Key Learning Objectives
- Calculate weight using W = m × g (mass in kg, gravitational field strength g = 9.8 or 10 N/kg).
- Explain how air resistance increases with velocity until it balances weight.
- Describe the motion of a falling object throughout its acceleration to terminal velocity.
Topic Summary & Core Notes
Understand gravity, mass vs weight (W = mg), drag forces, and how skydivers reach terminal velocity when downward weight equals upward air resistance.
Key Rules, Formulas & Definitions
Weight Formula
Weight (N) = Mass (kg) × Gravitational Field Strength (N/kg) [W = m × g]
Terminal Velocity Condition
Upward Air Resistance = Downward Weight → Resultant Force = 0 N → Acceleration = 0 (Constant Speed)
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Calculating Weight on Earth vs Moon
Question: An astronaut has a mass of 80 kg. Calculate their weight on Earth (g = 9.8 N/kg) and on the Moon (g = 1.6 N/kg).
Step-by-Step Solution:
- Step 1: Weight on Earth: W = m × g = 80 kg × 9.8 N/kg = 784 N.
- Step 2: Weight on Moon: W = m × g = 80 kg × 1.6 N/kg = 128 N.
- Step 3: Mass check: Mass remains 80 kg in both locations.
Final Answer: Earth: 784 N | Moon: 128 N (Mass remains 80 kg)
Senior Examiner Insight: Mass never changes with location; weight changes depending on local gravitational field strength (g).
Worked Example 2
Worked Example 2: Explaining Terminal Velocity in 3 Stages
Question: Describe the forces acting on a parachutist immediately after jumping from an aircraft until reaching terminal velocity.
Step-by-Step Solution:
- Stage 1 (Initial jump): Only downward weight acts. Drag = 0. Object accelerates downwards at maximum rate (g).
- Stage 2 (Speed increasing): As speed increases, upward air resistance increases. Resultant force decreases, so acceleration decreases.
- Stage 3 (Terminal velocity): Air resistance increases until it exactly EQUALS weight. Resultant force = 0 N, so speed remains constant.
Final Answer: Air resistance increases with speed until it equals weight, making resultant force zero at constant terminal velocity.
Senior Examiner Insight: State clearly that acceleration decreases as drag grows, even while speed continues to increase.
Common Pitfalls & Examiner Warnings
Common Mistake: Confusing Mass (kg) with Weight (Newtons)
Mass is matter quantity in kg; Weight is gravitational force pulling mass in Newtons (N).
Common Mistake: Saying 'forces run out' at terminal velocity
Forces do not disappear; downward weight and upward drag become EQUAL and BALANCED (Resultant = 0 N).
Common Mistake: Assuming heavy objects always fall faster in a vacuum
In a vacuum with no air resistance, ALL objects fall with the exact same acceleration (g).
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What is the weight of a 5 kg bowling ball on Earth (g = 9.8 N/kg)?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) 49 N
Detailed Explanation: W = m × g = 5 kg × 9.8 N/kg = 49 Newtons.
Mark Scheme & Scoring Points: 1 mark for 49 N.
Question 2 of 3
When a falling skydiver reaches terminal velocity, what is the resultant force acting on them?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) 0 N (Balanced forces)
Detailed Explanation: At terminal velocity, upward air resistance equals downward weight, resulting in 0 N net force.
Mark Scheme & Scoring Points: 1 mark for 0 N / balanced forces.
Question 3 of 3
What happens to a skydiver's terminal velocity immediately after opening their parachute?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) Massive surface area increases air resistance, creating an upward resultant force that decelerates them to a new, much slower terminal velocity.
Detailed Explanation: Increased surface area increases drag above weight, causing rapid deceleration to a safe landing speed.
Mark Scheme & Scoring Points: 1 mark for increased drag causing deceleration to a lower terminal speed.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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