02 mechanics by hei man kwok. 2.1 kinematics definitions displacement: distance moved in a...
TRANSCRIPT
02 Mechanics
BY HEI MAN KWOK
2.1 KINEMATICS
Definitions
• Displacement: distance moved in a particular direction – vector; SL Unit: m; Symbol: s
• Velocity: rate of change of displacement (has a direction) – vector; SL Unit: ms¯¹; Symbol: v or u
• Speed: rate of change of distance – scalar; SL Unit: ms¯¹; Symbol: v or u
• Acceleration: rate of change of velocity (change of direction or velocity) – vector; SL Unit: ms¯²; Symbol: a
Instantaneous vs Average
• Instantaneous: at a particular point in time • Average: taken over a period of time
Average Instanteous
Speed
Velocity
Acceleration Doesn’t exist
Conditions of SUVAT/ Uniformly Accelerated Motion
• Acceleration must be uniform/ constant • Acceleration, velocity and displacement are all
in the same plane (same direction or opposite)
Free-fall
• Acceleration of a body in a vacuum near the Earth’s surface has an acceleration of of free-fall
• Air resistance increases as an object free falls as the velocity increases until the force of air resistance is equal to the weight where the object reaches terminal velocity
Sketch + label, calculate and interpret
• Distance - time • Displacement – time • Velocity – time • Acceleration – time
• Eg. Bouncing ball and free-fall
Relative Velocity in 1D and 2D
9.1 PROJECTILE MOTION
2.2 FORCES AND DYNAMICS
Weight (a type of force)
• Weight = mass x gravity • W = mg • W = mass x 9.81
Forces
Force in Newtons (N); a vector quantity
1. Tension 2. Contact (Normal if at 90 degrees)3. Weight 4. Friction 5. Upthrust 6. Air resistance
Determining Resultant Force
• Use sin cos tan and phytagorous if needed • Make sure the forces are only in opposite
directions before final calculation
Newton’s First Law of Motion
• A body will remain at rest or moving with constant velocity unless acted upon by an unbalanced force
• Eg. Mass on a string, parachutist,
Translational Equilibrium
• All forces are balanced • Center of force will not move however the
mass can rotate around the center
Newton’s Second Law of Motion
Acceleration of a body is proportional to the force applied and inversely proportional to its mass
Rate of change of momentum of a body is directly proportional to the unbalanced force acting on the body and takes place in the same direction
Linear Momentum and Impulse
Linear Momentum = mass x velocity ; vector quantity
Force-time graph
Law of Conservation of Linear Momentum
• Linear momentum is conserved (always the same) for a system of isolated bodies without any external forces acting upon the two objects
Newton’s Third Law of Motion
If body A exerts a force on body B, then body B will be exert an equal and opposite force on body A Eg. Falling body, box at rest Gun recoiling, water cannon
2.3 WORK, ENERGY AND POWER
Work
• Work done = force x distance moved in the direction of the force
• If displacement is not in the direction of the force – change one of them
• Interpret a force – displacement graph
Energy and Principle of Conversation of Energy
• Energy is the quantity that enables body A to do work on body B
• When body A does work on body B, energy is transferred from body A to body B
• (be able to describe energy transformation)• Principle of Conversation of Energy – Energy
cannot be created nor destroyed – it can only be changed and transferred from one form to another
Kinetic Energy and Gravitational Potential Energy Kinetic Energy = energy a body has due to its movement
Gravitational Potential Energy = energy a body has due to its position above the earth
Elastic and Inelastic Collisions
• Elastic Collisions: KE and mv (p) are conserved • Inelastic Collisions: two bodies stick tgt and
according to the law of energy conservation; wd squashing the balls = energy lost as KE
• Eg. Explosion – energy to initiate an explosion comes from the chemical energy contained in the explosive = energy gained in KE of the ball
Power and Efficiency
2.4 UNIFORM CIRCULAR MOTION