# Newton's Laws of Motion in Everyday Situations

This topic is part of the HSC Physics course under the section Forces.

### HSC Physics Syllabus

• using Newton’s Laws of Motion, describe static and dynamic interactions between two or more objects and the changes that result from:

– a contact force
– a force mediated by fields
• apply Newton’s first two laws of motion to a variety of everyday situations, including both static and dynamic examples, and include the role played by friction f_{friction}=\mu F_N (ACSPH063)

### Newton's First Law of Motion: The Law of Inertia

Seatbelts and airbags: When a car suddenly stops, the passengers tend to continue moving forward due to their inertia. Seatbelts and airbags are designed to counteract this by providing an external force to gradually slow down the passengers, reducing the risk of injury.

Sports: In sports like soccer, players must overcome the ball's inertia to make it move. Additionally, when a player stops running suddenly, their body will want to continue moving forward due to inertia, which is why they need to decelerate gradually to maintain balance.

### Newton's Second Law of Motion: The Law of Acceleration

Rocket propulsion: Engineers must consider the mass of a rocket and the force required to achieve a specific acceleration when designing rocket propulsion systems. By increasing the force (thrust) generated by the rocket's engines, they can overcome the force of gravity and launch the rocket into space.

Elevators: Elevator systems apply Newton's second law by using a counterweight to balance the force required to lift the elevator car. By adjusting the counterweight, the motor can provide the necessary force to accelerate the elevator car up or down at a constant speed, ensuring a comfortable ride for passengers.

### Newton's Third Law of Motion: Action and Reaction

Swimming: When a swimmer pushes against the water with their arms and legs, the water exerts an equal and opposite force on the swimmer, propelling them forward. This application of Newton's third law allows swimmers to move efficiently through the water.

Space travel: In space, there is no friction or air resistance, making conventional propulsion methods ineffective. Instead, spacecraft use Newton's third law by expelling propellant in one direction, causing the spacecraft to move in the opposite direction. This principle, known as "action-reaction," is the basis of rocket propulsion in space.