Lesson - 3

• The unit of force is the newton (N).

• Forces appear when two objects interact with each other.

• A force can be represented by an arrow to show its direction, while length is proportional to the size of the force.

• Solid friction, air resistance, and drag act in the opposite direction to the object’s motion and can produce heating.

• The resultant force is the single force that has the same effect as two or more forces.

• A resultant force can change the speed and/or direction of an object.

• The force of gravity pulls objects downwards and is normally called the weight of the object.

• The acceleration caused by the pull of the Earth’s gravity is called the acceleration of free fall or the acceleration due to gravity. It is given the symbol g and its value is 9.8 m/s2 close to the surface of the Earth.

• The mass of an object, measured in kilograms, tells you how much matter that object is composed of.

• The weight of an object, measured in newtons, is the gravitational force that acts on that object.

• Terminal velocity is the name for the maximum constant speed reached when the resultant force acting on an object becomes zero. It is often applied to parachutists when the upwards force of air resistance becomes equal and opposite to weight.

  
  

• If an object moves in a circle, a force must be acting towards the centre of the path, perpendicular (at right angles) to the speed of the object.

  
  

• For motion in a circular path, a bigger force is required if the body is more massive, moving faster or moving in a tighter circle.

  
  

• Force = mass × acceleration, F = ma.

  
  

• Momentum is the quantity mass × velocity, p = mv.

  
  

• The principle of the conservation of momentum means that the total momentum after an interaction between bodies (for example, a collision) is the same as it was before the interaction.

  
  

• The impulse (of a force) can be defined as the change in an object’s momentum (mv ‒ mu) or the force acting on an object multiplied by the time for which the force acts (Ft), so impulse = FΔt.

  
  

• Force can be defined as the rate of change of momentum, F=ΔpΔt.

  
  

• The resultant of two vectors that do not act along the same line can be found by drawing a vector triangle or by calculation.