GED Science: Motion, Forces, and Work Concepts
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Overview
This chapter covers how things move and the forces that change their motion. On the GED test, a question gives you any formula it needs, so practice using a formula and reading motion from a table or graph.
Speed is distance divided by time, and acceleration is the change in speed divided by time. Momentum is mass times velocity, so a light object moving fast can have more momentum than a heavy object moving slowly. Newton's first law says objects keep their motion unless a force acts (inertia). The second law says force = mass × acceleration. The third law says every action has an equal and opposite reaction.
Mass is the amount of matter and stays the same everywhere; weight is the pull of gravity on that mass (weight = mass × g, with g about 9.8 m/s² on Earth). Without air resistance, all objects fall at the same rate. Friction acts against motion.
Work is force times distance, and power is work divided by time. Simple machines, such as levers, pulleys, ramps (inclined planes), wedges, screws, and wheels and axles, make work easier by trading force for distance. Mechanical advantage = output force ÷ input force.
Key concepts
Speed and acceleration
Speed = distance ÷ time. Acceleration = change in speed ÷ time.
Memory tipWrite the formula first, then put in the numbers.
Newton's laws
1st: inertia. 2nd: force = mass × acceleration. 3rd: equal and opposite reaction.
Memory tipFirst = keeps moving; second = F = ma; third = push back.
Mass and weight
Mass (kg) does not change from place to place. Weight (N) = mass × g and depends on gravity; g is about 9.8 m/s² on Earth.
Memory tipOn Earth, weight = mass × 9.8 m/s². On the Moon, weight drops but mass stays.
Momentum
Momentum = mass × velocity.
Memory tipMultiply before you compare; the heaviest object may not win.
Work and power
Work = force × distance (joules). Power = work ÷ time (watts).
Memory tipWork multiplies; power divides by time.
Simple machines
Lever, pulley, wheel and axle, inclined plane, wedge, screw. Mechanical advantage = output force ÷ input force.
Memory tipLess force always means more distance.
Often tested
- On a distance-time table, distance that stays the same means the object is stopped.
- Average speed for an interval uses only the distance and time for that interval.
- A common mistake is dividing when the formula multiplies, or the reverse.
3 sample questions
Verified practice questions from this unit.
Question 1. A delivery truck travels 180 miles in 3 hours. What is its average speed?
- ① 540 miles per hour
- ② 90 miles per hour
- ③ 183 miles per hour
- ④ 60 miles per hour
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Answer: ④ 60 miles per hour
Key point: Speed = distance ÷ time
- Speed = distance ÷ time.
- 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.
Question 2. Using force = mass × acceleration, how much force is needed to give a 1,200-kilogram car an acceleration of 2 m/s²?
- ① 600 newtons
- ② 24,000 newtons
- ③ 1,202 newtons
- ④ 2,400 newtons
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Answer: ④ 2,400 newtons
Key point: F = m × a (newtons = kg × m/s²)
- F = m × a.
- 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.
Question 3. A bus stops suddenly, and a standing passenger lurches forward. What explains this?
- ① Inertia: the passenger's body keeps moving forward (Newton's first law)
- ② Gravity pulling the passenger forward
- ③ Friction pushing the passenger forward
- ④ Action and reaction (Newton's third law)
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Answer: ① Inertia: the passenger's body keeps moving forward (Newton's first law)
Key point: Inertia: objects keep their motion unless a force acts
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.
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