Forces and Motion

Forces and Motion

1. Speed

   • Speed= Distance/Time
     • Unit= m/s
   • Distance-Time Graphs
     • 1) LABEL AXIS with a quantity and unit. 2) Scale should fill 3/4. 3) ALWAYS INCLUDE THE LINE OF BEST FIT
       • Gradient (the slope of the line best fit) = Rise/Run= Distance/Time= Speed

2. Displacement,Velocity and Acceleration

   • VECTOR QUANTITY= has BOTH magnitude and direction. EG) Weight, displacement, velocity, force, acceleration and momentum.
     • SCALAR QUANTITY= has magnitude ONLY. EG) Mass, distance, speed, area and energy.
   • Displacement =it's the shortest distance travelled, drawn as a straight line, between two points in a stated.
   • VELOCITY = DISPLACEMENT/TIME
     • Units= km/hr or m/s
     • It's the displacement travelled per unit time.
   • ACCELERATION= CHANGE IN VELOCITY/ TIME
     • It's the change in velocity per unit of time.
       • Unit = m/s
     • ACCLERATION OF GRAVITY ( ACC OF FREEFALL) = gravity acts towards centre of Earth on ALL objects. Its value is INDEPENDENT of the MASS or VOL of obj and is taken as 10m/s i.e will travel at 10m/s at 1s, 20m/s at 2s etc.
   • Equations of Motion
     • S= 1/2 (v+u)t| V= u+ at| S= ut+1/2 at | V =u +2as

3. Newton's Laws

   • WHEN THE FORCES ACTING ON AN OBJ ARE UNBALANCED WE SAY THEY PRODUCE A RESULTANT FORCE.
     • RF = weight-friction
       • RF= ma
   • Friction is a forcce that opposes the motion of an obj.
   • 1ST MOTION LAW= A BODY REMAINS IN A STATE OF REST OR UNIFORM MOTION (CONSTANT SPEED) IN A STRAIGHT LINE UNLESS IT'S ACTED UPON BY A RF.
     • 2ND MOTION LAW= THE ACC OF A BODY IS INVERSELY PROPORTIONAL TO THE RF.
     • 3RD MOTION LAW= IF BODY A EXERTS A FORCE ON A BODY B THEN BODY B EXERTS AN EQUAL BUT OPPO FORCE ON A BODY A.
   • Examples of Newtons 1st and 2nd laws: 1)Sitting on a chair there are 2 forces acting on you. Weight acts downwards and the normal reaction reacts upwards. The 2 forces are equal= no rf= at rest. 2)Two peps pulling on a rope. 3) See back of pg.

4. Circular Motion

   • The force that acts towards the centre of path is centripetal force. AS THE SPEED ISN'T CHANGING THE ACCELERATING FORCE IS ALWAYS TOWARDS THE CENTRE OF THE CIRCLE = CF.
     • • When a mass moves in a circle it's ALWAYS CHANGING DIRECTION and therefore the VELOCITY IS ALWAYS CHANGING which means that there must be an ACC and from F=ma, there must be a RF.
         • The Cf increases if mass (m) of obj increases, speed(v) of obj increases and radius (r)of the circle decreases
   • Examples of Cm: 1) A car on the bend or roundabout is held by friction between tyres and the road surface.| 2) The moon is held in orbit around the earth by gravitational f.| 3) Clothes in washing machine held in orbit by drum.| 4) Satellites are held in orbit by gf of attraction of the Earth.

5. Momentum

   • MOMENTUM = MASS X VELOCITY
     • Unit= kgm/s
     • CHANGE IN MOMENTUM = FORCE X TIME
   • Momentum and Safety Features
     • Car Crumple Zones= spread the change in momentum over a long period of time and so reduce the force on the passengers and reduce injuries.
     • Airbags= increases the amount of time it takes the person to stop. Therefore, the change in momentum occurs over a longer period of time and so the force on the passengers is reduced, thus lessening the risk of serious injury.
     • Car Seat belts= increases the amount of time it takes to stop the person. Therefore, the change in momentum occurs over a longer period of time and so the force on the passengers is reduced, thus lessiening the risk of srs injury.
     • Crash Barriers= when car collides with barrier, it will crush steadily etc etc. (same as seat belts)
     • Force= change in momentum/ time.
   • Inelastic Collosions
     • KE isn't conserved- some or all of the KE is converted into other forms.
       • Conservation of Momentum= the total mom is conserved when 2 bodies collide or act on each other.
         • Examples:1) Rockets= Exhaust gases gain mom in backwards direction because they are forced out. The mom gains same amount of mom upwards direction, so no overall change in mom.| 2) Recoil of gun= gun and bullet at rest= mom o. Bullet of mass is shot to the right of vel. The gun mass recoils to the left with vel. These moms add to give 0.| 3) Stepping out of a boat= boat moves backwards to conserve mom.

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