Grade 8 Science Practice Test: Speed, Acceleration and Newton's Laws

Speed = distance / time = 120 m / 40 s = 3 m/s. Dividing time by distance gives 0.33, and multiplying gives 4,800.

Acceleration = change in speed / time = (18 - 6) / 4 = 3 m/s². Using only the final speed gives 18 / 4 = 4.5, which forgets the starting speed.

Speed tells how fast; velocity tells how fast and in which direction, such as 20 m/s north. Both can use the same units.

A flat line means the distance does not change, so the object is not moving. A straight sloped line would mean constant speed.

Time = distance / speed = 100 / 5 = 20 s. Multiplying 100 by 5 gives 500, which is not a time for this trip.

From F = m x a, a = F / m = 12 / 3 = 4 m/s². Multiplying gives 36, and dividing the wrong way gives 0.25.

Acceleration is any change in velocity, and velocity includes direction. On a curve the direction changes all the time, even at constant speed.

Forces in opposite directions subtract: 50 - 30 = 20 N, in the direction of the larger force, to the right. Adding them gives 80 N, which would only be true if both pushed the same way.

Newton's first law says a moving object keeps moving unless a force acts on it. The bus stops, but the passenger's body tends to keep going forward.

By Newton's third law, when the rocket pushes the gas down, the gas pushes the rocket up. This works even in space, where there is no ground or air to push against.