BRAINCAKEResultant Forces
The resultant force on an object is the single force that has the same effect as all the forces acting on it together. It is the vector sum of the forces, and by Newton's second law it equals mass × acceleration: F1 + F2 + ... = ma.
To use this, choose a positive direction, add the forces in that direction and subtract those in the opposite direction. The result divided by the mass gives the acceleration.
For a rocket rising vertically, the resultant force is the thrust T minus the weight mg, so T - mg = ma.
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All 16 lessons in Newtons Laws of Motion · All AQA AS-level Physics topics