Leo's Fast Cart
Leo's cart rolls into a wooden block at 2 m/s. The block slides 8 cm and stops.
Leo makes the cart go twice as fast. How far will the block slide now?
Page 1
Science · Grade 4 Science
Chapter 1: Energy and Motion
Twice as fast is four times as much.
Leo's cart rolls into a wooden block at 2 m/s. The block slides 8 cm and stops.
Leo makes the cart go twice as fast. How far will the block slide now?
Make your guess to turn the page.
Still to come: 9 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children expect twice as fast to mean twice the energy. The story shows the block sliding 8 cm at 2 m/s and 32 cm at 4 m/s, four times as far. Ask: if the energy had only doubled, how far would the block have gone? The answer to reach is that speed counts twice: the energy goes with speed times speed. That is why a small rise in car speed makes a crash so much worse.
Mass matters too — Double the mass and the energy doubles; double the speed and it goes up four times. A heavy object moving slowly can still carry as much energy as a light one moving fast, which is why a truck at 30 mph is more dangerous than a bicycle at 30 mph.
About the numbers — Kinetic energy is really one half × mass × speed × speed, in joules. The book leaves out the half so the child can multiply whole numbers, so one "unit" here is 2 joules. The block sliding 2 cm per unit is an idealised floor: a real block slides further with more energy in the same way, but not quite so neatly.