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Forces, vectors and resultants
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Forces are vectors. Combining them correctly tells you how an object will move.
Contact and non-contact forces
Contact: friction, air resistance, tension, normal contact force.
Non-contact: gravity, magnetic and electrostatic forces.
Non-contact: gravity, magnetic and electrostatic forces.
Resultant forces
Add forces in the same direction; subtract opposite ones.
Forces at right angles: use Pythagoras for the size and trigonometry for the direction, or a scale drawing.
Forces at right angles: use Pythagoras for the size and trigonometry for the direction, or a scale drawing.
Free body diagrams
Draw the object as a dot or box with an arrow for each force. The arrow length shows the size.
If the arrows balance, the resultant force is zero.
If the arrows balance, the resultant force is zero.
A boat is pushed 30 N east by its engine and 40 N north by the wind. Find the resultant force.
- The forces are at right angles.
- √302 + 402 = √2500
Answer: 50 N
Contact forces need touching; non-contact forces act at a distance. Add or subtract forces in a line; use Pythagoras for forces at right angles. Free body diagrams show every force on one object.
Check you have got it
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