# Newton's Laws and Forces

Computer Science I · Physics · https://tryals.app/learn/computer-science-i/newtons-laws-and-forces

## Why Things Move

Kinematics describes motion; **dynamics** explains it using **forces** (pushes or pulls, which are vectors). **Newton's three laws** form the foundation of mechanics:

| Law | Definition | Core Meaning |
| --- | --- | --- |
| **First law** | Object stays at rest or in uniform motion unless acted on by a net force | Motion needs no force to continue; only a **change** in motion does |
| **Second law** | **$F = ma$** (Net force equals mass times acceleration) | Acceleration is proportional to net force and inversely to mass |
| **Third law** | For every action, an equal and opposite reaction | Forces come in pairs acting on **different** objects; they never cancel |

The **net force** is the vector sum of all forces ($mg$, tension, friction, normal force). An object in **equilibrium** has zero net force and zero acceleration: it is at rest **or** moving at constant velocity.

## Forces in Action

The quantitative heart of mechanics is **$F_{net} = ma$**. Given the forces, you find the acceleration, and thereby the full motion. 

Consider a block driven by an unbalanced **net force**. The resulting acceleration $a$ is dictated directly by **$F_{net} = ma$**, while balanced opposing forces result in **equilibrium**.

> **Common Pitfalls:**
> - Thinking a force is needed to **keep** an object moving. Uniform motion needs **no** net force; a force is only required to **change** velocity (overturning the everyday illusion of friction).
> - Misapplying the **third law**. Action-reaction pairs act on **two different objects**, so they never cancel. Forces that cancel to create equilibrium are different forces acting on the **same** object.

## Practice questions

4 of this lesson's 9 practice questions, with answers. The full set is in the app.

### 1. A rocket accelerates in vacuum with nothing to push against. Which conservation law makes that unmysterious?

A. Conservation of energy, since chemical energy becomes kinetic energy
B. Conservation of mass, since the rocket loses mass as it burns fuel
C. Conservation of momentum, which needs no external medium
D. None, rockets push against residual gas present even in space

**Answer:** C. Conservation of momentum, which needs no external medium

**Why:** The system is rocket plus propellant, and its total momentum is conserved, throw mass backwards and the remainder moves forward. Nothing external is needed, which is why exhaust velocity matters so much and why the 1920 objection to Goddard was simply a misreading of the third law.

Page: https://tryals.app/practice/computer-science-i/newtons-laws-and-forces/a-rocket-accelerates-in-vacuum-with-nothing-to-push-against-which

### 2. Select every situation in which the net force on the object is ZERO.

A. A ball momentarily motionless at the peak of its arc
B. A puck gliding at constant velocity on frictionless ice
C. A car speeding up from a standstill
D. A book resting on a table
E. A parachutist whose drag has grown to match their weight

**Answer:** B. A puck gliding at constant velocity on frictionless ice; D. A book resting on a table; E. A parachutist whose drag has grown to match their weight

**Why:** Constant velocity and rest both mean zero acceleration and so zero net force. A ball at the top of its flight is the trap: it is momentarily motionless but gravity is still pulling on it, so the net force is its full weight.

Page: https://tryals.app/practice/computer-science-i/newtons-laws-and-forces/select-every-situation-in-which-the-net-force-on-the-object-is-zero

### 3. A book rests on a table and a puck glides at constant velocity across frictionless ice. Newton's first law treats these as the same case. Why?

A. Because the puck is decelerating imperceptibly, so both are effectively at rest.
B. Because the first law only applies to objects at rest, and the puck is an approximation to that case.
C. Because both objects are in contact with a surface, which is what determines the applicable law.
D. Because both have zero net force, and rest is not a special state.

**Answer:** D. Because both have zero net force, and rest is not a special state.

**Why:** The first law makes no distinction between rest and uniform motion, rest is the case where the constant velocity is zero. That is exactly what Aristotle got wrong: the intuition that motion needs a continuous cause comes from living in a world full of friction. It also implies no experiment distinguishes rest from constant velocity, which is Galilean relativity.

Page: https://tryals.app/practice/computer-science-i/newtons-laws-and-forces/a-book-rests-on-a-table-and-a-puck-glides-at-constant-velocity-across

### 4. Since Newton's third law says the ball pushes back on the bat with equal force, the two undergo equal accelerations.

**Answer:** False

**Why:** False. The forces are equal and opposite, but $a = F/m$, so the far lighter ball accelerates enormously while the bat barely changes speed. The pair acts on *different bodies*, which is why they never cancel, the common error is treating an action-reaction pair as a balanced pair on one object.

Page: https://tryals.app/practice/computer-science-i/newtons-laws-and-forces/since-newtons-third-law-says-the-ball-pushes-back-on-the-bat-with