الأسبوع 13 · المحور الثاني، الوحدة الثانية، المفهوم 2.3 «الطاقة والتصادم» (أ): التصادم والسرعة وطاقة الحركة · 3 دروس
2
Speed and the ramp race
Discovery Education Science P4: Term 1, Unit 2, pp. 158–171
GoalCalculate speed as distance divided by time and compare the speeds of two moving things.
New words
speed · how far something moves in one unit of timedistance · how far it is from one place to anothermetres per second (m/s) · a unit of speed
Explanation
Speed tells us how fast something is moving. It is the distance an object travels in one unit of time. To find speed, divide the distance by the time. A runner who goes 12 metres in 3 seconds has a speed of 12 ÷ 3 = 4 metres per second, written 4 m/s. A car that drives 60 kilometres in 1 hour has a speed of 60 km/h. To compare two objects, measure how far each goes in the same time: the one that goes farther is faster. Or measure how long each takes to go the same distance: the one that takes less time is faster. In a ramp race with toy trucks, a steeper ramp makes a truck faster. We measure the track with a metre ruler or tape and the time with a stopwatch.
Who or what
Distance
Time
Speed
runner
12 m
3 s
4 m/s
car
60 km
1 h
60 km/h
Worked examples
Marwan cycles 30 m in 10 seconds. His speed is 30 ÷ 10 = 3 metres per second.
Two runners run for one hour. Nour runs 6 km and Habiba runs 9 km. Habiba went farther in the same time, so Habiba is faster.
Books under ramp
Time for 3 m
Speed
2
6 s
0.5 m/s
4
3 s
1 m/s
6
2 s
1.5 m/s
Ramp race: Seif races a toy truck down a ramp and across 3 m of floor. With 2 books under the ramp the truck takes 6 s. With 4 books it takes 3 s, and with 6 books 2 s. The steeper the ramp, the less time, so the faster the truck. With 4 books: 3 ÷ 3 = 1 m/s.
Class activity
Ramp race. With a grown-up, lean a smooth board on 2 books. Mark a finish line 3 m from the ramp's foot on a clear floor, and keep feet and pets away. Let a toy truck roll from the top and time it with a stopwatch. Repeat with 4 books and with 6 books. Write the times in a table and work out each speed as 3 ÷ time. Never stand in front of the rolling truck.
Practice
1
A boy runs 20 m in 4 seconds. What is his speed?
20 ÷ 4 = 5 m/s.
2
A bus travels 100 km in 2 hours. What is its speed?
100 ÷ 2 = 50 km/h.
3
Two cars race over the same 1000 m. Car A takes 40 s and car B takes 50 s. Which is faster? Say the letter.
A, because it took less time for the same distance.
4
A truck rolls 8 m in 2 s on a steeper ramp and 8 m in 4 s on a gentle ramp. Which ramp gave the higher speed?
The steeper ramp: 8 ÷ 2 = 4 m/s, against 8 ÷ 4 = 2 m/s.
