physics skill check on work

Indicate whether there is positive (+) or negative (-) work being done on the object. At Great America, a roller coaster car is lifted to the peak of the first hill on the Shock Wave.

+

Indicate whether there is positive (+) or negative (-) work being done on the object. A freshman stands on his toes and lifts a World Civilization book to the top

+

Indicate whether there is positive (+) or negative (-) work being done on the object. A catcher puts out his mitt and catches the baseball.

-

Indicate whether there is positive (+) or negative (-) work being done on the object. A falling parachutist opens the chute and slows down.

-

Indicate whether there is positive (+) or negative (-) work being done on the object. An eastward-moving car skids to a stop across dry pavement.

-

in every example of work being done, there is...

In each case described here there is a force exerted upon an object to cause that object to be displaced.

the force must cause the

In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.

Which sets of units represent legitimate units for the quantity work? Circle all correct answers.

Joule Foot x pound N x m kg x m2/sec2

A waiter carries a tray full of meals across a dining room at a constant speed. is there work being done?

No There is a upward force applied to the tray but the tray moves horizontally. Upward forces do not cause horizontal displacement.

A teacher applies a force to a wall and becomes exhausted. is there work being done?

No While there is a force, the force does not cause a displacement. of any object.

A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. (Careful! This is a very difficult question that will be discussed in more detail later.)

No. This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement.

Joule (abbreviated J)

One Joule is equivalent to one Newton of force causing a displacement of one meter The Joule is the unit of work. 1 Joule = 1 Newton * 1 meter 1 J = 1 N * m In fact, any unit of force times any unit of displacement is equivalent to a unit of work.

For each situation below, calculate the amount of work done by the applied force. An upward force is applied to lift a 15 kg object to a height of 5 meters at constant speed.

The F value is equal to m•g since the speed is constant. W = (147 N)•(5 m)•cos(0°) W = 735 J

negative work

The Meaning of Negative Work On occasion, a force acts upon a moving object to hinder a displacement. Examples might include a car skidding to a stop on a roadway surface or a baseball runner sliding to a stop on the infield dirt. In such instances, the force acts in the direction opposite the objects motion in order to slow it down. The force doesn't cause the displacement but rather hinders it.

info on work formula

The amount of work (W) done on an object by a given force can be calculated using the formula W = F d cos Q where F is the force and d is the distance over which the force acts and Q is the angle between F and d. It is important to recognize that the angle included in the equation is not just any old angle; it has a distinct definition that must be remembered when solving such work problems.

For each situation below, calculate the amount of work done by the applied force. A 100 N force is applied to move a 15 kg object a horizontal distance of 5 meters at constant speed.

W = (100 N)•(5 m)•cos(0°) W = 500 J

For each situation below, calculate the amount of work done by the applied force. A 100 N force is applied at an angle of 30o to the horizontal to move a 15 kg object at a constant speed for a horizontal distance of 5 m.

W = (100 N)•(5 m)•cos(30°) W = 433 J.

A shot-putter launches the shot. is there work being done?

Yes The shot-putter applies a force to displace the shot from a compressed arm position to an outstretched arm position.

A weightlifter lifts a barbell above her head. is there work being done?

Yes There is a force applied to the barbell and the force causes a displacement of the barbell.

Before beginning its initial descent, a roller coaster car is always pulled up the first hill to a high initial height. Work is done on the car (usually by a chain) to achieve this initial height. A coaster designer is considering three different angles at which to drag the 2000-kg car train to the top of the 60-meter high hill. Her big question is: which angle would require the most work?

__All angles result in the same work.__ Show your answers and explain. This conclusion is supported by the calculations below. In each case, the angle Q in the work equation is 0°; this is the angle between the F vector and the displacement (not the incline angle); since these two directions are parallel to each other, the angle is 0°. The work value is just force•distance•cosine(0°). There is little to no difference between the three resulting values. The difference falls outside the level of precision to which the given F and d values have been expressed. Angle Force Distance Work 35° 1.15 * 104 N 105 m 1.21 * 106 J 45° 1.41 * 104 N 84.9 m 1.20 * 106 J 55° 1.64 * 104 N 73.2 m 1.20 * 106 J

Non-standard Units of Work

foot•pound kg•(m/s2)•m kg•(m2/s2

work is a ( ) acting over some amount of ( ) to cause a change in ( )

force distance energy

what are the three main things needed for work

force, displacement, and cause

when the force and displacement are:

in the same direction, the angle (O) is zero, making cos 0=1. w=F dcos(1) exactly opposite directions, the angle (0) is 180, making cos 0= -1. W=Fdcos(-1) perpendicular, the angle (0) is 90, making cos 0= 0. W=Fd(0). No work. no work is done when the force is PERPENDICULAR to displacement. EX: A force acts upward on an object as it is displaced rightward. In such an instance, the force vector and the displacement vector are at right angles to each other. Thus, the angle between F and d is 90 degrees. A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!!

Work is a ______________; a + or - sign on a work value indicates information about _______.

scalar; whether the work adds or removes energy from the object

how do you know work was done?

when there's a displacement

W = F • d • cos Θ

where F is the force, d is the displacement, and the angle (theta) is defined as the angle between the force and the displacement vector. Perhaps the most difficult aspect of the above equation is the angle "theta." The angle is not just any 'ole angle, but rather a very specific angle. The angle measure is defined as the angle between the force and the displacement.