(455) Forces and Newton’s laws [IB Physics SL/HL]

Introduction to Forces and Newton's Laws

• Importance of understanding forces and Newton's laws for exams

• Fun phrase: "May the mass times acceleration be with you" referring to Force (F = ma)

Free Body Diagrams

• Free body diagrams help visualize forces acting on an object

• Calculate net force (F_net) by summing all vectors

• Equal and opposite forces result in F_net = 0, leading to no acceleration

• Motion can occur with no acceleration (constant speed)

Examples of Forces

• Example of two forces:

  • 2 Newton opposing forces result in zero net force.

  • Forces can also cancel in different directions (e.g., friction vs. thrust in a car)

• Concept of equilibrium:

  • Resultant force equals zero.

  • Example: a plane where lift equals gravity and thrust equals drag

Newton's Laws

First Law (Law of Inertia)

• An object at rest stays at rest, and an object in motion stays in motion unless acted on by an unbalanced force

• Example of inertia: cyclist falling off after hitting an obstacle

• Demonstration with a ketchup bottle to illustrate inertia and forces

Second Law (F = ma)

• Key formula: Force (F) equals mass (m) times acceleration (a)

  • Forces are vectors; analyzing changes in momentum also important

• Example scenario: 52N force from a velociraptor versus 12N friction

  • Net force calculation: 52N - 12N = 40N

  • Acceleration calculation given mass (e.g., 75 kg) results in 0.533 m/s²

Third Law (Action and Reaction)

• For every action, there is an equal and opposite reaction

• Examples:

  • Pushing against a wall moving oneself in the opposite direction.

  • Jumping involves pushing down against the ground, resulting in an upward reaction.

  • Rockets utilizing gas ejection to propel in space.

Conclusion

• Understanding Newton's laws is essential for physics applications and problem-solving in real-world scenarios.