Newton's Laws

The three big laws of motion.

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1. Newton's Laws Explained

This video explains Newton's Laws for A Level Physics. Normally you start at number 1 and work through to number 3, but in this video I start with Newton's 3rd Law, then Newton's 2nd Law and finally Newton's 1st Law (also mentioning Newton's Law of Gravitation and big G).

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2. Newton's 1st Law

Newton's First Law states that a body will remain stationary or move at a constant velocity unless acted upon by a resultant force. I also show you a few examples including why the reaction/normal/contact force is not equal to the weight.

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3. Newton's 2nd Law

F=ma is good but not always correct. A better definition of Newton's 2nd Law is to say that the force is directly proportional to the rate of change of momentum and acts in the same direction (this last part is really important). This covers examples where the mass or acceleration may not remain constant.

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4. Newton's 3rd Law

Newton's 3rd Law: If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type on object A (this is better than 'every action has an equal and opposite reaction'). This sounds simple but is what many people find difficult identifying in questions.

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5. Newton's 2nd Law and F=ma

You should remember most of this from GCSE Physics, but it's good to refresh your knowledge. F=ma is a special case of Newton's Second Law provided you have a constant mass and a uniform acceleration, and it used throughout this subject to calculate how objects change their motion when a net force is applied.