EJC Physics H203 Dynamics 2023_1.Notes (FULL)

Content • Newton’s Law of motion • Linear momentum and its conservation Learning Outcomes Candidates should be able to: (a) State each of Newton’s laws of motion (b) Show an understanding that mass is the property of a body which resists change in motion (inertia) (c) Describe and use the concept of weight as the effect of a gravitational field on a mass (d) Define and use linear momentum as the product of mass and velocity (e) Define and use impulse as the product of force and time of impact (f) Relate resultant force to the rate of change of momentum (g) Recall and solve problems using the relationship F = ma, appreciating that resultant force and acceleration are always in the same direction (h) State the principle of conservation of momentum (i) Apply the principle of conservation of momentum to solve simple problems including inelastic and (perfectly) elastic interactions between two bodies in one dimension (knowledge of the concept of coefficient of restitution is not required) (j) Show an understanding that, for a (perfectly) elastic collision between two bodies, the relative speed of approach is equal to the relative speed of separation (k) Show an understanding that, whilst the momentum of a closed system is always conserved in interactions b etween bodies, some change in kinetic energy usually takes place. In dynamics, we study the forces that act on a body to cause motion . The vector sum of the se forces gives a resultant force that causes the body to accelerate. This resultant force causes change in motion. Dynamics explain the reasons behind k inematics where we describe how a body move s under constant acceleration. In the 17th century, Sir Isaac Newton formulated his 3 laws of motion. The process required Newton to deploy the concept of an “external agent” that can transport action at a distance, provide instantaneous motion and not be subject to resistive forces. Newton’s person al reading habits at that time resulted in him imagining these “external agents ” as angels. He gradually trimmed their wings and transformed this new agent into a purely objective “force”. Today, Newtonian mechanics is useful for many engineering efforts in our everyday scale, like how an artillery shell travels in air, and it describes many phenomena observed. Occasionally, we can take time off to marvel at the inspiration behind “force” – that the wings of angels are ever beating invisibly and constantly providing instant messaging between objects - so that forces may exist in our modern world. The Tesla Model 3 did very well in crash-testing because its engineers understood Dynamics very well in ensuring the safety of the passengers.

What the 3rd law refers to as “a body” can also refer to a collection of bodies. We can regard these bodies as “a system”. Example 1 Newton’s 1 st Law gives rise to the idea inertia: that a body is reluctant to change its “status quo” of motion. True or Fals