الأسبوع 13 · المحور الثاني، الوحدة الثانية، المفهوم 2.3 «الطاقة والتصادم» (أ): التصادم والسرعة وطاقة الحركة · 3 دروس
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Faster crash, bigger change
Discovery Education Science P4: Term 1, Unit 2, pp. 158–171
GoalExplain that a faster object has more kinetic energy, so a faster collision causes a bigger change, and draw before-and-after pictures.
New words
before and after · pictures that show a collision at two momentsdent · a hollow made when something presses into a soft thingdamage · harm that breaks or spoils something
Explanation
The faster an object moves, the more kinetic energy it has. When a fast object hits something, it passes more energy on, so the change is bigger: a bigger dent, a harder push, a louder bang. Some of the energy may also leave as heat or sound. This is one reason why fast driving is dangerous: a fast car in a collision causes much more damage than a slow one. Scientists show a collision in two pictures. In the picture before, we draw the objects and how fast they move. In the picture after, we draw what changed and say how the kinetic energy changed. We can test the idea with a clay ball dropped onto a card base from three heights. The higher it starts, the faster it is going when it lands, and the bigger the dent.
Drop height
Speed on landing
Size of the dent
low
slow
small
medium
faster
bigger
high
fastest
biggest
Worked examples
Mina's clay ball is dropped on a card base from 25 cm, 50 cm and 100 cm. The dents are 1 cm, 2 cm and 4 cm wide. The highest drop made the widest dent: 4 − 1 = 3 cm wider than the lowest.
before: fast truck, cup still→hit→after: cup slides far
Before: a toy truck rolls slowly toward a paper cup. After: the cup slides a short way. Next time, before: the truck rolls fast. After: the cup slides much farther. The fast truck had more kinetic energy to pass on.
On a road, a car at slow speed bumps a bin and leaves a small mark. A car at high speed would hit it with more energy and cause much more damage. This is why there are speed limits near schools.
Class activity
Dent test. A grown-up helps you roll a ball of clay about the size of a walnut and put a piece of card on the floor as a base. Hold the clay 25 cm above the card, open your fingers and let it fall. Flatten it and repeat from 50 cm and 100 cm. Measure each dent with a ruler and record the widths in a table. Draw a before picture and an after picture for the highest drop. Clay stays away from mouths and faces.
Practice
1
Which has more kinetic energy: a ball rolling fast or the same ball rolling slowly?
The fast ball.
2
Clay dents: 2 cm, 3 cm and 5 cm for drops from low, medium and high. How much wider is the biggest dent than the smallest?
5 − 2 = 3 cm wider.
3
Why is fast driving more dangerous than slow driving?
A fast car has more kinetic energy, so a collision causes more damage.
4
What do we draw in the picture before and in the picture after?
Before: the objects and how fast they move. After: what changed and how the kinetic energy changed.
