Unit 4: Newton's Laws, Friction, and Inclines
The three rules that connect force to motion, why surfaces drag on each other, and what a tilted ramp does to both.
9 topics · 34 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Newton's First Law and Inertia
- How it worksHow does an object's inertia keep it moving in a straight line unless something pushes or pulls it off course?
- Common mistakeWhy isn't a constant force needed just to keep an object moving at a steady velocity?
- Common mistakeWhy doesn't a sliding hockey puck eventually gliding to a stop on grass disprove Newton's first law?
- Going furtherWhat happens to a hockey puck sliding on frictionless ice with no forces acting on it at all?
Your turn1 of 2
A car slamming on its brakes suddenly decelerates, yet the passengers inside continue lurching forward toward the windshield for a moment before their seatbelts stop them. Why does your body lurch forward the instant a car slams on its brakes?Newton's Second Law
- How it worksHow does net force equal mass times acceleration link every force problem to one equation?
- Worked exampleFind the acceleration of a 10 kg cart pushed with a net force of 30 N.
- Worked exampleFind the net force needed to accelerate a 1200 kg car at 3 m/s2.
- Common mistakeWhy doesn't a bigger force always produce a bigger acceleration all on its own?
- Going furtherWhat happens to the force required when the same acceleration is demanded of triple the mass?
Your turn1 of 2
Find the acceleration of a 34.97 kg cart pushed with a net force of 195 N.Mass and Weight
- How it worksHow does weight come from gravity pulling on mass, while mass itself never changes?
- Worked exampleFind the weight of a 70 kg astronaut on Earth and on the Moon, where gravity is 1.6 m/s2.
- Common mistakeWhy isn't mass and weight the same quantity just measured in different units?
Your turn1 of 2
Find the weight of a 96 kg astronaut on a world where gravitational acceleration is 16.4 m/s2.Newton's Third Law
- How it worksHow does every force come paired with an equal and opposite force acting on a different object?
- Common mistakeWhy don't the two forces in a Newton's third law pair cancel each other out?
- Going furtherWhat happens to the size of the reaction force when a small car crashes head-on into a large truck?
Your turn1 of 2
A swimmer pushes backward against a pool wall with their feet and immediately glides forward away from the wall, rather than remaining stuck against it despite pushing on it. Why doesn't a swimmer push off the pool wall and just stay stuck to it?Free-Body Diagrams
- How it worksHow do you build a free-body diagram for a book resting still on a table, one force at a time?
- Worked exampleFind the normal force on a 5 kg box sitting at rest on a flat table.
- Worked exampleFind the normal force on a 5 kg box when an extra 10 N force pushes down on it at 30 degrees below horizontal.
- Common mistakeWhy isn't the normal force always equal to an object's weight?
Your turn1 of 2
Find the normal force on a 70 kg box sitting at rest on a flat table.Static and Kinetic Friction
- How it worksHow does friction come from the microscopic roughness of two surfaces catching on each other?
- Worked exampleFind the friction force on a 20 kg crate with a coefficient of kinetic friction of 0.3.
- Common mistakeWhy doesn't friction depend on how much surface area is touching?
- Going furtherWhat happens to the friction force the instant an object stops sliding and comes to rest?
Your turn1 of 2
Find the friction force on a 105 kg crate with a coefficient of kinetic friction of 0.602.Forces on an Incline
- How it worksHow does tilting the coordinate axes split gravity into a slice that pulls down the slope and a slice that presses into it?
- Worked exampleFind the acceleration of a block sliding down a frictionless 30 degree incline.
- Worked exampleFind the minimum angle at which a block with a friction coefficient of 0.4 begins to slide down a ramp.
- Common mistakeWhy isn't the normal force on an incline equal to an object's full weight?
- Going furtherWhat happens to the component of gravity pulling an object down the slope as the incline gets steeper?
Your turn1 of 2
Find the acceleration of a block sliding down a frictionless 57 degree incline.Tension and Connected Objects
- How it worksHow does tension transmit a pull through a rope connecting two objects moving together?
- Worked exampleFind the tension in a rope pulling two boxes of 4 kg and 6 kg across a floor with 20 N of force.
- Going furtherWhat happens to the tension in the rope between two connected blocks when the trailing block is removed?
Your turn1 of 2
A 141 N force pulls two connected boxes of mass 26 kg, leading, and 37 kg, trailing, across a frictionless floor. Find the tension in the rope connecting them, the force accelerating the trailing box.Equilibrium and Net Force
- How it worksHow does an object in equilibrium end up with a net force of exactly zero even with several forces acting on it?
- Worked exampleFind the tension in each of two ropes holding a 50 N sign, with each rope at 30 degrees from horizontal.
- Common mistakeWhy doesn't equilibrium mean that no forces at all are acting on an object?
Your turn1 of 2
Find the tension in each of two ropes holding a 282 N sign, with each rope at 55 degrees from horizontal in a symmetric setup.