A-Level Physics · guided topic map
Forces and momentum for Cambridge International AS & A Level Physics
Forces and momentum for A-Level Physics, organized into 2 syllabus topics and 2 mapped concept guides.
- Syllabus topics
- 2
- Mapped concept guides
- 2
- Educational level
- Cambridge International AS & A Level
Syllabus to lesson
Choose the exact concept
Work in order or jump to the concept named in your specification, course outline, or assignment.
3Dynamics
AS Level foundations
1 guide+
Dynamics
AS Level foundations
- 01Free-body diagrams and net-force modelsMapped lesson
4Forces, density and pressure
AS Level foundations
1 guide+
Forces, density and pressure
AS Level foundations
- 01Equilibrium and free-body diagramsMapped lesson
Diagrams
Forces and momentum as A-Level Physics draws it
The figures from the A-Level Physics practice papers that sit on these syllabus points — the apparatus, circuits and graphs an exam question actually puts in front of you.
01Fig. 7.1Kinematics · DynamicsA Level
Figure comment
Fig. 7.1The falling skydiver is drawn as a block with a dot at her centre of mass, labelled as having a total mass of 85 kg. A long arrow labelled W starts at that dot and points vertically downwards. A shorter arrow labelled F starts at her upper surface and points vertically upwards. To one side, a separate arrow labelled v points downwards to show her direction of motion.
Read the comment once, then trace every arrow, label, axis or component in the drawing before opening the questions.
Guided questions 5 parts
Reading cue. W is drawn from the dot at her centre of mass and F from the surface meeting the air; v is a velocity, not a third force, and W is drawn longer than F for a reason.
aDeduce Deduce from the relative lengths of the arrows W and F in Fig. 7.1 whether the skydiver has yet reached terminal velocity.
Check answer 2 marks
- F is drawn shorter than W, so there is a resultant force downwards
- she is therefore still accelerating and has not reached terminal velocity
bCalculate At the instant drawn, the drag force F is 3.4 × 10² N. Calculate the weight of the skydiver and her acceleration at that instant.
Check answer 3 marks
- W = 85 × 9.81 = 8.3 × 10² N
- resultant force = 834 − 340 = 4.9 × 10² N downwards
- a = 494 / 85 = 5.8 m s⁻² downwards
cDetermine The skydiver falls 250 m from rest and is then moving at 50 m s⁻¹. Determine the average drag force acting on her over that fall.
Check answer 4 marks
- loss of gravitational potential energy = 85 × 9.81 × 250 = 2.08 × 10⁵ J
- gain in kinetic energy = ½ × 85 × 50² = 1.06 × 10⁵ J
- work done against drag = 2.08 × 10⁵ − 1.06 × 10⁵ = 1.02 × 10⁵ J
- average drag force = 1.02 × 10⁵ / 250 = 4.1 × 10² N
dExplain The skydiver is falling at constant velocity when she turns into a head-down dive, presenting a much smaller area to the airflow. Explain how the two arrows of Fig. 7.1 change, and describe her subsequent motion.
Check answer 4 marks
- the drag is reduced, so F becomes shorter while W is unchanged
- there is now a resultant downward force, so she accelerates again
- as her speed rises the drag increases until F is once more equal to W
- she then falls at constant velocity at a higher terminal speed than before
Transfer challenge
A steel ball is released at the surface of a tall jar of oil and, after a short distance, falls at constant speed. Describe how the forces on the ball change from release until it moves at constant speed, and state the resultant force on it while the speed is constant.
Check answer 3 marks
- at release the drag is zero, so the resultant is weight minus upthrust and the acceleration is a maximum
- as the speed increases the viscous drag increases, so the resultant force and the acceleration both decrease
- when drag + upthrust = weight the resultant force is zero and the speed stays constant