Week 15 · Term 1 review (Units 1–4) and Darslab’s own review tasks · 3 lessons
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Review: electric, magnetic and gravitational forces
Science Prep 1 Term 1, Units 1–4: Review
GoalReview static electricity, magnets, gravity, mass and weight with new tasks.
New words
static charge · an electric charge that stays on an objectfield force · a force that acts at a distanceweight · the pull of gravity on an object, in newtons
Explanation
Like charges repel and unlike charges attract. Rubbing can move electrons from one object to another, so one becomes positive and the other negative. Like magnetic poles repel and unlike poles attract, and a magnet always has two poles. Electric, magnetic and gravitational forces are field forces: they act without touching. Mass is the amount of matter, in kg, and is the same everywhere. Weight = mass × g, in newtons, with g about 10 N/kg near the Earth. On the Moon g is about one sixth as large, so the weight is smaller but the mass is the same. Astronauts in orbit still feel gravity; they seem weightless because they are falling around the Earth.
Worked examples
Pair
Result
+ and −
attract
− and −
repel
N and S
attract
S and S
repel
Attract or repel? Positive and negative: attract. Two negative: repel. N pole and S pole: attract. S pole and S pole: repel.
A comb rubbed on wool picks up small pieces of paper. The comb has a static charge. It attracts the neutral paper pieces. A lightning rod gives charge a safe path to the ground.
Place
Mass
Weight
Earth
6 kg
60 N
Moon
6 kg
about 10 N
A bag has a mass of 6 kg. On the Earth, weight = 6 × 10 = 60 N. On the Moon g is about one sixth, so the weight is about 10 N. The mass is still 6 kg.
Two objects of 5 kg and 20 kg attract each other. The pulls are equal and opposite, although the masses differ.
Class activity
Quiz game. Write ten cards, five on forces at a distance and five on mass and weight, with a small calculation on each, such as mass 4 kg and g = 10 N/kg. Shuffle and take turns to answer. Keep the numbers small and check each other’s sums.
Practice
1
What is the weight of a 4 kg book near the Earth (g = 10 N/kg)?
4 × 10 = 40 N.
2
Two N poles of magnets are brought together. What happens?
