الأسبوع 12 · المحور الثاني، الوحدة الثانية، المفهوم 2.2 «الطاقة والحركة» (ب): طاقة الوضع وطاقة الحركة وتحولات الطاقة · 3 دروس
2
Stored energy and moving energy
Discovery Education Science P4: Term 1, Unit 2, pp. 145–157
GoalTell potential (stored) energy from kinetic (motion) energy and show how one becomes the other.
New words
potential energy · stored energy that an object has because of its height or its stretched or squeezed shapekinetic energy · the energy an object has because it is movingspring · a coil of metal that can be stretched or squeezed
Explanation
Scientists split energy into two big kinds. Kinetic energy is the energy of an object that is moving. A rolling ball, a running child and a flying bird all have kinetic energy. Potential energy is stored energy: the object is not moving yet, but it is ready to do work. A book held high above the floor has potential energy. So does a stretched rubber band, a squeezed spring, and a child sitting at the top of a slide. Let go of the book and it falls: its potential energy changes into kinetic energy. The higher an object is, and the heavier it is, the more potential energy it has. The faster an object moves, the more kinetic energy it has.
Worked examples
A climber stands at the top of a tower. She is still, but she is high up, so she has potential energy. If she slides down a rope, that energy changes into kinetic energy.
Seif winds up a toy car with a key and puts it down. The wound spring inside has potential energy. When he lets go, the energy changes into movement and the car rolls away.
top: potential→sliding: changing→bottom: kinetic
A girl sits at the top of a slide, then slides down. At the top: potential energy. On the way down: the potential energy changes into kinetic energy. She goes faster and faster.
Class activity
Rubber-band launch. A grown-up gives you a rubber band and a small paper ball. Stretch the band a little over two fingers and let the ball fly gently to the side, away from faces. Never aim it at anyone. Then stretch it more. Measure how far the ball goes each time with a tape. More stretch means more stored energy. What did you find? Keep the same ball and the same starting place.
Practice
1
Is a ball resting on top of a hill kinetic or potential energy? Why?
Potential energy: it is high up and ready to roll.
2
Which two objects have kinetic energy? Say the letters.
B and C: the cyclist and the bus.
3
A book is lifted from the floor to a high shelf. Does its potential energy get bigger or smaller?
Bigger, because it is higher.
4
What happens to the energy of a stretched rubber band when you let go?
Its potential energy changes into kinetic energy and the band snaps back.
