Summary AQA GCSE triple higher physics study notes key topic 5

Physics – Key topic 5
4.5 Forces
4.5.1 Forces and their interactions
4.5.1.1 Scalar and vector quantities
 Scalar quantities have magnitude only whereas vector quantities have magnitude and
an associated direction
- Energy, distance, time, mass, pressure, volume, power and speed
 A vector quantity may be represented by an arrow. The length of the arrow
represents the magnitude, and the direction of the arrow, the direction of the vector
quantity
- velocity, weight, displacement, acceleration, momentum and force

4.5.1.2 Contact and non-contact forces
 A force is a push or pull that acts on an object due to the interaction with another
object. All forces between objects are either:
- Contact forces – when two objects have to be touching for a force to act
- Non-contact forces – the objects are physically separated – they do not have to be
touching for a force to act
 Examples of contact forces include friction, air resistance, tension and normal contact
force
 Examples of non-contact forces are gravitational force, electrostatic force and
magnetic force
 Force is a vector quantity – it has both magnitude and direction

4.5.1.3 Gravity
 Weight is the force acting on an object due to gravity. The force of gravity close to the
earth is due to the gravitational field around the earth. The weight of an object
depends on the gravitational field strength at the point where an object is
 The weight of an object depends on the gravitational field strength at the point
where the object is
- Gfs varies with location – it is stronger the closer you are to the mass causing the
field, and stronger for larger masses

,  Gravity attracts all masses. Anything near a planet or star is attracted to it very
strongly. This has two important effects:
- On the surface of a planet, it makes all things fall towards the ground
- It gives everything a weight
 The weight of an object can be calculated using:
- Weight (N) = mass(kg) x gravitational field strength (N/kg)
- The weight of an object may be considered to act at a single point referred to as
the object’s centre of mass. For a uniform object, this will be at the centre of the
object
- Mass is the amount of “stuff” in an object whereas the weight is the force acting
on an object due to gravity
- The weight of an object and the mass are directly proportional
- Weight is measured using a calibrated spring balance (a newton-meter)

4.5.1.4 Resultant forces
 A number of forces acting on an object may be replace by a single force that has the
same effect as all the original forces acting together. This force is the resultant force
- If the force is in the direction the object is moving, it speeds up
- If the force is opposite to the direction it is moving, it slows down
 If the forces are balanced, the resultant force is zero and the object with either stay
stationary or move with constant speed. If there is a resultant force, it accelerated
depending upon the size of the resultant force. If the resultant force is in the opposite
direction to the motion, the object will decelerate
 When a resultant force moves an object, work is done on the object and energy is
transferred from one store to another
 A single force can be resolved into two components acting at right angles to each
other. The two component forces together have the same effect as a single force
 Forces can be shown in a free body diagram:
- the sizes of the arrow show the relative magnitudes of the forces
- in a car, the forwards arrow is driving force, the backwards arrow is friction, the
upwards arrow is normal contact force and the downwards arrow is weight