Unit 5 / Practice Drills
Unit 5 Practice Drills
Kinetics
Newton second law, work-energy, and impact. This unit scores well if you pick the right method family for the question.
Drill 1 - Newton on an Incline
Problem
An 18 kg block slides down a rough plane inclined at 25° to the horizontal. The coefficient of kinetic friction between the block and the plane is 0.20. Find the acceleration of the block.
Figure 1A
Block on a 25° rough incline
Step 1 - Resolve weight parallel and perpendicular to the plane.
Weight component down the plane = 18 × 9.81 × sin 25° = 74.63 N
Normal reaction = 18 × 9.81 × cos 25° = 160.04 N
Step 2 - Find the friction force.
Friction = μN = 0.20 × 160.04 = 32.01 N
Step 3 - Apply F = ma along the plane.
Net force down the plane = 74.63 − 32.01 = 42.62 N
42.62 = 18a
a = 2.37 m/s² down the plane
Worked Diagram Hidden
Reveal solution figure
Open this only after you have attempted the setup yourself.
Worked Diagram Hidden
Reveal solution figure
Open this only after you have attempted the setup yourself.
Figure 1B
Worked incline resolution and net force
Where students lose marks
Do not write friction as μmg. On an incline the friction force is μN, and the normal reaction is mg cos θ, not the full weight.
Drills 2 to 5 stay premium
The incline-dynamics drill stays free as the sampler. The work-energy, impact, rotating-platform, and curvilinear motion drills unlock with the premium pass.