Forces and Dynamics

1. Forces and Dynamics Free Body Force Diagrams

2. 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 Earth i. Situation Diagram (this shows what’s going on):

3. 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

4. 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: Contact, action force from the box on the Moon = 8N Gravitational force from the moon on the box = 8N Contact, reaction force from the moon on the box = 8N Gravitational force from the box on the Moon = 8N

5. E.g. 3 A cat on a box, on Earth i. Situation Diagram: ii. FBFD for cat: Contact force from the box on the cat Gravitational force from the Earth on the cat

6. E.g. 3 A cat on a box on Earth i. Situation Diagram: iii. FBFD for box: Contact force from the cat on the box Gravitational force from the Earth on the box Contact force from the Earth on the box

7. E.g. 3 A cat on a box on Earth i. Situation Diagram: iii. FBFD for Earth: Contact force from the box on the Earth Gravitational force from the cat on the Earth Gravitational force from the box on the Earth

8. E.g. 4 A skier moving at constant speed down a piste: i. Situation Diagram: ii. FBFD for skier: Gravitational force from Earth on skier Frictional force from Earth on skier Contact force from Earth on skier

9. Translational Equilibrium … Gravitational force from Earth on skier Frictional force from Earth on skier Contact force from Earth on skier

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