Newton's 2nd Law
Newton's 1st law of motion told us why objects accelerate (or don't accelerate) due to a net force. But how do we calculate the actual value of the acceleration? And how exactly is it related to the net force? That's where we need Newton's 2nd law of motion.
The famous equation that we get from Newton's 2nd Law, ΣF = ma, is the foundation for this unit and a lot of physics. It also takes care of Newton's 1st law of motion conceptually: if there's no net force then there's no acceleration, so an object at rest will remain at rest and an object in motion will have a constant velocity.
We're going to start with a conceptual understanding of Newton's 2nd law and introduce a new concept: mass. An object's mass is a measure of its inertia, so objects with more mass have more inertia and are more resistant to a change in their state of motion. Think about pushing an empty grocery cart and a full grocery cart - which one is harder to move and turn around corners?
Then we'll learn how to draw a free body diagram and apply Newton's 2nd law equation in 1D and 2D. Just like we learned in kinematics, the x and y directions are independent, so we can write ΣF = ma for the x direction and the y direction to get two equations. Then we can use those equations to solve for any value we want to find.
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Intro to Newton's 2nd law of motion
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Net force, mass and acceleration
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Calculate the acceleration and net force
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3 key points about Newton's 2nd law of motion
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Proportional reasoning: how changing one variable affects the others
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How Newton's 2nd law replaces Newton's 1st law
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Newton's 2nd law in 2 dimensions
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How to find the x and y acceleration components
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Combining the net force and acceleration components
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Why do objects move in the x or y direction?
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Example problem 1: multiple forces in each direction
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Example problem 2: working with forces at an angle
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Steps for drawing a FBD and applying Newton's 2nd law
Answers
Free-Response Questions
Forces
- 2024 Q1 - - Block sliding on a track with loops, forces, FBDs, circular motion, energy
- 2024 Q3 - - Beam attached to a wall with a string, forces, FBDs, tension, rotational motion, torque
- 2024 Q5 - - Collision of two blocks, momentum, energy, forces
- 2022 Q1 - - Pulley system with 2 blocks and a spring, kinematics, forces, energy, work
- 2019 Q1 - - Plunger pushes block and sphere across surface, kinematics, forces, energy, work, torque, angular momentum
- 2019 Q2 - - Pulley system with 2 blocks, kinematics, forces, FBDs, tension, rotational motion
- 2018 Q1 - - Spacecraft in circular orbit, circular motion, forces, FBDs, law of gravitation
- 2017 Q2 - - (Experimental design) Coefficient of friction between block and board, forces, friction
- 2016 Q1 - - Wheel rolls down an incline, forces, FBDs, energy, friction
- 2015 Q1 - - Pulley system with 2 hanging blocks, forces, FBDs, tension
Free body diagrams
- 2024 Q1 - - Block sliding on a track with loops, forces, FBDs, circular motion, energy
- 2024 Q3 - - Beam attached to a wall with a string, forces, FBDs, tension, rotational motion, torque
- 2023 Q3 - - Block and spring rotating about axle, circular motion, centripetal force, FBDs
- 2022 Q2 - - Gravitational force between planet and moons, law of gravitation, FBD's
- 2019 Q2 - - Pulley system with 2 blocks, kinematics, forces, FBDs, tension, rotational motion
- 2018 Q1 - - Spacecraft in circular orbit, circular motion, forces, FBDs, law of gravitation
- 2016 Q1 - - Wheel rolls down an incline, forces, FBDs, energy, friction
- 2015 Q1 - - Pulley system with 2 hanging blocks, forces, FBDs, tension
- 2015 Q4 - - Projectile motion of two spheres, projectile motion, FBDs
- Newton's 2nd Law of Motion
- Khan Academy - Newton's 2nd law of motion
- Professor Dave - Newton's Second Law of Motion: F = ma
- Khan Academy - Newton's 2nd law in two dimensions
- Free body diagrams
- Free Body Diagram... What is it?
- Bozeman Science - Free-Body Diagrams
- Khan Academy - Inclined plane force components
- Khan Academy - Free body diagram with angled forces
Newton's 1st law of motion told us why objects accelerate (or don't accelerate) due to a net force. But how do we calculate the actual value of the acceleration? And how exactly is it related to the net force? That's where we need Newton's 2nd law of motion.
The famous equation that we get from Newton's 2nd Law, ΣF = ma, is the foundation for this unit and a lot of physics. It also takes care of Newton's 1st law of motion conceptually: if there's no net force then there's no acceleration, so an object at rest will remain at rest and an object in motion will have a constant velocity.
We're going to start with a conceptual understanding of Newton's 2nd law and introduce a new concept: mass. An object's mass is a measure of its inertia, so objects with more mass have more inertia and are more resistant to a change in their state of motion. Think about pushing an empty grocery cart and a full grocery cart - which one is harder to move and turn around corners?
Then we'll learn how to draw a free body diagram and apply Newton's 2nd law equation in 1D and 2D. Just like we learned in kinematics, the x and y directions are independent, so we can write ΣF = ma for the x direction and the y direction to get two equations. Then we can use those equations to solve for any value we want to find.
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