Grade 10 Physical Sciences · Mechanics

Use conservation of mechanical energy

Use the idea that mechanical energy stays the same without friction to find a speed or height (a ball dropped from 5 m hits the ground at about 9,9 m/s).

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Grade 10 Mechanical Energy Past Paper Practice Part 1Miss Martins Maths and Science

Conservation of energy | Work and energy | Physics | Khan AcademyKhan Academy

Conservation of energy | Physics | Khan AcademyKhan Academy

Conservation of Mechanical Energy - Physics #grade10Edu-ca-te

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Question 5.2 · May/June 2024 · Paper 1 · National (DBE) · 3 marks

A crate of mass

18

kg, initially at rest, slides down a frictionless slope from point A to point B. The crate then moves along a rough horizontal surface from point B towards point C. Point A is

3

m above the horizontal surface. See the diagram below. Diagram for this question

5.2 Using ENERGY PRINCIPLES only, calculate the speed of the crate at point B.(3)

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More past paper questions: use conservation of mechanical energy

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3. Question 1.5 · May/June 2024 · Paper 1 · National (DBE) · 2 marks
Various options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10).
1.5 A stone of mass m is dropped from a height h above the ground. Ignore the effects of air friction.

Which ONE of the following combinations CORRECTLY represents the kinetic energy and the total mechanical energy of the stone at the instant the stone has fallen through a distance of $\frac{1}{4}$ h?(2)
1. A Kinetic energy: $\frac{3}{4}$ mgh; Total mechanical energy: $\frac{1}{4}$ mgh
2. B Kinetic energy: $\frac{1}{4}$ mgh; Total mechanical energy: $\frac{3}{4}$ mgh
3. C Kinetic energy: $\frac{3}{4}$ mgh; Total mechanical energy: mgh
4. D Kinetic energy: $\frac{1}{4}$ mgh; Total mechanical energy: mgh
Show the memo (1 tick)Memo
1. ✓
  Choose the correct optionWith no air friction, mechanical energy is conserved: it stays equal to its starting value, $E_{m ec h} = m g h + 0 = m g h$ .
  
  After falling $\frac{1}{4}$ h the stone is at height $\frac{3}{4}$ h, so $E_{P} = \frac{3}{4} m g h$ and $E_{K} = m g h - \frac{3}{4} m g h = \frac{1}{4} m g h$ .
  
  Answer: D
  2 marks · Use conservation of mechanical energy
4. Question 1.3 · November 2023 · Paper 1 · National (DBE) · 2 marks
Various options are provided as possible answers to the following questions. Each question has only ONE correct answer.
1.3 A ball is projected vertically upwards from the top edge of a building. Points P, Q, R and S represent different positions during the motion of the ball, as shown in the diagram below.

Ignore the effects of air friction.

Between which two points will the ball undergo the greatest change in kinetic energy?(2)
1. A P and S
2. B P and R
3. C R and S
4. D Q and S
Show the memo (1 tick)Memo
1. ✓
  Correct optionWithout friction mechanical energy is conserved, so the change in kinetic energy equals the change in potential energy, $m g Δ h$ . The largest height difference is between Q (highest point, $E_{k} = 0$ ) and S (ground, largest speed).
  
  Answer: D
  2 marks · Use conservation of mechanical energy
5. Question 1.3 · November 2022 · Paper 1 · National (DBE) · 2 marks
Various options are provided as possible answers to the following questions. Each question has only ONE correct answer.
1.3 A ball is projected vertically upwards from the ground and reaches its maximum height after a while.

Ignore the effects of air friction.

How will the ACCELERATION and TOTAL MECHANICAL ENERGY of the ball at its maximum height compare to that immediately after it was projected?(2)
1. A Acceleration: Equal to; Total mechanical energy: Equal to
2. B Acceleration: Greater than; Total mechanical energy: Smaller than
3. C Acceleration: Equal to; Total mechanical energy: Greater than
4. D Acceleration: Smaller than; Total mechanical energy: Equal to
Show the memo (1 tick)Memo
1. ✓
  Acceleration and mechanical energy of a projectileWith no air friction the only force is gravity, so the acceleration is $g$ (downwards) at every point, including the top: EQUAL TO.
  Only gravity (a conservative force) does work, so total mechanical energy is conserved: EQUAL TO.
  
  Answer: A
  2 marks · Use conservation of mechanical energy
6. Question 1.7 · November 2023 · Paper 1 · Provincial · 2 marks
1.7 An object of mass $m$ is dropped from the top of the building and strikes the ground with kinetic energy $E$ . Another object of mass $2 m$ is dropped FROM THE SAME HEIGHT and strikes the ground. Its kinetic energy will be equal to ...(2)
1. A $\frac{1}{4} E$ .
2. B $\frac{1}{2} E$ .
3. C $E$ .
4. D $2 E$ .
Show the memo (1 tick)Memo
1. ✓
  Kinetic energy at the ground equals mghMechanical energy is conserved: $E_{K}$ at the ground $= E_{P}$ at the top $= m g h$ . For the same height, doubling the mass doubles $m g h$ , so the kinetic energy at the ground is $2 E$ .
  
  Answer: D
  2 marks · Use conservation of mechanical energy

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