ENGLISH VERSION: CHAPTER 7: ENERGY & POWER

[Pages:8]SCIENCE FORM 3 2021 BY CIKGU HEERY ENGLISH VERSION:

CHAPTER 7: ENERGY & POWER

7.1 WORK, ENERGY, & POWER

1. Meaning of energy = ability to do work 2. The more energy, the more work can be done 3. Meaning of work = product of force & displacement

= Force x displacement = F (N) x s(m) - SI of both energy & work is Joule (J) - direction of force & displacement must be similar - no work is done if no motion - Example of calculation:

Work = Force x disp. = Fs = 100N x 2m = 200J

*He is doing work of moving up against gravity Work = Fs = (600+100) x 2m = 700N x 2m = 1400J

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2. Meaning of power = work over time = Work done, W (J) / time taken, t (s)

- SI unit of power is Watt (W) - the shorter the time, the higher the power - Calculation example:

Box is pushed in 5 second. Calculate power.

Power = Work/time

= 100N x 2m 5s

= 40W

7.2 POTENTIAL ENERGY & KINETIC ENERGY

1. Gravitational potential energy (GPE) = energy contained by object due to height from Earth's surface

2. Formula of GPE = mass x gravitational acceleration x heigh = m x g x h = mgh

- Earth's gravitational acceleration is contant at 10ms-2 - The higher the height, the more the GPE

3. Calculation example:

A 500kg object is lifted for 5m. Calculate its GPE.

= mgh = 500kg x 10 x 5m = 25,000J

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4. Elastic potential energy (EPE) = energy contained in object being stretched or compressed

5. Formula of EPE = ? x force x spring displacement = ? x F x X = ? FX

*remember, displacement must be in meter(m)

6. Calculation example:

A spring is being stretched for 0.2m using a 100N load. Calculate the EPE of the spring.

= 1/2 x F x X = ? x 100N x 0.2m = 10J

7. Kinetic energy (KE) = energy contained in moving object

8. Formula of KE = ? x mass x velocity x velocity = ? x m x v2 = ? mv2

*small mass object can have high KE if moving very fast (bullet, arrow)

9. Calculation example:

A 20kg cannonball is launched as 10ms-1 velocity. Calculate its KE.

= ? mv2 = ? x 20kg x 10ms-1 x 10ms-1 = 1000J

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7.3 PRINCIPLE OF CONSERVATION OF ENERGY (POCOE)

1. POCOE states that: - energy cannot be created or destroyed - energy can only be converted

2. Example:

- EPE at A is converted to GPE at B - GPE at B is converted to KE at C - Assuming no energy loss, we can conclude that:

Energy at A = B = C ? Fx = mgh = ? mv2

3. Calculation example:

A 0.02kg ball is pushed into toy gun at distance of 0.2m using a 50N of force. Calculate the velocity when the ball is launched. (assume no energy loss)

Elastic potential energy = kinetic energy ? Fx = ? mv2

? x 50N x 0.2m = ? x 0.02kg x v2 v2 = 500

v = 22.4ms-1

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BAB 7: TENAGA & KUASA

7.1 KERJA, TENAGA, & KUASA

1. Definisi tenaga = keupayaan utk melakukan kerja 2. Semakin banyak tenaga, semakin banyak kerja yang dapat dilakukan 3. Definisi kerja = hasil darab daya & sesaran = Daya x sesaran = F(N) x s(m) - unit SI utk kedua-dua tenaga & kerja ialah Joule (J) - arah daya & sesaran mesti sama - tiada kerja dilakukan jika tiada pergerakan berlaku - Contoh pengiraan:

Kerja = Daya x sesar = Fs = 100N x 2m = 200J

*Kerja sdg dilakukan (naik tangga ke atas) utk melawan graviti Kerja = Fs = (600 + 100) x 2m = 700N x 2m = 1400J

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2. Definisi kuasa = kadar kerja dilakukan = Kerja, W (J) / masa yang diambil, t (s)

- Unit SI kuasa ialah Watt (W) - Semakin pendek masa, semakin tinggi kuasa! - Contoh pengiraan:

Kotak ditolak dlm masa 5 saat. Kira daya.

Kuasa = Kerja / masa

= 100N x 2m 5s

= 40W

7.2 TENAGA KEUPAYAAN & TENAGA KINETIK

1. Tenaga keupayaan graviti (TUG) = tenaga yang terkandung dlm objek kerana ketinggian dari permukaan Bumi

2. Formula TUG = jisim x pecutan graviti x tinggi = m x g x h = mgh

- Pecutan graviti Bumi adalah malar pada 10ms-2 - Semakin tinggi jarak dari permukaan Bumi, semakin tinggi TUG

3. Contoh pengiraan:

Objek 500kg diangkat setinggi 5m. Hitung TUG dlm objek tersebut.

= mgh = 500kg x 10 x 5m = 25,000J

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4. Tenaga keupayaan kenyal (TUK) = tenaga dalam objek yang diregang atau dimampatkan

5. Formula TUK = ? x daya x jarak regangan atau mampatan = ? x F x X = ? FX * ingat, jarak mestilah dalam meter (m)

6. Contoh pengiraan:

Spring diregangkan sebanyak 0.2m menggunakan beban 100N. Hitung TUK dlm spring itu

= 1/2 x F x X = ? x 100N x 0.2m = 10J

7. Tenaga kinetik (TK) = tenaga yang terkandung dalam objek bergerak

8. Formula TK = ? x jisim x halaju x halaju = ? x m x v2 = ? mv2

* objek berjisim kecil memiliki TK besar jika halaju tinggi (peluru, anak panah)

9. Contoh pengiraan:

Peluru meriam 20kg dilancarkan dgn halaju 10ms-1. Hitungkan TK.

= ? mv2 = ? x 20kg x 10ms-1 x 10ms-1 = 1000J

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7.3 PRINSIP KEABADIAN TENAGA

1. Prinsip ini menyatakan bahawa: - tenaga tidak boleh dicipta atau dimusnahkan - tenaga hanya dapat bertukar ke bentuk tenaga lain

2. Contoh:

- TUK di A ditukar menjadi TUG di B - TUG di B ditukar menjadi TK di C - Anggap bahawa tiada kehilangan tenaga berlaku, kesimpulannya:

Tenaga pada A = B = C ? Fx = mgh = ? mv2

3. Contoh pengiraan:

Peluru 0.02 kg ditolak ke dlm pistol mainan pada jarak 0.2 m menggunakan daya 50N. Hitung halaju bola dilancarkan. (anggap tiada kehilangan tenaga)

Tenaga keupayaan kenyal = tenaga kinetik ? Fx = ? mv2

50 x 50N x 0.2m = ? x 0.02kg x v2 v2 = 500

v = 22.4ms-1

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