Watt (W)
• 1 hp = 746 W
• Power is not a measure of how much work you can do but how fast you can do it.
𝑃=𝑊𝑡
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MORE POWA
• 𝑃=𝐹𝑣𝑐𝑜𝑠𝜃
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EXAMPLE• An elevator has a mass of 600 kg. The elevator is designed to move upward at a constant speed a distance of 20 m in 16 sec. What is the minimum power rating on a motor to accomplish this task?
• If you used a 40 hp motor, how many people can be in the elevator if the average passenger mass is 65 kg?
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EXAMPLE• The engine of a motorboat delivers 30 kW to the propeller while the boat is moving at 15 m/sec. What is the resistive force acting on the boat?
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STAIR ACTIVITY:
•Get a half sheet of paper: 1. Convert your weight into mass (1 kg = 2.2 lbs)
2. How much work would you have to do to lift yourself up a flight of stairs 8 meters in height? Only consider vertical displacement.
3. We will go to the stairs in the cafeteria (h = 4.3 meters). Time how long it takes for you to run or walk up the stairs. Calculate your power output in hp.
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THE MOST POWERFUL IN ALL OF AP PHYSICS
Will Jordan1.87 hp
Juliana Kolb0.92 hp
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HORSEPOWER!
•How unemployment skyrocketed amongst horses in the 18th century.
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1 calorie of heat is required to warm 1 g of water by 1°C.
1 calorie = 4.184 J1 Calorie (kcal) = 4184 J
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GRAPHS •Work:
𝑊=𝐹 ∆𝑥𝑐𝑜𝑠 𝜃