5: Newton’s Laws of Motion
The Golden Gate Bridge, one of the greatest works of modern engineering, was the longest suspension bridge in the world in the year it opened, 1937. It is still among the 10 longest suspension bridges as of this writing. In designing and building a bridge, what physics must we consider? What forces act on the bridge? What forces keep the bridge from falling? How do the towers, cables, and ground interact to maintain stability?
When you drive across a bridge, you expect it to remain stable. You also expect to speed up or slow your car in response to traffic changes. In both cases, you deal with forces. The forces on the bridge are in equilibrium, so it stays in place. In contrast, the force produced by your car engine causes a change in motion. Isaac Newton discovered the laws of motion that describe these situations.
Forces affect every moment of your life. Your body is held to Earth by force and held together by the forces of charged particles. When you open a door, walk down a street, lift your fork, or touch a baby’s face, you are applying forces. Zooming in deeper, your body’s atoms are held together by electrical forces, and the core of the atom, called the nucleus, is held together by the strongest force we know—strong nuclear force.
Dynamics refers to the connection between forces and motion. (It answers the “why” of kinematics’ “what”…) The key concept is force, which is nothing more than any kind of physical push or pull. Force is a vector, it has size and direction. We’ll want to quantify and compare forces: the “standard metric pull” will be called 1 Newton, or 1 N. (We’ll figure out how big this standard should be, soon)
Forces are pushes or pulls.
A force can:
- Make something start to move
- Speed something up that was already moving
- Slow something down
- Bring something to a stop
- Keep something from moving
When you APPLY forces to objects, the objects respond by accelerating. No force at all means NO acceleration at all. This is NOT obvious to many people! Aristotle – and still many people today – think you must apply a net force to KEEP an object moving along steadily. But NO, you don’t. Experimentally, that’s 100% incorrect.
Steady motion means no acceleration: no net force is required!
5.1 Forces
- Distinguish between kinematics and dynamics
- Understand the definition of force
- Identify simple free-body diagrams
- Define the SI unit of force, the newton
- Describe force as a vector
Forces and Motion Simulation
Engage the simulation to predict, qualitatively, how an external force will affect the speed and direction of an object’s motion. Explain the effects with the help of a free-body diagram. Use free-body diagrams to draw position, velocity, acceleration, and force graphs, and vice versa.
5.2 Newton’s First Law
- Describe Newton’s first law of motion
- Recognize friction as an external force
- Define inertia
- Identify inertial reference frames
- Calculate equilibrium for a system
5.3 Newton’s Second Law
- Distinguish between external and internal forces
- Describe Newton’s second law of motion
- Explain the dependence of acceleration on net force and mass
5.4 Mass and Weight
- Explain the difference between mass and weight
- Explain why falling objects on Earth are never truly in free fall
- Describe the concept of weightlessness
5.5 Newton’s Third Law
- State Newton’s third law of motion
- Identify the action and reaction forces in different situations
- Apply Newton’s third law to define systems and solve problems of motion
5.6 Common Forces
- Define normal and tension forces
- Distinguish between real and fictitious forces
- Apply Newton’s laws of motion to solve problems involving a variety of forces
5.7 Drawing Free-Body Diagrams
- Explain the rules for drawing a free-body diagram
- Construct free-body diagrams for different situations
Physics Education Research at UNG 