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Grade 9 / Middle 3 (age 14-15)

grade 9 Work and Energy

Work and Energy

In physics, work is done only when a force moves an object in the force's direction, and work W equals force F times distance d. If you push a wall hard but the wall does not move, the work is zero. Doing work transfers energy, raising kinetic or potential energy, and without friction their sum (mechanical energy) is conserved. Change the force and the distance and the size of work W = Fd appears. Kinetic and potential energy continue in the text boxes below.

What is "Work" in Physics?

📦 Intuition — Everyday "Work" vs Physics "Work"
①Everyday: studying, thinking — those count
②Physics: a force must move the object to count as work
③No matter how hard you push a stationary wall, work = 0!
50 N
5 m

Work and Energy

⚡ Energy = Capacity to Do Work
①Doing work on an object → its energy increases
②Lifting it up → potential energy
③Speeding it up → kinetic energy

Kinds of Energy

🔋 Kinetic and Potential Energy
①Kinetic: energy of a moving object (more with speed)
②Potential: energy of a raised object (more with height)
③Mechanical energy = KE + PE
④Without friction, mechanical energy is conserved

Summary

Definition of Work
W = F × d
Work (J) = force (N) × distance (m)
Kinetic Energy
Ek = 12mv2
Proportional to mass and to speed²
Potential Energy
Ep = mgh
mass × gravity × height
🎯 Exam Points
①Work = force × distance (unit: J = N·m)
②Force and motion must be in the same direction to do work
③Doing work transfers energy
④Mechanical energy conservation: KE + PE = constant
⑤1 J = work done by 1 N over 1 m

Examples and Unit Test

Example 1
A force of 10 N is applied to an object, moving it 5 m in the direction of the force. How much work is done, in joules?
1
Work = force × distance moved (when the force and motion are in the same direction).
2
Work = 10 N × 5 m = 50 J.
▸ 50 J (10 × 5 = 50)
1 J is the work done moving an object 1 m with a force of 1 N.
Example 2
An object of mass 2 kg moves at 3 m/s. What is its kinetic energy, in joules? (kinetic energy = ½mv²)
1
Kinetic energy = ½ × mass × speed².
2
½ × 2 kg × (3 m/s)² = ½ × 2 × 9 = 9 J.
▸ 9 J (½ × 2 × 3² = 9)
Kinetic energy is proportional to the square of speed, so doubling the speed makes it four times as large.
Unit Test
Which statement about work and energy is correct?
①No matter how hard you push a wall, if it does not move the work done is zero
②The unit of work is the newton (N)
③Kinetic energy is directly proportional to speed
④Potential energy is the same regardless of height
⑤Work is done even when the force is perpendicular to the motion
▸ ① No matter how hard you push a wall, if it does not move the work done is zero
1
The unit of work is the joule (J) (② wrong), and kinetic energy is proportional to the square of speed (③ wrong).
2
Potential energy mgh is proportional to height (④ wrong), and when the force is perpendicular to the motion the work is zero (⑤ wrong); with no movement the work done is zero (① correct).
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Inertia & Action-Reaction
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Free Fall
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