Forces

Forces

1. intro

   • measured in newtons (N)
   • makes an object move (e.g. tension)
   • makes an object stop moving (e.g. friction)
   • forces can change the direction of a moving object
   • forces can change the shape of an object

2. contact forces

   • impact force
     • when 2 objects bump into each other
     • when a particle in the air hits your skin
     • when a tennis ball hits a racket
   • strain force (tension)
     • when an object is stretched
     • when an elastic band or string is stretched
     • when a ball hits the strings on a tennis racket
   • friction

3. friction

   • slows down a moving object
     • e.g. goalkeeper gloves
   • surfaces
     • smooth surfaces
       • reduce friction
     • rough surfaces
       • increase friction
   • uses
     • good uses of friction
       • between foot and ground
       • studs on trainers
       • football gloves
       • brake pads
     • bad uses of friction
       • generates heat
         • rubbing hands together
         • moving parts in car rub together
       • causes particles to heat up
         • causes dust particles in comets
         • can be a problem when space shuttles are sent into space because they get very hot

4. non contact forces

   • magnetic force
     • exerts a pulling force on some materials without pulling it
     • closer the magnet is, stronger the force is
     • 2 magnets can also exert a pushing force on each other
   • electrostatic force
     • created when 2 forces are rubbed together
     • rub a comb on a jumper
     • 2 charged objects exert a force on each other without touching each other
     • e.g. if you hold a charged comb near water running from a tap, you can bend the flow of the water
   • gravitational force
     • force of attraction between any 2 masses in universe
     • e.g. the force between body and ground
     • gravitational force between planets
     • on earth, all masses are attracted to the earth itself
     • size of force depends on mass
       • greater mass
         • stronger force
       • lower mass
         • weaker force
     • gravitational force acting on any mass is called weight and is measured in newtons (N)
     • weight on earth = mass x 9.82N/kg

5. speeding up, slowing down

   • when force is applied to stationary object it may begin to move
     • e.g. footballer kicks the ball
   • when a force is applied to a moving object in the opposite direction of motion, it will slow down or even stop
     • e.g. football may be stopped quickly by the impact force of the goalkeeper
     • e.g. football may be stopped slowly by the contact force of friction as it rolls on the ground
   • when a force is applied to a moving object in the same direction of motion, it will speed up

6. upthrust

   • when an object is in aliquid it experiences an upwards force called upthrust
   • caused by water being pushed out of the way
   • determined by volume of the object and density of the liquid
   • upthrust makes things weigh less when in a liquid
   • if upthrust is large enough to balance the wight of the object, it will float

7. balanced forces

   • a skydiver experiences 2 forces
   • gravitational force and friction with the air (drag) upwards
   • if 2 forces are equal size the skydiver neither slows down or speeds up
   • the forces are said to be balanced

8. density

   • measure of how heavy something is for its size
   • calculated with this equation: density = mass/volume