Forces, motion and Newton's laws
Resultant force, F = m × a and Newton's three laws.
Resultant force
When several forces act on an object, the resultant force is the single force that has the same effect. If forces balance, the resultant is zero.
Newton's laws
1st: if the resultant force is zero, an object stays still or keeps moving at a steady speed in a straight line. 2nd: resultant force = mass × acceleration (F = m × a). 3rd: when two objects interact, they exert equal and opposite forces on each other.
Weight
Weight = mass × gravitational field strength (W = m × g). On Earth, g ≈ 9.8 N/kg.
✏️ Worked example
A 1500 kg car accelerates at 2 m/s². What resultant force acts on it?
F = m × a = 1500 × 2 = 3000 N.
🧠 Test yourself
What is the weight of a 50 kg person on Earth (g = 9.8 N/kg)?
490 N.
If forces are balanced, what happens to a moving object?
It keeps moving at a steady speed in a straight line.
A 2 kg ball has a resultant force of 10 N. Acceleration?
a = 10 ÷ 2 = 5 m/s².
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