Forces and Movement

• When an object (body) experiences a force, at least one of the following happens:

  1. Change speed (accelerate or decelerate)
  2. Change shape
  3. Change direction
• Normal reaction force

  When an object rests or is in contact with a solid surface it feels a reaction force 90 degrees to the surface.

  This is equal and opposite to the force of the object pushing on the surface.
• Applied force

  A push or pull

  Also known as contact force and pull force
• Gravitational force

  Masses are attracted to each other. Therefore objects are attracted to the centre of the Earth. This is called weight.

  Gravitational force or weight is proportional to the mass and gravitational field strength (10N/kg for Earth at GCSE)
• Friction (surface on surface)

  Acts in the opposite direction, friction can be reduced if surfaces are smoother.

  Friction can be helpful or a nuisance
• Air resistance or drag

  Drag occurs when an object moves through a fluid- it is called air resistance in air.

  Drag is affected by the shape of the object, the fluid it travels through and its speed.

  Drag opposes motion.
• Spring force

  A force is required to stretch or compress a spring.

  Spring force is proportional to the extension of the spring.
• Tension

  Tension acts in strings, chains and cables when they are stretched.

  The force is always parallel to the string.

  The opposite of tension is compression.
• Thrust

  Thrust is a reaction force. It occurs when mass is pushed out the back of something, causing the object to move forwards.

  Rockets push gas out and downward and the rocket moves upward.
• Up-thrust

  Up-thrust can only occur in fluids and is the reason things float. It is not the same as normal reaction force.

  Up-thrust is equal the weight of the fluid the object displaces.
• Lift

  Lift is the force produced due to the flow of a fluid over an aerodynamic surface.

  In aircraft, lift is upward on the wings and in a F1 car it acts downward to hold the car on the track.
• Magnetic

  Magnetic force of attraction pulls two opposite poles together. Magnetic force of repulsion pushes two like poles apart.

  The magnetic force increases as the distance between the poles decreases.
• Electrostatic

  Electrostatic force of attraction pulls two opposite charges together. Electrostatic force of repulsion pushes two like charges apart.

  Electrostatic forces can be increased by decreasing the distance between the charges or by increasing the magnitude of the charges.
• A scalar quantity only has a magnitude (size), it doesn't have direction.
• A vector quantity has a magnitude (size) and a direction
• Examples of a scalar


  Mass, distance, speed, volume, density, temperature, energy
• Examples of a vector


  Weight, displacement, velocity, acceleration, force, momentum
• Vector quantities have both magnitude and direction. Arrows are a perfect way to depict vectors and forces are labelled using arrows for this reason. The arrow can be made bigger or smaller to represent the magnitude of the force and can be rotated to indicate the direction of the force.
• Resultant force is the overall force acting on an object


  Opposite forces: Big number take away small number

  Same direction forces: Added together
• Friction is the force that opposes the motion of an object.

  Friction occurs when two or more objects are rubbing against each other.

  Friction also occurs in fluids, we call this drag.
• Unbalanced force = mass * acceleration (F=ma)

  Force is measured in Newtons (N)

  Mass is measured in Kilograms (kg)

  Acceleration is measured in meters per second squared (m/s^2)
• Weight is a force due to gravity. Its unit is Newton (N). Weight is the results of gravity. The Earth's gravity attracts objects towards the centre of the Earth and the force you feel is called weight.
• Weight = mass * gravitational field strength

  W = mg

  Weight in Newtons (N)

  Mass in kilograms (kg)
• Stopping distance is the total distance required for a car to stop moving.

  Thinking distance is the distance it takes for a driver to react and press the brake pedal

  Braking distance is the distance it takes between when the brake pedal is pressed and the car halts

  Stopping distance = thinking distance + braking distance
• Factors that affect breaking distance

  Mass of vehicle

  State of vehicle's breaks

  State of the road

  Friction between tyre and road
• Factors that affect thinking distance

  Driver's reaction time

  Drug use

  Distractions to driver
• Factors that affect both breaking and thinking distances

  Speed of vehicle
• When objects fall:

  1. Weight accelerates object.
  2. Weight is greater than air resistance.
  3. As velocity increases, air resistance increases, and acceleration decreases.
  4. Eventually weight = air resistance.
  5. The object reaches a maximum speed called terminal velocity.
• Practical: investigate how extension varies with applied force for helical springs, metal wires and rubber bands

  Extension = new length - original length

  • Measure original length of spring using a ruler.
  • Measure new length using a ruler for a range of masses.
  • Measuring between the same two points on the spring.
  • Avoiding parallax by reading at eye level and using a pointer.
  • Clamping ruler vertically.
  • Repeat and take an average.
  • Measure extension with decreasing mass as well.
• When a force is applied to an elastic object, the object is stretched in a linear fashion.

  This means that there will be a straight-line relationship between force and extension.

  After a certain point, the object will become permanently stretched, and non-linear.
• When two or more forces act on the same object, the object can be stretched, compressed or bent.

  A change of shape is called distortion.

  • Elastic distortion is temporary
  • Inelastic distortion is permanent
• mass, energy, speed, force

  The vector quantity is force.
• Before the fuse is lit, the total weight of a rocket including fuel is 0.7N

  The gravitational field strength is 10N/kg

  The total mass of the rocket including fuel is

  0.07 kg
• When the velocity of the car is constant, the force of the friction on it is

  the same size as the driving force
• A car has a mass of 1200kg. Calculate the force needed to give this car an acceleration of 2.4 m/s^2 (2 sig fig)

  2900 N
• A car accelerates at a constant rate of 1.83 m/s^2 along a flat straight road.

  The force acting on the car is 1.870 kN.

  Calculate the mass of the car (3 sig figs).

  1020 kg
• Explain why it is safer for drivers to drive at a slower speed in wet weather.

  Wet weather:

  • Less/no friction
  • Increased stopping distance

  Slower speed:

  • Shorter braking/stopping distance
  • Reduces possibility of skidding/sliding/losing control/crashing
• Describe how a student could measure the extension of a rubber when he adds another weight to the weight hanger.

  • Measure a length or a specific distance related to the rubber or weights on a hanger
  • Use a metre rule/stick/ruler/measuring tape
  • Note position of a fixed point on rubber/weight carrier
  • Extension calculated/measured as the change in or difference between two positions or lengths or extensions
• Describe how a student could measure the extension of a spring when a load is added.

  Measure unstretched length of spring

  Measure stretched length of spring

  Subtract
• Name the two forces acting on an object as it falls.

  Weight and air resistance/drag