5- Forces

• Scalar quantities: magnitude only (size)

  • Mass, temp, speed, energy, distance, time 
• Vector quantities: have magnitude and direction 

  • Displacement, weight, force, velocity, acceleration, momentum 
• You can represent vector using an arrow:

  
• All forces have both magnitude and direction- they’re vector quantities (Newtons= unit) 

  • Forces take place when two objects interact  
• Contact forces: two objects are physically interacting 

  • Tension
  •  friction
  • air resistance
  • normal contact force: An object at rest on a surface exerts a force on each other, table would exert upwards force on lamp and lamp exerts a downwards force on table. 
• Non-contact forces: two objects are physically separated 

  • Gravitational force: attracts objects toward other objects
  • Electrostatic force: force between two charged objects : opposite charges experience and electrostatic force of attraction 
  • Magnetic force: force experienced by objects in a magnetic field 
• Mass: the amount of matter and object has - in Kg, a scalar quantity, mass is the same wherever it is
• Weight: the force acting on it due to gravity - Newtons (N) 

  • Gravitational field strength is the force of gravity in a particular location (depends on where you are) - eg: on the moon the gravitational field strength is not the same 
  • Weight of an object is directional proportional to the mass of the object, so if you double the mass then the weight also double
• Weight (N) = mass (kg) x Gravitational field strength (N/kg)
• Centre of mass: a position defined relative to an object or system of objects.
• Resultant force: a single force that has the same effect as all of the original forces acting together

  • to work it out: ‘large force - smaller force’  
• Free body diagram: object is shown as a point, forces are drawn as arrows starting at the point and direction of the force  

  
• Vector diagrams

  • draw out the forces (eg 10N = 10cm etc) as lines connecting according to the angle given
  • then draw those line in the opposing position
  • measure the middle line from both corners and that's the answer (resultant force)

  
• 1. draw faint lines to show the horizontal and vertical axes

  2. using a protactor measure 35° from the object and use a ruler to draw the vector showing the 100N force- this will be 10cm

  3. draw a dotted line from the head of the vector to the horicontal and vertical axes (now u can draw the horizontal and vertical components

  4. measure the vertical and horizontal lines and use the scale to find the results

  
• Work done: a force is used to move an object and energy is transferred 

  Work done (J) = Force (N) x distance (m) 
• Elastic deformation: a temporary deformation of a material's shape that is self-reversing after removing the force or load

  • In order to change an object's length of shape we have to apply more than one force (or the forces wont be balanced) 
  • Inelastic materials:  do not return to original shape/length when force is removed 
• Force (N) = spring constant (k) x extension (e)

  
• required practical

  1. Clamp stand, two bosses and two clamps - place a heavy weight on clamp stand to stop it falling over
  2.  Attach a metre ruler and a spring (ruler at the 0 point) 

  • Metre ruler should be vertices or readings will be inaccurate 
  • End of spring has a point and this should be horizontal (gives us readings)

  1. Mean the pointers current positive (the unstretched length with no force attached) 
  2. Add a 1N weight and read position of pointer, repeat process taking down the readings 
  3. To work out the extension, subtract length of the unstretched length from the reading 

  
• Hooke's law: The extension of a spring is directly proportional to the force applied,  provided that the limit of proportionality is not exceeded

  
• Speed of an object tells us the distance the object travelled in a given time (scalar quantity) 
• Speed (m/s) = distance (m) / time (s)

  
• Typical speeds: 

  1. Normal walking speed: 1.5 m/s
  2. Running speed: 3 m/s
  3. Cycling speed: 6 m/s 

  • Speed depends on age and fitness level & terrain (flat ground = fast)  
  • car on main road: 13 m/s, fast train in UK: 50 m/s, Cruising aeroplane: 250 m/s, sound in air 330: m/s 
• Velocity: speed in a given direction eg: and object is moving 20 m/s south (vector quantity) 

  • Calculate with speed formula but you must add in the direction
• Velocity

  
• If velocity ismoving in a circle: if an object moves at a constant speed in a circle then its velocity is constantly changing, even if speed is constant 
• Calculate the speed: 

  Gradient = distance travelled / time taken IF its curved find the tangent (change in y/change in x) 

  
• Acceleration: the change in its velocity over a given time

  acceleration= change in velocity / time

  
• constant acceleration:

  
• Gradient of distance time graph = acceleration 

  
• acceleration

  
• If an object is acceleration at a constant rate then use:

  Final velocity2 - initial velocity2  = 2(acceleration x distance) 

  V2 - U2 = 2as

  • might need to rearrange (this is given in the exam)

  
• Acceleration towards the earth:

  • When any object falls towards the surface of the Earth, it initually accelerates at around 9.8 m/s2- this is due to the fore of gravity acting on the object 
  • As the object falls an upward force of frictions with the air particles (air resistance), after some time the force of air resistance balances due to gravity
  • The object stops accelerating and moves at a constant velocity- the terminal velocity (only when an object is falling through fluid eg: air or liquid) 
• terminal velocity- the maximum velocity that an object can attain when it is falling through a fluid, such as air or water

  • this depends on the object- some objects experience a greater force of friction due to their shaoe so they have a lower termincal velocity
• Newtons 1st law of Motion: if the resultant force acting on a stationary object is zero, then the object will remain stationary

  • If the resultant force acting on a moving object is zero the the object will continue moving in the same direction at the same speed (with same velocity) 
  • The velocity of an object is a resultant force is acting on the object resistive forces; friction (of the air, road etc)
• ! When a resultant force acts on an object it increases its speed- it accelerates 

  • BUT if the resultant force is against the object then it will slow down- decelerate
  • Resultant force can also change the objects direction (if the force is upwards- will go up) 
• 2nd Law of Motion: the acceleration of an object is proportional to the resultant force acting in the object and inversely proportional to the mass of an object 

  • Greater force = greater acceleration
  • greater mass = acceleration will be low

  Force (N) = Mass (kg) x Acceleration (m/s2)

  Road transport:

  • Cars tavel at 13m/s on main road and 30m/sd on motorway
  • To acceleration from main road to motorway involves acceleration of 2m/s (2000N)
• Force (N) = Mass (kg) x Acceleration (m/s2)
• Inertia: an object will stay stationary or continue moving at the same speed and direction unless a resultant force is applied 

  • Object will stay still or keep same motion unless you apply a resultant force 
  • Inertial mass= measure of how difficult it is to change the velocity of an object, defined as a ratio of the force need to accelerated over acceleration produced 

  Large inertial mass = greater force needed to produce given acceleration

  small interior mass = lower force needed
• 3nd Law of Motion: whenever two objects interact, the force they exert on each other are equal and opposite (equal in magnitude but opposite in direction) 

  • Eg: “if you push a wall, the wall pushes you back” 
•  Required Practical: Acceleration

  1)Attach car to a piece of string and loop around a pulley- the other end is attached to a 100g mass

  2)The weight of the mass will provide the force acting on the toy car, on the desk draw chalk lines at equal intervals (eg every 10cm) hold the toy car at the starting point, let go of car when ready - as there's a resultant force acting through the string the car will accelerated along the bench, must record the time the car crosses each mark

  3)repeat experiment but decrease the mass at end of the string (80, 60, 40 g) - the force it decreases each time