Please pick-up a new packet and worksheet Section #10:

¡  Work ¡  Power ¡  Energy (Potential and Kinetic) ¡  Conservation of Energy ¡  Impulse and Momentum ¡  Collisions (Elastic and Inelastic) ¡  Quiz – Part #1 (Work, Power and Energy) ¡  Quiz – Part #2 (Momentum, Impulse, and Collisions)

Work is done only when a constant force is applied on an object causing the object to move in the same direction as the applied force. Objects that are at rest may have many forces acting on them, but no work is done if there is no movement.

Formula for work: Work = Force x Distance W = F x D

The SI unit of work is the Joule Named in honor of James Prescott Joule

Work = F (N) x D (m) = Joules (J) One Joule, J, of work is the work done when 1 Newton of force is applied through a distance of 1 meter.

What IS considered as work done in Physics? §  Pushing a cart for 10 meters §  Lifting your book bag upwards by 1 meter

What is NOT considered as work in Physics?

§  Pushing against a wall §  Carrying your book bag to your next class §  Waiter carrying a tray above his head across the room

Is the weightlifter doing work as he holds the barbell above his head?

§  Weightlifter is not actually doing work when holding barbell above his head

§  Force is applied to barbell §  If no movement, no work done

Work Depends on Direction…

•  All of the force does work on the suitcase. •  The horizontal part of the force does work. •  The force does no work on the suitcase.

Force

This force does work

This force does no work

Force

Direction of motion Direction of motion Direction of motion Force and motion in the same direction Part of force in

direction of motion Lifting force not in direction of motion

Example #1: You are helping to push your mother’s heavy shopping cart with a force of 50 N for 200 m. What is amount of work done? W = F x d W = 50 N x 200 m W = 10,000 J

Example #2 A man slides a milk can, weighing 100 N, 10 meters across a concrete floor by applying 50 N of force. He then lifts it 3.0 meters onto a truck. What was the total work he did? WTotal = WS + WL = FS x dS + FL x dL

= (50 N)(10 m) + (100 N)(3 m) = 500 J + 300 J

WTotal = 800 J

What if force and direction are not the same? We have to “resolve” the situation into components

If the force and displacement are not in the exact same direction, then: Work = F x d (cosθ) Where θ is the angle between the force direction and the displacement direction.

FA = 40 N

d = 3 m

Example #3 What is the work done in pulling a block 3 m by a 40 N force at an angle of 35° to the horizontal?

W = F x d (cosθ) W = (40 N)(3 m)(cos35°) W = 98.3 J

35°

Example #4 A child pulls a sled 30 m across a level snow slope, exerting a force of 100 N on a rope inclined at 30° to the horizontal. How much work is done by the force?

W = F x d (cosθ) W = 100 N x 30 m x cos(30°) W = 2,598.08 J

Example #5 Mr. Holbrook carries a 200-N suitcase up three flights of stairs (a total height of 10 m) and then pushes it with a horizontal force of 50 N at a constant speed of 0.5 m/s for a horizontal distance of 35 meters. How much work does Mr. Holbrook do on his suitcase during this entire motion?

W = 3,750 J

Power is the rate at which work is done. Formula for Power P = work / time P = W / t P = (F x d) / t

The SI unit of power is Watts Named in honor of James Watt

One Watt, W, of power is the power achieved when 1 J of work is done or 1 J of energy is transferred in a time of 1 second.

Taking a closer look at power…. P = (F x d) / t P = F x (d / t) P = F x V (Force x Velocity) The net force applied on an object and its speed.

Horsepower: The term "horsepower" was coined by the James. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine horse could pull (lift by means of a pulley) 22,000 ft-lbs/minute.

Horsepower: Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 ft-lbs/minute or: 746 watts/second

Horsepower Conversions: hp = power / (746 watts)

Example #6: A 70 kg student runs up a long flight of stairs in 4 seconds. The vertical height of the stairs is 5.5 meters. What is the power and horsepower of the student? P = 944.21 w hp = 1.27

Example #7: A car traveling on a level road needs 700 N of force to overcome friction and air resistance. What power and horsepower is expended by the car to keep it traveling at a constant velocity of 30 m/s. P = 21,000 w hp = 28.15

Example #8: With what constant velocity can a 2 hp motor raise a mass of 150 kg? v = 1.01 m/s

Work Problem #1 An electric motor drives a fan, and supplies energy at the rate of 0.25 hp for 3 hours. How many joules does it supply in this period? (2,014,200 J)

Work Problem #2 Calculate the horsepower required to raise a 150 kg drum to a height of 20 meters in a time of 1 minute. (0.658)