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2.5Force and Translational Dynamics

Newton’s Second Law — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 36 1 quiz

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Lesson summary

2.5 Newton's Second Law — AP PHYSICS 1 notes: Apply a = Fnet/m to find acceleration in one and two dimensions, including inclined planes.Newton's second law — the acceleration of a system's center of mass is proportional to the net force and in the same directionEquation — asys =…

  • Velocity change
  • Independent axes
  • Net external force
  • Acceleration is inversely proportional to the mass of the system
  • Define the system and draw the free-body diagram
  • Choose a positive direction

Part 1 of 6Statement

Newton's second law
  • Newton's second law the acceleration of a system's center of mass is proportional to the net force and in the same direction
Equation
  • Equation asys = Fnet/msys
  • Acceleration is directly proportional to net external force.
  • Acceleration is inversely proportional to the mass of the system.
Velocity change
  • Velocity change the velocity of the center of mass changes only if a net external force acts

Part 2 of 6Using the law

1
Define the system and draw the free-body diagram.
2
Choose a positive direction.
3
Add the forces with signs to find Fnet.
4
Divide by the system mass to find a.

Part 3 of 6Pattern: rocket

Forces
  • Forces thrust +15 MN, weight −8 MN, air resistance −0.5 MN
Net force
  • 6.5 MN upward
Acceleration
  • 6.5 × 106/(0.8 × 105) = 81 m s-2 upward

Part 4 of 6Two dimensions

Independent axes
  • Independent axes forces may be balanced in one direction and unbalanced in another
Velocity change
  • Velocity change occurs only in the direction of the unbalanced force
Inclined plane
  • Inclined plane balanced perpendicular to the slope, unbalanced along it

Part 5 of 6Pattern: water drop on a windscreen

Angle
  • 30° with weight 8.0 × 10-5 N
Net force
  • Fg sin 30° = 4.0 × 10-5 N along the slope
Acceleration
  • Acceleration a = 4.0 × 10-5/(8.0 × 10-6) = 5 m s-2

Part 6 of 6More from the specification

1Gravitational weight near surface of Earth
  • Gravitational weight near surface of Earth accelerating downward toward the centre of Earth at 9.81 m/s² defines weight as mass times g.
2Resultant
  • Vector sum of two or more vectors.
3Inertia
  • Resistance of an object to changes in its motion.
4Parallel
  • Components on separate branches, each with the full e.m.f.
5Resultant force
  • Another name for net force.
6Reference frame
  • Coordinate system used by an observer.
7Contact force
  • Force requiring touching objects.
8Driving force
  • Force applied by an engine or motor to produce motion.
9Constant
  • Not changing.
10Normal
  • Line at 90° to a surface.

Key terms

TermMeaning
Newton's second lawFnet = ma.
Net external forceSum of forces from outside the system.
Center of massPoint whose acceleration describes the system.
NewtonUnit of force, kg m s-2.

Avoid these mistakes

1
Using weight instead of net force.
2
Using mass in the wrong units (kg required).
3
Mixing the perpendicular and parallel components on an incline.
4
Forgetting that acceleration is in the direction of the net force.

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