Forces and Dynamics

Free Body Force Diagrams

Free Body Force Diagrams

We draw free body force diagrams in order to make it easier to see what forces are acting on a single body involved in a more complex mechanics problem.

E.g. An asteroid approaching Earthi. Situation Diagram (this shows what’s going on):

ii. Free body force diagram for the Asteroid:

iii. Free body force diagram for the Earth:

Gravitational force from Earth on Asteroid = 1000N

Gravitational force from Asteroid on Earth = 1000N

E.g. 2

A 5kg box on the Moon (g=1.6 ms-2)

i. Situation Diagram: ii. FBFD for box: iii. FBFD for Moon:

Gravitational force from the moon on the box = 8N

Gravitational force from the box on the Moon = 8N

Contact, reaction force from the moon on the box = 8N

Contact, action force from the box on the Moon = 8N

E.g. 3

A cat on a box, on Earth

i. Situation Diagram:

Gravitational force from the Earth on the cat

Contact force from the box on the cat

ii. FBFD for cat:

E.g. 3

A cat on a box on Earth

i. Situation Diagram:

Gravitational force from the Earth on the box

Contact force from the Earth on the box

Contact force from the cat on the box

iii. FBFD for box:

E.g. 3

A cat on a box on Earth

i. Situation Diagram:

Gravitational force from the box on the Earth

Contact force from the box on the Earth

Gravitational force from the cat on the Earth

iii. FBFD for Earth:

E.g. 4

A skier moving at constant speed down a piste:

i. Situation Diagram: ii. FBFD for skier:

Contact force from Earth on skier

Gravitational force from Earth on skier

Frictional force from Earth on skier

Contact force from Earth on skier

Gravitational force from Earth on skier

Frictional force from Earth on skier

Translational Equilibrium

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