GED Science Motion, Forces, and Work Summary

1. Speed = distance ÷ time.
2. 180 ÷ 3 = 60 miles per hour.

Check: 60 × 3 = 180 miles.

Wrong choices

• 540 miles per hour: 540 comes from multiplying 180 by 3. Speed is distance divided by time.
• 90 miles per hour: 90 comes from dividing by 2 instead of 3.
• 183 miles per hour: 183 comes from adding 180 and 3.

1. F = m × a.
2. F = 1,200 × 2 = 2,400 newtons.

Check: 2,400 ÷ 1,200 = 2 m/s².

Wrong choices

• 600 newtons: 600 comes from dividing 1,200 by 2. The formula multiplies.
• 24,000 newtons: 24,000 has an extra zero. 1,200 × 2 = 2,400.
• 1,202 newtons: 1,202 comes from adding 1,200 and 2.

Newton's first law says an object in motion stays in motion unless a force acts on it. The bus stops, but the passenger's body tends to keep moving forward. This is inertia.

Wrong choices

• Gravity pulling the passenger forward: Gravity pulls down, not forward.
• Friction pushing the passenger forward: Friction from the floor acts against the slide. It does not push the person forward.
• Action and reaction (Newton's third law): The third law is about pairs of forces between two objects. The lurch happens because the body keeps its motion.

Mass is the amount of matter in an object, so it does not change from place to place. Weight is the pull of gravity on that mass. The Moon's gravity is weaker, so she weighs less there, but her mass is still 70 kg.

Wrong choices

• 420 kilograms: 420 is 70 × 6. Mass does not increase on the Moon.
• 0 kilograms: She still has matter in her body, so her mass is not zero.
• About 12 kilograms: About 12 kg is one-sixth of 70. Her weight drops on the Moon, but her mass does not.

Newton's third law says that when one object pushes on another, the second object pushes back with an equal force in the opposite direction. The swimmer pushes water back, and the water pushes the swimmer forward.

Wrong choices

• The law of conservation of mass: Conservation of mass is about matter in chemical reactions.
• Newton's second law: The second law links force, mass, and acceleration (F = ma). It does not describe paired forces.
• Newton's first law: The first law is about inertia, objects keeping their motion.

Friction is a force between two surfaces that are touching. It acts against the direction of motion, so it slows the sliding box until it stops.

Wrong choices

• Buoyant force: Buoyant force is the upward push of a liquid or gas. It does not stop a sliding box.
• Magnetic force: There are no magnets involved.
• Gravity: Gravity pulls the box down toward the floor, not backward against its motion.

1. Change in speed = 20 − 0 = 20 m/s.
2. Acceleration = 20 ÷ 4 = 5 m/s².

Check: speeding up by 5 m/s each second for 4 seconds gives 20 m/s.

Wrong choices

• 80 m/s²: 80 comes from multiplying 20 by 4. Acceleration is the change in speed divided by time.
• 24 m/s²: 24 comes from adding 20 and 4.
• 16 m/s²: 16 comes from subtracting 4 from 20.

1. Work = force × distance.
2. Work = 50 × 10 = 500 joules.

Check: 500 ÷ 10 = 50 newtons.

Wrong choices

• 5 joules: 5 comes from dividing 50 by 10. The formula multiplies.
• 60 joules: 60 comes from adding 50 and 10.
• 40 joules: 40 comes from subtracting 10 from 50.

From 30 s to 50 s, the distance from the start stayed at 150 m. If the distance does not change, the cyclist is not moving.

Wrong choices

• The cyclist was speeding up.: Speeding up would make the distance grow faster. Here the distance did not grow at all.
• The cyclist was riding back toward the start.: Riding back toward the start would make the distance from the start get smaller. It stayed at 150 m.
• The cyclist moved at a steady 5 meters per second.: 5 m/s was the speed during the first 30 seconds. After 30 s, the distance did not change.

1. Distance in the first 30 s = 150 − 0 = 150 m.
2. Speed = 150 ÷ 30 = 5 m/s.

Check: 50 m every 10 s is also 5 m each second.

Wrong choices

• 3 meters per second: 3 m/s is 150 m ÷ 50 s, the average over the whole 50 seconds, not the first 30.
• 150 meters per second: 150 is the distance, not the speed.
• 50 meters per second: 50 is the distance covered in each 10-second interval, not the speed per second.