CIE AS Physics (9702) 2019-2021

Revision Notes

5.1.1 Types of Force

Forces in Gravitational & Electric Fields

  • A gravitational field provides a force of attraction between masses
  • A uniform gravitational field is one where the gravitational field lines are parallel and always the same distance apart – this is almost true close to the Earth’s surface

Force in gravitational field, downloadable AS & A Level Physics revision notes

Force on a mass in a uniform gravitational field

 

  • The force depends on the mass m and the gravitational field strength g which varies depending on the planet
  • On Earth, g is 9.81 N kg-1
  • An electric field provides a force of attraction, or repulsion, between charges
  • A uniform electric field is one where the electric field lines are parallel and always the same distance apart – this is true in two parallel conductive plates such as in a capacitor

Force in an electric field, downloadable AS & A Level Physics revision notes

Force on a charge in a uniform electric field

 

  •  The force depends on the size of the charge q and the electric field strength E

Gravitational v electric fields table

Comparing-Gravitational-&-Electric-Fields-table, downloadable AS & A Level Physics revision notes

Upthrust

  • Upthrust is a force which pushes upwards on an object submerged in a fluid i.e. liquids and gases
  • Also known as buoyancy force, upthrust is due to the difference in hydrostatic pressure at the top and bottom of the immersed object
  • The force of upthrust is significantly larger in liquids than in gases, this is because liquids are much denser than gases
  • Recall that hydrostatic pressure depends on the height (h) or depth that an object is submerged in from P = ρgh
  • Therefore, the water pressure at the bottom of an object is greater than the water pressure at the top, as shown in the diagram below:

 

Upthrust, downloadable AS & A Level Physics revision notes

This can will experience upthrust due to the hydrostatic pressure difference

 

  • Upthrust is a force and is directly proportional to the pressure. The force on the bottom of the can will be greater than the force on top of the can
  • This resultant pressure causes a resultant upward force on the can known as upthrust
  • Upthrust is why objects appear to weigh less when immersed in a liquid. If the upthrust is greater than the weight of the object, the object will rise up
  • For an object to float, it must have a density less than the density of the fluid its immersed in

 

Worked example

Worked example - upthrust, downloadable AS & A Level Physics revision notes

Exam Tip

Since upthrust is force it is influenced by pressure, not by the density of the object as commonly misunderstood.

Drag Forces

  • Drag forces are forces acting the opposite direction to an object moving through a fluid (either gas or liquid)
  • Examples of drag forces are friction and air resistance
  • A key component of drag forces is it increases with the speed of the object. This is shown in the diagram below:

Drag force increases with speed, downloadable AS & A Level Physics revision notes

Frictional forces on a car increase with its speed

 

Worked Example

Worked Example - frictional force on car, downloadable AS & A Level Physics revision notes

Using Newton’s Second Law to find the drag force

 

Exam Tip

Remember to consider drag forces in your calculation for the resultant force. More details of this are in the notes “Force and acceleration”.

Centre of Gravity

  • The centre of gravity of an object is the point at which the weight of the object may be considered to act
  • For example, for a person standing upright, their centre of gravity is roughly in the middle of the body behind the navel, and for a sphere, it is at the centre
  • For symmetrical objects with uniform density, the centre of gravity is located at the point of symmetry

Point of symmetry, downloadable AS & A Level Physics revision notes

The centre of gravity of a shape can be found by symmetry

 

Stability

  • The position of the centre of gravity of an object affects its stability
  • An object is stable when its centre of gravity lies above its base

Stability, downloadable AS & A Level Physics revision notes

The object on the right will topple, as its centre of mass is no longer over its base

 

  • The wider base an object has, the lower its centre of gravity and it is more stable
  • The narrower base an object has, the higher its centre of gravity and the object is more likely to topple over if pushed

Stability and base, downloadable AS & A Level Physics revision notes

The most stable objects have wide bases and low centres of mass

 

Centre of gravity v centre of mass

  • In a uniform gravitational field, the centre of gravity is identical to the centre of mass
  • The centre of mass does not depend on the gravitational field
  • Since weight = mass ✕ acceleration due to gravity, the centre of gravity does depend on the gravitational field
  • When an object is in space, its centre of gravity will be more towards the object with larger gravitational field for example, the Earth’s gravitational field on the Moon

Centre of gravity vs centre of mass, downloadable AS & A Level Physics revision notes

The Earth’s stronger gravitational field pushes the Moons centre of gravity closer to Earth

 

Exam Tip

Since the centre of gravity is a hypothetical point, it can lie inside or outside of a body. The centre of gravity will constantly shift depending on the shape of a body. For example, a human body’s centre of gravity is lower when learning forward than upright

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