Work A force applied in the direction of a displacement Area under Force-Displacement graph A scalar...
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WorkWork A force applied in the direction of a
displacement Area under Force-Displacement graph A scalar quantity
Work = Force x Displacement x cos θ
W = Fdcosθ (units: 1 Nm = 1 Joule)
How much work is done by a crane that lifts 120 kg up a distance of 75 m?
Example 2:A student pulls a box with a force of 25 N at an angle of 25o with the horizontal. The box moves 12 m along the ground. How much work is done?
Power Rate of doing work (or rate of using energy) Units: Watt (1 W = 1 J/s) or hp (1 hp = 746 W)
Power = Work = Energy used time time
P = W = Fd = Fv t t
ExampleExample::Find the power output of a 60.0 kg athlete who climbs the Grouse Grind in 40.0 Find the power output of a 60.0 kg athlete who climbs the Grouse Grind in 40.0 minutes. (elev. gain = 800.0m)minutes. (elev. gain = 800.0m)
Example:Find the power output of a 60.0 kg athlete who climbs the Grouse Grind in 40.0 minutes. (elev. gain = 800.0m)
Power = Worktime
= (mg)d t
= 60 x 9.8 x 800 2400 = 196 W
196 W * (1 hp / 746 W)
= 0.26 hp
Pg. 204 Lab – Your Power
What do you think? What do you think? Which of the following statements are true about work?
A. Work is a form of energy.B. A Watt is the standard metric unit of work.C. Units of work would be equivalent to a Newton times a meter.D. A kg∙m2/s2 would be a unit of work. E. Work is a time-based quantity; it is dependent upon how fast a force displaces an object.F. Superman applies a force on a truck to prevent it from moving down a hill. This is an example of work being done.
A. TRUE - Work is a form of energy, and in fact it has units of energy.B. FALSE - Watt is the standard metric unit of power; Joule is the standard metric unit of energy.C. TRUE - A N•m is equal to a Joule.D. TRUE - A kg•m2/s2 is a mass unit times a speed squared unit, making it a kinetic energy unit and equivalent to a Joule.E. FALSE - Work is not dependent on how rapidly the force displaces an object; power is time-based and calculated by force multiplied by speed.F. FALSE - Since Superman does not cause a displacement, no work is done; he is merely holding the car to prevent its descent down the hill.
G. An upward force is applied to a bucket as it is carried 20 m across the yard. This is an example of work being done.H. A force is applied by a chain to a roller coaster car to carry it up the hill of the first drop of the Shockwave ride. This is an example of work being done.I. The force of friction acts upon a softball player as she makes a headfirst dive into third base. This is an example of work being done.J. An eraser is tied to a string; a person holds the string and applies a tension force as the eraser is moved in a circle at constant speed. This is an example of work being done.
G. FALSE - The upward force does not cause the horizontal displacement so this is a NON-example of work.H. TRUE - There is a component of force in the direction of displacement and so this is an example of work.I. TRUE - There is a force and a displacement; the force acts in the opposite direction as the displacement and so this force does negative work.J. FALSE - For uniform circular motion, the force acts perpendicular to the direction of the motion and so the force never does any work upon the object.
Which of the following statements are true about power? Include all that apply.A.Power is a time-based quantity.B.Power refers to how fast work is done upon an object.C.Powerful people or powerful machines are simply people or machines which always do a lot of work.D.A force is exerted on an object to move it at a constant speed. The power delivered by this force is the magnitude of the force multiplied by the speed of the object.
E. The standard metric unit of power is the Watt.
F. If person A and person B do the same job but person B does it faster, then person A does more work but person B has more power.
G. The Newton•meter is a unit of power.H. A 60-kg boy runs up a 2.0 meter staircase
in 1.5 seconds. His power is approximately 80 Watt.
I. A 300-Newton force is applied to a skier to drag her up a ski hill at a constant speed of 1.5 m/s. The power delivered by the toe rope is 450 Watts.
Answer: ABDEIA. TRUE - Power is a rate quantity and thus time-based.B. TRUE - This is the definition of power.C. FALSE - This is not always the case. A machine can do a lot of work but if it fails to do it rapidly, then it is not necessarily powerful. In fact two machines can do the same task (and therefore the same work), yet they can have drastically different power ratings.D. TRUE - An equation for computing work in constant speed situations is P=F•v.E. TRUE - Watt is the unit of power? Yes!!F. FALSE - Vice versa. If two people do the same job, then they're doing the same amount of work. The person who does it fastest generates more power.G. FALSE - A N•m is a Joule and that is a unit of work (not power). Think force (N) times distance (m); that's work (J).H. FALSE - The work would be (m•g)•d or approximately 1200 J. The power is work divided by time - 1200 J/1.5 s = 800 W.I. TRUE - Since force and speed are given, use Power = F•v. The calculation yields 450 W.
3. Positive, Negative, or No work?3. Positive, Negative, or No work?
A.A.A cable is attached to a bucket and the A cable is attached to a bucket and the force of tension force of tension is used to pull the is used to pull the bucket out of a well.bucket out of a well.
B.B.Rusty Nales uses a hammer to exert an Rusty Nales uses a hammer to exert an applied force applied force upon a stubborn nail to upon a stubborn nail to drive it into the wall.drive it into the wall.
C.C.Near the end of the Shockwave ride, a braking system exerts an Near the end of the Shockwave ride, a braking system exerts an applied force applied force upon the coaster car to bring it to a stop.upon the coaster car to bring it to a stop.
D.D.The The force of friction force of friction acts upon a baseball player as he slides into third base.acts upon a baseball player as he slides into third base.
E.E.A busy spider hangs motionless from a silk thread, supported by the A busy spider hangs motionless from a silk thread, supported by the tensiontension in in the thread.the thread.
F.F.In baseball, the catcher exerts an abrupt In baseball, the catcher exerts an abrupt applied force applied force upon the ball to stop it upon the ball to stop it in the catcher's mitt.in the catcher's mitt.
G.G.In a physics lab, an In a physics lab, an applied force applied force is exerted parallel to a plane inclined at 30-is exerted parallel to a plane inclined at 30-degrees in order to displace a cart up the incline.degrees in order to displace a cart up the incline.
H.H.A pendulum bob swings from its highest position to its lowest position under A pendulum bob swings from its highest position to its lowest position under the influence of the the influence of the force of gravityforce of gravity..
Answers:A.The force is upwards and the displacement is upwards. When the force and the displacement act in the same direction, positive work is done.B.The force is horizontal and the displacement is horizontal. When the force and the displacement act in the same direction, positive work is done. (It is true that the wall is doing negative work upon the nail but this statement is about the hammer's force on the nail.)C.The force is backwards and the displacement is forwards. When the force and the displacement act in the opposite direction, negative work is done.D.The force is backwards and the displacement is forwards. When the force and the displacement act in the opposite direction, negative work is done.
E. If the force does not cause the object to be displaced (the object hangs motionless), then no work is done.
F. The force is backwards and the displacement is forwards. When the force and the displacement act in the opposite direction, negative work is done.
G. The force is upwards and parallel to the incline and the displacement is in the same direction parallel to the incline. When the force and the displacement act in the same direction, positive work is done.
H. As the bob swings downwards from its highest position, the motion is downwards (and rightwards); the force is also downwards and as such there is a component of force in the direction of motion. When the force and the displacement act in the same direction, positive work is done. (Note that if the bob was swinging upwards from its lowest position to its highest position, then gravity would be doing negative work.)
Simple MachinesA machine is a device that helps make work easier (not less of it!) to perform by accomplishing one or more of the following functions:
transferring a force from one place to another
changing the direction of a force decreasing the magnitude of a force increasing the distance or speed of a force
The 6 Simple Machines
Pulley Wheel and Axle
Mechanical Advantage (MA)
Wout = Win
Fout x Dout = Fin x Din
Fout = Din
MA = Output = Fout
3 Types of levers: The class of a lever is determined by the location of the effort force (E) and the load (R) relative to the fulcrum (F).
First Class Lever.
Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws.
Set up a class 1 lever with the fulcrum at the middle of the meter stick
Hang a 1 kg mass near one end. This will be your Load force
Balance the meter stick by adding a 200 g mass on the other side of the fulcrum. This is your Effort force
What is mechanical advantage of your lever? Record the distance from the fulcrum to each force
Torque ()is equal to the product of force and distance from a fulcrum
= Fd1. What is the Torque due to your effort force
2. What is the Torque due to your load force (2)?
Set up 2 other examples of class 1 levers using different masses and distances from the fulcrum.
Repeat steps 1 and 2 above. Compare 1 and 2
What do you notice?
Trial Effort Force
1 Load Force
Second Class Lever Examples of
second-class levers include nut crackers, wheel barrows, doors, and bottle openers.
Set up a Class 2 lever similar to what is shown below
Hang a 500 g mass (Load) between the Effort force and the fulcrum
Using a string and a force meter, measure the Effort force required to lift the Load
What is the mechanicaladvantage of the lever?
1. What is the Torque due to the effort force?
2. What is the Torque due to the load force?
Set up 2 other class 2 levers. Repeat steps 1 and 2. What do you notice about the Torques?
Trial Effort Force
1 Load Force
Third Class Lever Examples of
third-class levers include tweezers, biceps, and hammers
A flat surface that is higher on one end
Makes the work of moving things easier because you need less force
Wedges Two inclined planes Two inclined planes
joined back to back.joined back to back. Wedges are used to split Wedges are used to split
Pulleys Pulley are wheels and
axles with a groove around the outside
A pulley needs a rope, chain or belt around the groove to make it do work
Diagrams of Pulleys
Fixed pulley: A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage.
Movable Pulley: The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.
The effort needed to lift the load is less than half the weight of the load.
The main disadvantage is it travels a very long distance.
Rube Goldberg Machines
Rube Goldberg machines are examples of complex machines.
All complex machines are made up of combinations of simple machines.
Rube Goldberg machines are usually a complicated combination of simple machines.
By studying the components of Rube Goldberg machines, we learn more about simple machines