2Newton’s 1st LawAn object continues in uniform motion in a straight line or at rest unless a resultant external force acts
3Newton’s 1st LawAn object continues in uniform motion in a straight line or at rest unless a resultant external force actsDoes this make sense?
4Newton’s 1st law Newton nicked it! Newton’s first law was actually discovered by Galileo.Newton nicked it!
5Newton’s first lawGalileo imagined a marble rolling in a very smooth (i.e. no friction) bowl.
6Newton’s first lawIf you let go of the ball, it always rolls up the opposite side until it reaches its original height (this actually comes from the conservation of energy).
7Newton’s first lawNo matter how long the bowl, this always happens
8Newton’s first law No matter how long the bowl, this always happens. constant velocity
9Newton’s first lawGalileo imagined an infinitely long bowl where the ball never reaches the other side!
10Newton’s first lawThe ball travels with constant velocity until its reaches the other side (which it never does!).Galileo realised that this was the natural state of objects when no (resultant ) forces act.constant velocity
11Other examplesImagine a (giant) dog falling from a tall building
12Other examplesAir resistanceTo start the dog is travelling slowly. The main force on the dog is gravity, with a little air resistancegravity
13Other examplesAs the dog falls faster, the air resistance increases (note that its weight (force of gravity) stays the same)Air resistancegravity
14Other examplesEventually the air resistance grows until it equals the force of gravity. At this time the dog travels with constant velocity (called its terminal velocity)Air resistancegravity
20InertiaA stationary object only starts to move when you apply a resultant force.A moving object keeps moving at a steady speed in a straight line.To change the speed or direction you need to apply another resultant force
21This reluctance to change velocity is called INERTIA The inertia of an object depends on its massA bigger mass needs a bigger force to overcome its inertia and change in motion
23MomentumMomentum is a useful quantity to consider when thinking about "unstoppability". It is also useful when considering collisions and explosions. It is defined asMomentum (kg.m/s) = Mass (kg) x Velocity (m/s)p = mv
24An easy exampleA lorry has a mass of kg and a velocity of 3 m.s-1. What is its momentum?Momentum = Mass x velocity= x 3= kg.m.s-1.
25The Law of conservation of momentum “in an isolated system, momentum remains constant”.
26momentum before = momentum after In other words, in a collision between two objects, momentum is conserved (total momentum stays the same). i.e.Total momentum before the collision = Total momentum afterMomentum is not energy!
27A harder example!A car of mass 1000 kg travelling at 5 m/s hits a stationary truck of mass 2000 kg. After the collision they stick together. What is their joint velocity after the collision?
28A harder example! Before 2000kg1000kg5 m/sMomentum before = 1000x x0 = 5000 kg.m/sCombined mass = 3000 kgAfterV m/sMomentum after = 3000v
29Momentum before = momentum after A harder exampleThe law of conservation of momentum tells us that momentum before equals momentum after, soMomentum before = momentum after5000 = 3000vV = 5000/3000 = 1.67 m/s
30Momentum is a vectorMomentum is a vector, so if velocities are in opposite directions we must take this into account in our calculations
31An even harder example!Snoopy (mass 10kg) running at 4.5 m/s jumps onto a skateboard of mass 4 kg travelling in the opposite direction at 7 m/s. What is the velocity of Snoopy and skateboard after Snoopy has jumped on?I love physics
32An even harder example!Because they are in opposite directions, we make one velocity negative10kg-4.5 m/s4kg7 m/sMomentum before = 10 x x 7 = = -1714kgv m/sMomentum after = 14v
33Momentum before = Momentum after An even harder example!Momentum before = Momentum after-17 = 14vV = -17/14 = m/sThe negative sign tells us that the velocity is from left to right (we choose this as our “negative direction”)
34I thought of this law myself! Newton’s second lawNewton’s second law concerns examples where there is a resultant force.I thought of this law myself!
35Let’s go back to Mr Dickens on his bike. Remember when the forces are balanced (no resultant force) he travels at constant velocity.Pushing forcefrictionConstant velocity
36Newton’s 2nd law Now lets imagine what happens if he pedals faster. Pushing forcefriction
37Newton’s 2nd law His velocity changes (goes faster). He accelerates! Remember from last year that acceleration is rate of change of velocity. In other wordsacceleration = (change in velocity)/timePushing forcefrictionacceleration
38Newton’s 2nd law Now imagine what happens if he stops pedalling. friction
39Newton’s 2nd lawHe slows down (decelerates). This is a negative acceleration.friction
40Newton’s 2nd lawSo when there is a resultant force, an object accelerates (changes velocity)Ms Weston’s PorchePushing forcefriction
41It’s physics, there’s always a mathematical relationship! Newton’s 2nd lawThere is a mathematical relationship between the resultant force and acceleration.Resultant force (N) = mass (kg) x acceleration (m/s2)It’s physics, there’s always a mathematical relationship!FR = ma
42An example What will be Mr Dickens’ acceleration? Mass of Mr Dickens and bike = 100 kgPushing force (100 N)Friction (60 N)
43An example Resultant force = 100 – 60 = 40 N FR = ma 40 = 100a a = 0.4 m/s2Mass of Mr Dickens and bike = 100 kgPushing force (100 N)Friction (60 N)
48Free-body diagramsShows the magnitude and direction of all forces acting on a single bodyThe diagram shows the body only and the forces acting on it.
49ExamplesMass hanging on a ropeT (tension in rope)W (weight)
50Examples Inclined slope If a body touches another body there is a force of reaction or contact force. The force is perpendicular to the body exerting the forceInclined slopeR (normal reaction force)F (friction)W (weight)
51Examples String over a pulley T (tension in rope) T (tension in rope) W1W1