Grade 8 Science Practice Test: Forces, Waves, Genetics and Space

By Newton's third law, when the skater pushes on the wall, the wall pushes on her with a force of the same size in the opposite direction. That force from the wall is what sends her backward. The two forces are always equal in size, even though only the skater moves.

The force stayed at 6 N, and as the mass increased the acceleration went down: doubling the mass from 1 kg to 2 kg cut the acceleration in half. This matches Newton's second law, acceleration = force ÷ mass, so an object with more mass is harder to speed up.

The two forces point in opposite directions, so subtract them: 30 N − 12 N = 18 N. The net force points the same way as the larger force, to the right. Adding the forces to get 42 N would be correct only if both pointed in the same direction.

The gravitational force between two objects depends on their masses and the distance between them. It gets stronger when the masses increase or the objects move closer, and weaker when they move apart. Gravity acts across empty space, so a vacuum does not change it.

Speed = frequency × wavelength = 4 waves per second × 0.5 m = 2 m/s. The answer 8 m/s comes from dividing the frequency by the wavelength instead of multiplying them.

Sound is a mechanical wave: it travels by making the particles of a medium, such as air or water, vibrate. The space between the Sun and Earth has almost no particles, so sound cannot cross it, while light is an electromagnetic wave that can travel through a vacuum. Sound is also far slower than light, not faster.

White light contains all colors. The apple's skin absorbs most of them and reflects mainly red light, which reaches our eyes. An object that absorbed red light and reflected the rest would not look red.

A Punnett square for Bb × bb gives two Bb boxes and two bb boxes. Only bb guinea pigs have white fur, so 2 out of 4, or 50%, are expected to be white. The answer 25% belongs to a different cross, Bb × Bb.

Growing from runners is asexual reproduction: the new plants develop from the parent's own cells, so they have the same genes, apart from rare mutations. Getting half of the genes from each of two parents happens in sexual reproduction, for example when strawberry flowers are pollinated and form seeds.

Similar bone patterns in limbs used for different purposes are called homologous structures. They are evidence that these mammals inherited the pattern from a shared ancestor and then changed over many generations. The animals live in very different habitats and use their limbs differently, so neither of those explains the match.

The Sun always lights half of the Moon. As the Moon travels around Earth, we see more or less of that lit half, from almost none at new moon to all of it at full moon. Earth's shadow falls on the Moon only during a lunar eclipse.

From Mercury to Saturn, every planet that is farther from the Sun takes longer to complete one orbit, from about a quarter of a year to more than 29 years. The orbit time is not equal to the distance: Jupiter is about 5 times as far from the Sun as Earth but takes almost 12 years to go around once.