A-Level Physics · guided topic map

Fluids, density and pressure for Cambridge International AS & A Level Physics

Fluids, density and pressure for A-Level Physics, organized into 1 syllabus topic and 4 mapped concept guides.

Syllabus topics: 1
Mapped concept guides: 4
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.

Forces, density and pressure

AS Level foundations

4 guides

01Density and relative densityMapped lesson
02Pressure in solidsMapped lesson
03Hydrostatic pressureMapped lesson
04Upthrust and Archimedes' principleMapped lesson

Diagrams

Fluids, density and pressure 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.

2 Kinematics 3 Dynamics 4 Forces, density and pressure Open A Level question · AS Level foundations Q7

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.
recall2 marks
Check answer 2 marks
1. F is drawn shorter than W, so there is a resultant force downwards
2. 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.
routine3 marks
Check answer 3 marks
1. W = 85 × 9.81 = 8.3 × 10² N
2. resultant force = 834 − 340 = 4.9 × 10² N downwards
3. 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.
demanding4 marks
Check answer 4 marks
1. loss of gravitational potential energy = 85 × 9.81 × 250 = 2.08 × 10⁵ J
2. gain in kinetic energy = ½ × 85 × 50² = 1.06 × 10⁵ J
3. work done against drag = 2.08 × 10⁵ − 1.06 × 10⁵ = 1.02 × 10⁵ J
4. 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.
top of the paper4 marks
Check answer 4 marks
1. the drag is reduced, so F becomes shorter while W is unchanged
2. there is now a resultant downward force, so she accelerates again
3. as her speed rises the drag increases until F is once more equal to W
4. 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