Why this unit matters
Dynamics explains why motion changes, not just how it looks, and supports quantitative problem solving across physics.
What you will learn
- Draw complete free-body diagrams with labeled interaction forces.
- Apply Newton's laws to single objects and connected systems.
- Analyze friction, tension, normal force, and inclined-plane situations with justified assumptions.
Understand the core ideas
Translational dynamics explains motion changes by connecting forces to acceleration. The most important representation is the free-body diagram, which includes only forces acting on the chosen object. Typical forces include weight, normal force, tension, applied pushes or pulls, and friction. Newton's first law describes constant velocity when net external force is zero. Newton's second law, F_net = ma, sets both direction and magnitude of acceleration from the vector sum of forces.
Equation setup depends on clean coordinate choices. On flat surfaces, horizontal and vertical axes are convenient. On inclines, axes parallel and perpendicular to the slope often reduce algebra and make normal force easier to identify. Static friction adjusts up to a maximum to prevent slipping, while kinetic friction has approximately constant magnitude opposite motion once sliding occurs. Newton's third law pairs are equal and opposite interaction forces on different objects, so they never cancel within one object's free-body diagram.
A reliable method is: isolate object, draw forces, choose signs, write component equations, then solve and unit-check. This sequence prevents most sign mistakes and misidentified forces in multi-object systems.
Key terms
- free-body diagram
- Diagram showing all external forces acting on one selected object.
- net force
- Vector sum of all external forces on an object.
- normal force
- Contact force exerted by a surface perpendicular to the surface.
- Newton's third law
- Interaction forces come in equal-magnitude, opposite-direction pairs on different objects.
Net force and acceleration with friction
A 5.0 kg block on a horizontal floor is pulled right by 30 N. Kinetic friction is 8.0 N left. Take right as positive.
- Draw the free-body diagram: weight down, normal up, pull right, friction left.
- Write horizontal net force: F_net,x = 30 N - 8.0 N = +22 N.
- Apply Newton's second law: a_x = F_net,x/m = 22 N / 5.0 kg .
- If the block starts from rest and the force acts for 3.0 s, use v = v0 + at = 0 + (4.4)(3.0) = +13.2 m/s.
- Check units and sign: N/kg gives , and positive values indicate motion and acceleration to the right.
A common misconception
Claim: Action-reaction forces cancel each other on the same free-body diagram.
Correction: Third-law pairs act on different objects. A single-object diagram includes only forces on that object.
Lessons in this unit
- Interactions and free-body diagramsTranslate physical scenarios into force representations for analysis.
- Newton's first and second lawsConnect net force to acceleration direction and magnitude.
- Newton's third law pairsIdentify equal-and-opposite interaction forces on different objects.
- Dynamics of connected systemsSet up equations for multi-object systems including friction and tension.
Study task
Unit checkpoint
A 4.0 kg object has a net horizontal force of 12 N. What is its acceleration?
From Fnet = ma, a = Fnet.