McGill University · AEPH 113

McGill AEPH 113 tutoring

Physics 1: send your chapter so we can check whether we can help before you book.

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Course overview

AEPH 113 at Macdonald Campus covers calculus-based mechanics as well as oscillations, waves and sound. Start below by choosing a system and identifying its external forces. Send your chapter or a question, even if you haven’t started it, so we can confirm support before booking.

Restricted to Bioresource Engineering students.

AEPH 113 topics to review

Kinematics and Newton’s lawsCircular motion and rigid-body rotationMomentum, work, energy and conservationSimple harmonic motionWaves and sound

Common difficulties

Drawing useful free-body diagrams
Choosing axes and vector components
Selecting a conservation law or equation of motion

Choose the system before cancelling forces

Two skaters push against each other on level ice. Model them as particles and neglect horizontal ice resistance. At one instant, skater A, on the left, has mass $m_A=50$ kg and skater B, on the right, has mass $m_B=75$ kg. The contact force has magnitude $C=150$ N. Take right as positive. Find each horizontal acceleration and explain why the net horizontal force on the two-skater system is zero even while they accelerate.

Hint: draw one boundary at a time

A receives a leftward contact force; B receives a rightward contact force. These form a third-law pair on different bodies. Weight and the ice’s support balance vertically for each skater in this model.

Write separate equations, then combine

For A alone, contact from B is external and $-C=m_Aa_A$ , giving $a_A=-150/50=-3$ m/s². For B alone, $+C=m_Ba_B$ , giving $a_B=150/75=2$ m/s². The opposite forces are equal; the acceleration magnitudes need not be equal because the masses differ.

For the system containing A and B, their mutual contact is internal. Adding the equations gives

m_Aa_A+m_Ba_B=-150+150=0.

This says the system’s total horizontal momentum has zero rate of change. It does not say either skater has zero acceleration or that the skaters share one acceleration. Do not cancel the contact force on an individual skater’s diagram.

Retry: change the mass, boundary and external force

In a new trial, A is still 50 kg but B is 100 kg. At the instant when their mutual contact force is 150 N, an external horizontal force of 50 N also acts leftward on B. Use B alone as your first system: which horizontal forces are external, and what is $a_B$ ? Then use A and B together: what remains external?

Check both system choices

For B alone, contact contributes $+150$ N and the additional force contributes $-50$ N. Both are external to B, so $a_B=(150-50)/100=1$ m/s². For A, $a_A=-150/50=-3$ m/s². For the pair, contact is internal but the added force remains external. Check: $50(-3)+100(1)=-50$ N, exactly the external resultant. The system’s total horizontal momentum now changes.

Continue with choosing a system for connected objects. That lesson also explains when geometry justifies shared acceleration; the separating skaters here do not have that constraint. Oscillations and waves remain other parts of the AEPH 113 course.

How tutoring can help

Send the question, with or without a force diagram. Your tutor can help you choose the system and begin the diagram. If support is confirmed, your tutor can check which object receives each force, what becomes internal when the boundary changes and whether any shared-acceleration assumption is justified. Then change the system and account for each remaining external force.

Exam preparation for AEPH 113

For each mechanics question, choose the system and identify its external forces before choosing a principle. Redo the skater retry with one skater, then both; explain whether total momentum changes. Mix in the rotation, oscillation and wave topics assigned for your exam, checking units and any conservation assumptions.

Use your section’s outline to choose the chapters to review. We work through permitted practice and comparable problems, without completing graded assignments or exams on your behalf.

Need help with AEPH 113?

Send your topic and preferred times. We’ll confirm whether we can help and, if so, agree on a time by text or email before payment.

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AEPH 113 · official course reference