HS Energy Lesson 1

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d) Look for a pattern of what was done to the objects that we studied to give them the

chalk-smashing potential. Then, devise a new physical quantity to describe this

pattern.

1.2 Observe and find a pattern

Now, suppose that a friend decides to save the chalk in the first two experiments by

exerting, with her hands, an opposing force on the block or on the cart after they are

released. In each case, she pushes on the moving object opposite to the direction of its

velocity. Below, give the direction of the force your friend exerts on the moving object

relative to its displacement as she stops it, thus causing the system to lose its potential to

break the chalk.

a) After lifting the block, you release the block and it starts falling. Your friend then

starts pushing upward on the falling block, slowing it down, and the block does not

break the chalk.

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b) You push the cart so that it rolls faster and faster. You then stop pushing. Just before

the cart reaches the chalk, your friend pushes it in a direction opposite to its

direction of motion. This causes the cart to slow down and stop so that it does not

break the chalk.

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Complete this table.

Experiment a) b)

Draw an arrow indicating the direction of the force your friend exerted on the system object that

you studied ( ).

Draw an arrow indicating the displacement of the system object while your friend was exerting the

force ( ).

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c) How could you modify the definition of the quantity you devised in the previous

activity to account for the system’s loss of the chalk-breaking potential due to your

friend’s intervention?

1.3 Observe and find a pattern

Consider the Earth and a 1-kg block.

a) You hold a string tied to a block so that it stays about 1 cm

above a table. A piece of chalk is placed on the table under the

block. If you release the block and it falls on the chalk, the

chalk will not break (it’s too close to the chalk).

Next you slowly walk about 2 m beside the table, continually

keeping the block 1 cm above the surface. After walking the 2

m, the block hangs over a second, identical piece of chalk.

Draw the force exerted by the string on the block and the

displacement of the block as you walked the 2 m.

b) Discuss whether the vertical force the string exerted on the block while moving it

horizontally above the tabletop caused the Earth and block to have a better chance of

breaking the second piece of chalk than the first piece.

c) Revise the quantity you devised in the last two activities to account for this result.

Your revision will involve the angle between the external force exerted on the

system and the system object’s displacement. We call this quantity work.

1.4 Observe and find a pattern (if you know trigonometry)

a) Consider a 1-kg dynamics cart being pulled at angle ! that can roll on a low-friction

horizontal dynamics track and a piece of chalk that is taped to the fixed, vertical end

of the track. You pull the cart so that it rolls faster and faster toward the chalk at the

end of the dynamics track and breaks the chalk when it hits it. Draw the force

exerted by you on the cart and the displacement of the cart while you were pulling it.

b) Discuss whether the angled force exerted on the cart while moving it horizontally

gave it a better chance of breaking the piece of chalk than the force exerted in

activity 1.1 part (b).

c) What trigonometric function would help you determine the system’s increase in

chalk-smashing ability? Is this consistent with the increase, decrease, and no change

in chalk-smashing potential for activities 1.1-1.3?

You are holding

the string

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d) Revise the quantity you devised in the last three activities to account for this result.

Your revision will involve the angle between the external force exerted on the

system and the system object’s displacement. We call this quantity work.

Students familiar with trig, proceed to page 8; those who are not, continue here.

1.5 Practice

Jeff did 573 J of work on a sled. He pulled the sled for a distance of 30 m. What is the

average force that he exerted on the system?

1.6 Practice

Steve slowly lifts a 20 kg barbell 1 meter vertically. How much work does he do on the

barbell?

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A woman pulls a box upwards. Since the force exerted by the woman on the box is in the same direction as the displacement, !

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A woman carries a box while walking at a constant pace. Since the force exerted by the woman on the box is perpendicular to the displacement,

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A woman catches a ball thrown at her. Since the force exerted by the woman on the ball is in the opposite direction as the displacement, !

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1.7 Practice

Jessica, at a constant slow speed, moved a 1 kg book from a 2 m high shelf to the floor.

How much work did she do on the book?

1.8 Practice

If Natasha slows a moving grocery cart by pulling on it exerting a force of 23 N over 2.3 m,

what will be the work she does on it?

Homework

1.9 Relate

Describe a situation when you have done:

a) +1 J of work on a system.

b) -1 J of work on a system.

c) 0 J of work on a system.

1.10 Regular problem

While working out, a man lifts a 10-kg object a vertical distance of 0.80 m. He then carries

it for 10 m where he sets it down a vertical distance of 0.80 m. How much work does he do

on the object when he picks the object up, when he carries it, and when he sets it back

down? What is the total work that he does on it?

1.11 Observe and explain

In another situation, you stretch a block-spring system and then release the block. The block

slides toward the wall and smashes a piece of chalk. Label whether the ability of the block-

spring-wall system to crush the chalk increases, decreases, or remains the same between

each step.

Is this process consistent with the pattern we observed today between the net force exerted

on an object and the displacement of the object?

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Work-Trigonometry Section: In this section, students will use trigonometry to express

work done for a more general range of situations.

Did You Know?

Work W: Work is a physical quantity that is equal to the product of the magnitude of the

average force FEx on O that an external environmental object exerts on a system object, the

magnitude of the system object’s displacement d, and the cosine of the angle between FEx on

O and d.

W = (FEx on O cos !) d

1.12 Regular Problem

Suzanne is pulling a sled up a hill that makes a 24° angle with the horizontal. She keeps the

rope parallel to the hill and exerts a 150-N force on it. How much work will she do if she

pulls the sled 150 m?


A 4 kg grocery cart rolls down an incline 3 m incline with an angle of 10°. How much

work does the Earth do on the cart?


Juan pushes a box at an angle to the horizontal 250 J of energy over a distance of 10m. If

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the force exerted is 30 N, what is the angle between the force exerted by Juan on the box

and the horizontal?


To clean the floor, David exerts a 40N force on a broom handle two push it 2m. If the mop

handle makes a 40° with the floor, what is the work done by David on the broom? If the

mop handle were angled at 65° would David do more or less work?

HS Energy Lesson 1

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Transcript of HS Energy Lesson 1