# Topic 2. Topic 1 The Killers LEARNING OBJECTIVES. Mechanics. 1. Percentage Uncertainties 2. Plotting graphs 3. Vector addition and subtraction

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1 Topic 1 The Killers 1. Percentage Uncertainties 2. Plotting graphs 3. Vector addition and subtraction ROOKIE MISTAKE: Don t underestimate the importance of this topic. It makes up 5-7% of your final IB Physics mark Topic 2 Mechanics LEARNING OBJECTIVES Download the complete checklist from this tutorial page.

2 SIMPLE.. ROOKIE MISTAKE! Not drawing a diagram is the worst thing you can do! Topic 2 The Killers 1. Projectile Motion 2. Forces in Equilibrium 3. Conservation of Momentum 1 Projectile Motion

3 Projectiles A projectile is an object upon which the only force acting is gravity. If there were any other force acting upon an object, then that object would not be a projectile. Air resistance is usually assumed to be negligible. Free-body diagram of a projectile. 1 - Oblique Projection An object is which thrown upwards at an angle is a projectile. It continues in motion by its own inertia and is influenced only by the downward force of gravity. 2 - Horizontal Projection An object also undergoes projectile motion, when thrown horizontally and is acted upon by only gravity.

4 Horizontal and Vertical Motion Horizontal and Vertical Motion When dealing with projectile motion, it is important to deal with horizontal and vertical motion separately. Horizontal Motion The horizontal velocity remains constant Vertical Motion The vertical motion uniformly accelerates a = 0ms -2 a = g = 9.8ms -2 Use: Speed = distance / time Use: Equations of Motion Rules 1. Draw a diagram 2. Choose which direction is positive(e.g. upwards). Anything upwards has a positive value and anything downwards has a negative value. 3. Draw/complete projectile table

5 ACTUAL EXAMINER FEEDBACK A common mistake was the incorrect calculation of resolving velocity for projectile motion 2 Forces in Equilibrium Newton s 1st Law An object will remain at rest or continue to move with a constant velocity unless acted upon by a resultant external force. An object will continue to move with a constant velocity unless acted upon by a resultant external force.

6 Newton s 1st Law An object will remain at rest or continue to move with a constant velocity unless acted upon by a resultant external force. An object will remain at rest unless acted upon by a resultant external force.

7 When the net force on a body is zero, the body is said to be in equilibrium. Equilibrium An object at rest, or travelling at a constant speed will be in equilibrium. ACTUAL EXAMINER FEEDBACK Very few candidates had the ability to treat a body as a single particle and analyse the forces acting Free-Body Diagram When considering a body in equilibrium, it is useful to draw a free-body diagram: Represent the object as a point mass Draw all forces arrows acting from this point. Ensure all forces are to scale and at the correct angle.

8 Rules 1. Look for phrases: constant speed / velocity equilibrium stationary at rest 2. Draw a free-body diagram 3. Make sure forces are equal in all directions 3 Conservation of Momentum Linear Momentum Linear momentum is defined as: The product of mass and velocity of an object. p =mv The units of momentum are: kgms -1. It is a vector quantity.

9 Rules 1. Look for the phrases: collision explosion momentum - obvious?? 2. Draw a diagram (incl. before and after collision) 3. Choose a direction as positive 4. Apply p=mv to each body (before and after) 5. momentum before = momentum after Force and Momentum Newton s 2nd Law can be stated as: The rate of change of momentum of a body is equal to the resultant force acting on the body. f = Δp Δt = mv mu Δt = m v u Δt Therefore it is clear that when a force acts upon a body over a period of time, a change of momentum occurs and hence an acceleration. = ma Conservation of Linear Momentum The Law of Conservation of Momentum states that: During any interaction where no external forces act, the total linear momentum is conserved, in any specified direction. Therefore, the total momentum before a collision (p before) equals the total momentum after a collision (p after)!! p before = p after

10 ACTUAL EXAMINER FEEDBACK Very few candidates seemed to know what was meant by an elastic collision Inelastic Collisions Momentum is conserved in all collisions, assuming no external forces apply; however, kinetic energy is not. An inelastic collision is when kinetic energy is lost during a collision and converted into other types of energy. When bodies stick together after a collision, it is totally inelastic. Max possible Ek is lost. Elastic Collisions There are very few collisions in real life where kinetic energy is not lost as sound or heat; however, An elastic collision is when kinetic energy is conserved in a collision.

11 SIMPLE.. ROOKIE MISTAKE! Not drawing a diagram is the worst thing you can do! Topic 2 The Killers 1. Projectile Motion 2. Forces in Equilibrium 3. Conservation of Momentum