Unit 5: Newton's Laws of Motion
Inertia, force, and action-reaction pairs, turned into free-body diagrams that solve connected-object problems.
6 topics · 20 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 Newton's first law explain why passengers lurch forward when a car suddenly stops?
- Common mistakeWhy isn't a constant force needed just to keep an object moving at a constant velocity?
- Going furtherWhat would happen to a spacecraft's motion in deep space if every one of its engines shut off?
Your turn1 of 2
A spacecraft drifts through deep space, far from any gravity, moving at 19 m/s, when its engines shut off. Ignoring negligible drag, what is its speed 328 seconds later, in meters per second?Mass, Weight, and Newton's Second Law
- How it worksHow does F = ma turn a net force and a mass into a predicted acceleration?
- Worked exampleFind the acceleration of a 1200 kg car pushed by a net force of 3000 N.
- Common mistakeWhy isn't your weight on the Moon the same number as your weight on Earth?
- Going furtherWhat happens to an object's weight, but not its mass, when you carry it from Earth to the Moon?
Your turn1 of 2
A net force of 4330 N acts on a 1368 kg object. What is its acceleration, in meters per second squared?Newton's Third Law
- How it worksHow does every push you make on a wall come paired with an equal push back on your own hand?
- Common mistakeWhy don't action-reaction force pairs cancel out and prevent anything from ever moving?
- Going furtherWhat happens to the reaction force pair when you push on an object much lighter than yourself?
Your turn1 of 2
A 5 kg rifle fires a 0.0147 kg bullet at 859 m/s. Using momentum conservation, the rifle's recoil momentum equals the bullet's forward momentum. What is the rifle's recoil speed, in meters per second?Free-Body Diagrams
- How it worksHow do you decide which forces belong on a free-body diagram and which ones don't?
- Worked exampleDraw the free-body diagram for a book resting on a table and find the normal force if its weight is 20 N.
- Common mistakeWhy doesn't the force of your hand pushing a book stay on that book's free-body diagram once you let go?
Your turn1 of 2
A book of weight 29 N rests motionless on a table. What is the magnitude of the normal force the table exerts on it, in newtons?Solving Newton's Second Law Problems
- How it worksHow do you write a separate F = ma equation for each object in a connected system and solve them together?
- Worked exampleFind the acceleration and tension for a 5 kg and a 3 kg block connected by a string over a frictionless pulley.
- Worked exampleFind the tension in a rope pulling a 10 kg sled at constant velocity across a frictionless surface.
- Going furtherWhat happens to the acceleration of an Atwood machine's two masses as one mass grows much larger than the other?
Your turn1 of 2
Two blocks, 3 kg and 8 kg, are connected by a light string over a frictionless, massless pulley (an Atwood machine), with the 8 kg block hanging lower. Taking g = 9.8 m/s2, what is the magnitude of their common acceleration, in meters per second squared?Newton's Laws on an Incline
- How it worksHow do you rotate the coordinate axes so gravity splits into components along and perpendicular to a ramp?
- Worked exampleFind the acceleration of a block sliding down a frictionless 30 degree incline.
- Common mistakeWhy isn't the normal force on an incline equal to the full weight of the object sitting on it?
Your turn1 of 2
A block slides down a frictionless incline tilted at angle theta above the horizontal, with gravitational acceleration g. Write an expression, in terms of g and theta, for its acceleration down the slope.