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Forces are vectors, so we can find a resultant force on an object, no matter how many forces are acting on it. If the resultant force is zero, the forces must be balanced.
Balanced forces cause no acceleration (This means that the object will remain stationary or carry on moving at a constant speed.)
If the resultant force is not zero the forces are unbalanced. Unbalanced forces cause acceleration in the direction of the resultant force.
Every force has a partner force that is the same size but actson another object and in the opposite direction. It is hard to imagine at first why all these forces don't just cancel out, so here are a couple of examples:
Theoretically, as you push yourself forward when you walk the world is being pushed backwards; luckily the Earth is so large that you don't notice. If you try propelling yourself forwards off a boat on the bank of a lake, you should be able to see the boat is pushed away from the bank as you move towards the bank!
So the acceleration of an object is related to the resultant force causing it. In fact we can calculate the force needed to cause an acceleration using:
F = ma
F = force (N)
m = mass (kg)
a = acceleration (m/s2)
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