GoalReview forces, speed, energy and collisions with new mixed problems.
New words
friction · a force that slows moving things where surfaces rubpotential energy · stored energykinetic energy · the energy of moving things
Explanation
Unit 2 was about motion. A force is a push or a pull. Balanced forces do not change motion; unbalanced forces do. Friction slows and stops moving things. Speed is distance divided by time: 12 m in 3 s is 4 m/s. Energy is the ability to do work. Stored energy is potential energy, for example in a lifted book or a stretched spring, and the energy of motion is kinetic energy. Energy changes form but is never lost: in a collision it passes from one object to another, and some becomes sound and heat. A faster object and a heavier object both have more kinetic energy, so they cause bigger changes when they collide. Seat belts, airbags, head restraints and helmets help keep people safe.
Idea
Remember
speed
distance ÷ time
potential energy
height or stretch
kinetic energy
more speed or more mass: more energy
energy
changes form, never lost
Worked examples
Vehicle
Distance
Time
Speed
bus
150 km
3 h
50 km/h
taxi
180 km
3 h
60 km/h
A bus travels 150 km in 3 hours. Speed = 150 ÷ 3 = 50 km/h. A taxi travels 180 km in 3 hours: 180 ÷ 3 = 60 km/h. The taxi is faster by 10 km/h.
top: potential→sliding: kinetic→stopping: heat
A girl sits at the top of a slide: potential energy. She slides down: the potential energy changes into kinetic energy. At the bottom she slows down: friction changes the kinetic energy into heat.
Which collision causes the bigger change? A small ball rolling slowly into a cup, or a heavy ball rolling fast into the same cup. The heavy, fast ball has more kinetic energy, so the cup moves farther.
Class activity
Term 1 poster. Choose one idea from Unit 2: friction, speed, energy changes or seat belts. Make a poster for younger children with a drawing, a short explanation and one real example from Egypt, such as a microbus braking, a football kick or a kite. Present it to your family.
Practice
1
A cyclist rides 24 m in 6 seconds. What is her speed?
24 ÷ 6 = 4 m/s.
2
A: a book held high. B: a runner sprinting. C: a child sitting at the top of a slide. Which two have potential energy? Say the letters.
A and C: both are high up and ready to move.
3
A driver brakes and the car stops. Which force stops it?
Friction between the brakes and the wheels, and between the tyres and the road.
4
In a collision 10 units of energy went in. 7 units went to the other object and 2 to sound. How many units went to heat?
