What is work ? (in physics)

Work

Do Now: In a physical sense, which do you think is more work?

Write the question including the choices then write your choice down. •Holding a 100 kg weight above your head. •Pushing really hard against a wall to the point of exhaustion•Lifting a 10 kg weight above your head from the floor• Pushing a 50kg box across the floor at a constant velocity•Studying all night long for a physics test

What is Work?Work is a force action over some distance to cause a change in energy.

The amount of work done depends on two things:

a.The amount of force exerted and

b.The distance over which the force is applied.

There are two factors to keep in mind when decidingwhen work is being done: something has to move and the motion must be in the direction of the applied force.

Work can be calculated by using the following formula:

Work=force x distance

Units of Work

Work = Force x Displacement

The unit of force is Newtons (N)The unit of displacement is meters (m)

The unit of work is newton-meters (N-m)One newton-meter is equal to one joule (J)

So, the unit of work is a joule.Work is a scalar, like energy.

Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally.

In which of the following cases was work done:

1. Someone applies a force to a wall and becomes exhausted.

2. A book falls off a table and free falls to the ground. 3. A waiter carries a tray full of meals above his head by

one arm straight across the room at constant speed.

4. A rocket accelerates through space.

Two students, Ben and Bonnie, are in theTwo students, Ben and Bonnie, are in theweightlifting room. Bonnie lifts the 50 kg weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 barbell the same distance over his head 10 times in 10 seconds. times in 10 seconds. Which student does the most work? Which student does the most work? Which student delivers the most powerWhich student delivers the most powerExplain your answers. Explain your answers.

Ben and Bonnie Ben and Bonnie do the same amount do the same amount of work; they apply of work; they apply the same force to lift the same force to lift the same barbell the the same barbell the same distance above same distance above their heads. their heads.

Yet, Ben is the Yet, Ben is the most powerful since most powerful since he does the same he does the same work in less time. work in less time.

Power and time Power and time are inversely are inversely proportional.proportional.

2. How much power will it take to move a 10 kg mass 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in use the formulas for force, work, and power all in the correct order.the correct order.

Force=Mass x Acceleration Force=Mass x Acceleration Force=10 x 2Force=10 x 2Force=20 NForce=20 N

Work=Force x DistanceWork=Force x DistanceWork = 20 x 10Work = 20 x 10

Work = 200 JoulesWork = 200 Joules

Power = Work/TimePower = Work/TimePower = 200/5Power = 200/5

Power = 40 wattsPower = 40 watts

What’s Up With Work?

Scenario A: A force acts rightward upon an object as it is displaced rightward.

Scenario B: A force acts leftward upon an object which is displaced rightward

Scenario C: A force acts upward on an object as it is displaced rightward.

The least steep incline (30-degree incline angle) will require the least amount of force while the most steep incline will require the greatest amount of force. Yet, force is not the only variable affecting the amount of work done by the car in ascending to a certain elevation. Another variable is the displacement which is caused by this force. A look at the animation above reveals that the least steep incline would correspond to the largest displacement and the most steep incline would correspond to the smallest displacement.

How much Work is being done?

Diagram A Answer:

W = (100 N) * (5 m)* cos(0 degrees) = 500 J

The force and the displacement are given in the problem statement. It is said (or shown or implied) that the force and the displacement are both rightward. Since F and d are in the same direction,the angle is 0 degrees.

Diagram B Answer:

W = (100 N) * (5 m) * cos(30 degrees) = 433 J

The force and the displacement are given in theproblem statement. It is said that the displacement is rightward. It is shown that the force is 30 degrees above the horizontal. Thus, the angle between F and d is 30 degrees.

Diagram C Answer:

W = (147 N) * (5 m) * cos(0 degrees) = 735 J

The displacement is given in the problem statement. The applied force must be 147 N since the 15-kg mass (Fgrav=147 N) is lifted at constant speed. Since F and d are in the same direction, the angle is 0 degrees.

A force of 50 N acts on the block at the angle shown in the diagram. The block moves a horizontal distance of 3.0 m. How much work is done by the applied force?

W = F * d * cos(Theta) W = (50 N) * (3 m) * cos (30 degrees) = 129.9 Joules

How much work is done by an applied force to lift a 15-Newton block 3.0 meters vertically at a constant speed?

To lift a 15-Newton block at constant speed, 15-N of force must be applied to it (Newton's laws). Thus,

W = (15 N) * (3 m) * cos (0 degrees) = 45 Joules

Calculate the work done by a 2.0-N force (directed at a 30° angle to the vertical) to move a 500 gram box a horizontal distance of 400 cm across a rough floor at a constant speed of 0.5 m/s. (HINT: Be cautious with the units.)

Here is a good example of the importance of understanding the angle between F and d. In this problem, the d is horizontal and the F is at a 60-degree angle to the horizontal. Thus, theta is 60 degrees.

W = (2.0 N) * (4.00 m) * cos (60 degrees) = 4.0 J