Let’s Get Physics-l: Dynamics

Previously, we have had a look at kinematics which is the study of motion of objects ignoring the forces that cause the motion. Dynamics, as some of you may have now worked out, is the study of motion with a consideration of the forces that causes this motion. We discussed forces a bit in a previous post but, when considering dynamics, there are 5 more forces we need to take into account.


Tension is a force that occurs in rope, string, cables, wire or spaghetti which is being pulled, hung, rotated or supported. It’s a really important force as it allows us to ascend buildings with relative ease in elevators, lift heavy objects via a crane and support bridges. Spider-man relies on tension a great deal whilst webbing his way around New York City.

A quick note on the diagram, you’ll notice tension works in both directions. Assuming we model the web as massless a net force would result in an infinite acceleration which simply can’t exist. Hence, we have two equal and opposite tension forces.

Tension ensures our beloved web head can take down crooks all over the city

Normal Contact Force

The Normal Contact or Reaction force is the force exerted on an object by a surface perpendicular to the point of contact. You experience a reaction force all the time, be it from a chair, mattress, the floor or a dragon. Even Time Lords experience reaction force.

The TARDIS would just keep going down without a reaction force (Though from the angle pictured it looks like it’s already too late (Oops))


Upthrust is an upwards force that liquids and gases exert on bodies moving through it as a result of the displacement of atoms and molecules within them. Objects falling through a fluid experience both upthrust and drag.  Star-lord knows all about falling through fluids, especially when he loses his jetpack.

Rocket warned Peter about losing his jetpack. In his defence, it doesn’t help when you’re being shot at by Kree warriors.


Friction is not what you experience with an irritating co-worker or classmate. It is a force that occurs between two surfaces in contact with each other. It occurs primarily as result of interatomic and intermolecular forces and results in an opposition to the motion of an object. Cars, such as Ecto 1, experience friction between their tyres and the road surface as well as their exterior and air. When we talk about friction in fluids we refer to it as drag (The fifth force).

‘If there’s somethin’ weird and it don’t look good’ covers a fair bit of Physics, truth be told. That being said, I still love it.


  • Dynamics – The study of motion and the forces that cause them.
  • Tension – Force experienced by rope (etc.) which is being hung, rotated, supported or pulled.
  • Normal Contact Force – Force exerted on an object by a surface perpendicular to the point of contact.
  • Upthrust – Force exerted on a body by a fluid as a result of the displacement of atoms or molecules.
  • Friction – Force that occurs between two surfaces in contact with each other.
  • Drag – Friction in fluids.

Dynamics, not too bad really (yet). In the next Forces and Motion post we will be taking a closer look at Drag and Terminal Velocity. As always, if you have any thoughts or questions please feel free to leave them in the comments.

(The characters and vehicles referenced are property of Marvel Comics, the BBC, Sony and the original Ghostbusters. They have simply been used here to help demonstrate a concept, I lay no claim to them.)

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