Overview
This lecture covers Newton's First Law of Motion, how to identify related exam questions, the formal definition required for tests, related safety considerations, and a step-by-step example calculation.
Identifying Newton's First Law Questions
- Look for phrases like "constant/uniform velocity or speed," "stationary," "still," "not moving," or "equilibrium."
- Newton's First Law applies when acceleration is zero and the object is either at rest or moving at constant velocity.
- Start such questions by stating: ( F_{\text{net}} = ma ); since ( a = 0 ), then ( F_{\text{net}} = 0 ).
Newton's First Law: Definition and Explanation
- Newton's First Law: A body remains at rest or in uniform motion unless a nonzero resultant/net force acts on it.
- Both at-rest (stationary) and constant velocity cases are included.
- The object changes motion only if a nonzero net/resultant force is applied.
Exam Tips for Definitions
- Full marks require stating "nonzero," "resultant," or "net force" in the definition.
- Always use the official exam guideline wording for definitions.
Application: Safety and Inertia
- Inertia is the property causing a body to resist changes in its state of motion and is proportional to mass.
- In a moving car, without a seatbelt, when the car stops suddenly, you continue moving forward due to inertia.
- Seatbelts provide the nonzero net force needed to change your motion and keep you safe.
Worked Example: Sliding a Box at Constant Velocity
- Example: Find the force required to slide a 100 kg box at constant velocity with a coefficient of kinetic friction 0.2.
- Draw a free body diagram: show normal force (( F_N )), gravity (( F_g )), applied force (( F_{\text{applied}} )), and kinetic friction (( F_k )).
- At constant velocity: ( F_{\text{net}} = 0 ) in the direction of motion; so ( F_{\text{applied}} + F_k = 0 ).
- Use vector addition; assign positive direction (e.g., right) and substitute negative for friction (left).
- Calculate normal force: ( F_N = F_g = m \times g = 100 \times 9.8 = 980,N ).
- Calculate kinetic friction: ( F_k = \mu_k \times F_N = 0.2 \times 980 = 196,N ).
- Thus, ( F_{\text{applied}} = 196,N ) rightward to maintain constant velocity.
Key Terms & Definitions
- Newton's First Law — An object remains at rest or in uniform motion unless acted upon by a nonzero net/resultant force.
- Inertia — The property of an object to resist changes in its state of motion; depends on mass.
- Equilibrium — State where the net force on an object is zero.
- Kinetic Friction (( F_k )) — The force opposing the motion of two surfaces sliding past each other; calculated as ( F_k = \mu_k F_N ).
- Normal Force (( F_N )) — The perpendicular force exerted by a surface on an object.
Action Items / Next Steps
- Practice identifying Newton's First Law scenarios in exam questions.
- Memorize the exact exam guideline definition of Newton's First Law, including the required keywords.
- Draw free body diagrams to assist in solving force problems.
- Review more examples and past paper questions on Newton's Laws.