Motion in a Gravitational Field: projectiles, energy and drag
Key idea: Use one uniform gravitational-field model consistently across force, component-motion and energy descriptions, then identify how drag changes the motion.
Before you start: Kinematics GraphsEnergy & Fields
By the end, you can
- Describe weight as the gravitational force on a mass and use W = mg in a uniform field.
- Explain projectile motion as uniform velocity in one direction and uniform perpendicular acceleration in the other.
- Derive ΔEₚ = mgΔh from work done and apply it with a consistent height change and sign.
- Describe falling with air resistance using resultant force, energy transfers and terminal velocity.
Starting-point self-check
1. Check your starting point
Attempt all three groups without notes and mark the first force, component or energy statement you could not justify. Use the recorded topic diagnostic above when you want scoring and a personalised repair plan.
Falling with air resistance 5(e)
Question 1
Describe how resultant force, acceleration, kinetic energy and energy transfer change as an object falls from rest through air until terminal velocity.
Check the model response
Initially weight exceeds drag, so the object accelerates and gains kinetic energy. As speed rises, drag rises, reducing resultant force and acceleration. At terminal velocity drag equals weight, acceleration is zero and kinetic energy is constant; gravitational potential-energy loss continues and is transferred to thermal stores by drag.
repair
2. Repair the five common breaks
Use only the repair matching an error, then rebuild the force, component or energy model.
Falling with air resistance 5(e)
Check this idea
Misconception: At terminal velocity no forces act and no energy is transferred.
Repair: Weight and drag both act but balance. Speed and kinetic energy stay constant while gravitational potential energy continues to be transferred to thermal stores in the object, air and surroundings.
worked example
3. Follow three worked models
Follow how each solution declares axes, system boundaries and the uniform-field assumption.
Falling with air resistance 5(e)
Model 1
A skydiver falls from rest, reaches terminal velocity, opens a parachute and later reaches a lower terminal velocity. Explain the complete force and energy sequence.
Check the model response
Before the first terminal speed, weight exceeds drag and speed rises; increasing drag reduces acceleration. At terminal speed drag equals weight. Opening the parachute makes drag exceed weight, so the upward resultant slows the downward motion and kinetic energy falls while energy is transferred thermally. As speed falls, drag falls until it again equals weight at a lower constant speed.
guided practice
4. Guided practice
Use each hint only to choose the force, component equation or energy-transfer direction.
Falling with air resistance 5(e)
Question 1
An 80 kg skydiver falls at a terminal speed of 50 m s⁻¹. Use g = 9.8 N kg⁻¹. Find drag and the rate at which gravitational potential energy is transferred.
Hint: At terminal speed the forces balance, but the downward displacement continues.
Check the model response
Drag = weight = mg = 784 N upward. The gravitational potential-energy loss rate is mgv = 784(50) = 39.2 kW, transferred mainly to thermal stores by drag.
independent practice
5. Independent practice
Solve without repair notes and state every sign convention and modelling assumption.
Falling with air resistance 5(e)
Question 1
Describe force, acceleration, kinetic-energy change and energy transfers immediately before and after a falling parachutist opens a parachute, and at the new terminal velocity.
Check the model response
Before opening at the first terminal speed, drag equals weight and kinetic energy is constant. Immediately after opening, drag exceeds weight, the resultant and acceleration are upward, and the downward speed and kinetic energy decrease while energy is transferred thermally. At the new lower terminal speed, drag again equals weight; kinetic energy is constant while gravitational potential energy continues to be dissipated.
Practice exit check
6. Practice assessment
Use this as extra closed-book practice, then complete the separate recorded assessment in your plan.
Falling with air resistance 5(e)
Question 1
A body is released from rest in air and eventually reaches terminal velocity. Give a continuous explanation using weight, drag, resultant force, acceleration, kinetic energy and gravitational potential-energy transfer.
Check the model response
At release drag is zero and weight gives a downward resultant, so acceleration is near g and kinetic energy rises. Increasing speed increases drag, reducing resultant force and acceleration. When drag equals weight, resultant force and acceleration are zero and terminal speed is constant. Kinetic energy then stays constant while gravitational potential-energy loss is transferred to thermal stores by drag.
Re-test practice
7. Delayed re-test practice
Return after at least three days and solve these fresh contexts without reopening earlier responses. The recorded plan enforces the delay and uses a separate re-test family for selected-response skill-group evidence.
Falling with air resistance 5(e)
Question 1
A falling object is first moving below its terminal speed and later is briefly moving above a new lower terminal speed after its area increases. Compare weight and drag, acceleration and energy change in both cases.
Check the model response
Below terminal speed, weight exceeds drag, so the resultant and acceleration are downward and kinetic energy rises. Above the new terminal speed, drag exceeds weight, so acceleration is upward while the object still moves downward and kinetic energy falls. In both cases gravitational potential energy decreases and some energy is transferred thermally by drag.