UnofficialAP Physics 1Unit 2 of 8

AP Physics 1 Unit 2: Force and Translational Dynamics

Newton's laws, friction and circular motion.

Unit overview

Dynamics explains why motion changes. Newton's second law, $\sum \vec{F} = m\vec{a}$ , links the net force on a system to its acceleration. Every analysis starts with a free-body diagram that shows each force acting on the object, drawn from the point where it acts and labelled by type: gravitational, normal, tension, friction or applied.

Newton's third law says forces come in pairs acting on different objects, so they never cancel on a single free-body diagram. Friction opposes relative sliding, with kinetic friction $f_k = \mu_k F_N$ and static friction up to $\mu_s F_N$ . Circular motion is dynamics too: a net inward force of $mv^2/r$ is needed to keep an object on a circle.

Key topics

Free-body diagrams: Every force on one system, with no components or net force drawn on the diagram itself.
Newton's second law: Choose axes, sum forces along each, set equal to $ma$ .
Friction and inclines: Normal force on an incline is $mg\cos\theta$ when nothing else pushes perpendicular to the surface.
Circular motion and gravitation: Centripetal acceleration $v^2/r$ and $F_g = Gm_1m_2/r^2$ .

Worked example

A 5 kg box is pulled across a level floor by a 30 N horizontal force. The kinetic friction coefficient is 0.4 and $g = 10$ m/s². Find the acceleration.

Normal force: $F_N = mg = 50$ N.
Friction: $f_k = 0.4 \cdot 50 = 20$ N.
Net force: $30 - 20 = 10$ N, so $a = 10/5 = 2$ m/s².

Answer: $a = 2$ m/s²

Common mistakes

Adding a separate centripetal force to the free-body diagram; it is the net inward result of real forces.
Assuming the normal force always equals $mg$ , even on inclines or with vertical pulls.
Placing third-law pairs on the same object and cancelling them.

Exam tips

Justify answers by naming the law you are using and the system you chose.
For connected objects, try treating them as one system first to find the acceleration, then isolate one object for tension.

See the AP Physics 1 exam format and every unit on the course page.