☰ خطة العام · العلوم (بالإنجليزية)

Mass, energy and keeping safe in collisions

الأسبوع 14 · المحور الثاني، الوحدة الثانية، المفهوم 2.3 «الطاقة والتصادم» (ب) ومشروع الوحدة «سلامة المركبة» · 3 دروس
2

Where does the energy go?

Discovery Education Science P4: Term 1, Unit 2, pp. 172–188
GoalExplain that energy is passed from object to object in a collision and is not destroyed, using a row of hanging balls.
New words
conservation of energy · energy is never created or destroyed; it only changes form or movesNewton’s cradle · a row of hanging balls that pass energy alongfriction · a force that slows moving things where surfaces rub
Explanation

A Newton’s cradle is a row of identical balls hanging side by side. Lift one ball on the end and let it go. It swings down and hits the row. The balls in the middle hardly move, but the ball on the other end swings up almost as high. The kinetic energy was passed from ball to ball along the row. If you lift two balls, two balls swing out on the other side. You also hear a click: some energy leaves as sound. Some energy leaves as a tiny bit of heat, and some is used against friction where the strings move. That is why the swinging slowly dies away. The energy has not been destroyed. It has moved on, to the air and to the other parts. Energy is conserved: the total before a collision equals the total after.

Worked examples
Mariam lifts one ball of a Newton’s cradle and lets go. After the click, one ball on the other end swings out. The energy of the first ball was passed along the row.
ItemEnergy
moving ball (before)10 units
far ball (after)8 units
sound1 unit
heat1 unit
Omar counts the energy in made-up units. Before the collision the moving ball has 10 units. After: the far ball has 8 units, sound takes 1 unit and heat takes 1 unit. 8 + 1 + 1 = 10. The total is the same, so no energy is lost.
A coin slides on a table and hits a row of coins. The coin at the far end slides away and the others stay. You hear a click. The coin’s kinetic energy was passed along, and some became sound.
Class activity

Coin row. Put four coins in a straight row, touching, on a smooth table. A grown-up flicks a fifth coin gently along the line so that it hits the row. Watch which coin moves. Try with two coins sliding together. Listen for the click. Write what you saw and what happened to the energy. Coins stay away from mouths.

Practice
1
In a Newton’s cradle, one ball is lifted and let go. How many balls swing out at the other end?
2
Why do we hear a click when the balls hit?
3
Before a collision a ball has 12 units of energy. After: the other ball has 9 units, sound has 2 units. How many units went to heat?
4
Is energy destroyed when the swinging balls slowly stop?