Applied Science AQA/Newton's Laws

USe Newton's Laws and Momentum

Setting Applied context

Many types of scientists and engineers use Newton’s laws of motion to predict the motion and interaction of objects. For instance, automotive engineers when designing crumple zones in cars and sports scientists in suggesting improvements in athletic abilities.

Syllabus Content	What you need to do
• application of Newton’s First Law of Motion to both stationary and moving objects • inertia	Say what Newton's First Law of Motion is. Define inertia. Work out the resultant force on an object.
• Newton’s Second Law of Motion • the formula: F = ma • weight = mg as an example of Newton’s Second Law of Motion	Say what Newton's Second Law of Motion is. Do calculations using F = ma; rearrange this equation. Work out the weight of an object from its mass.
• Newton’s Third Law of Motion including its relationship to the Law of Conservation of Momentum • the meaning of ‘momentum’ • the formulas: p = mv F = ∆p/t • applying the Law of Conservation of Momentum to a range of situations including collisions and/or the motion of objects	Say what Newton's Third Law of Motion is. Define momentum. Work out the momentum of a moving object. Work out the force needed to change the momentum of an object. Use the principle that momentum is conserved to work out how fast objects are travelling before or after a collision.

Explanation of key ideas (must be original text, not C&P) – level checked by AQA

Newton's First Law of Motion

"If the resultant force on an object is zero, its motion will remain constant."

Balanced forces - example of Newton's First Law

If the forces balance, it will stay doing whatever it was doing... either moving at a constant speed in a straight line or not moving. i.e. its not accelerating.

Conversely, if an object is not accelerating, you know that the forces balance... or cancel out.

Inertia can be thought of as a 'reluctance' to move... a large table has more inertia than a small chair. Newton's First Law uses the idea of inertia - an object won't change what it is doing without a reason.

(You might want to look up the difference between 'speed' and 'velocity' - one is a scalar and the other a 'vector' quantity i.e. the direction can matter too.)

Newton's Second Law of Motion

"If the resultant force on an object is not zero, it will accelerate."

Forces cause acceleration - which can show up as either a change in speed OR a change in direction. If an object is accelerating, you know that there must be an overall force acting on it (a non-zero resultant force.)

There is an equation that links closely to this law: 'F = m a' (Resultant Force = Mass x Acceleration).

This equation is very similar to 'w = m g' because weight is a force and 'g' can be called the acceleration due to gravity'. (At GCSE you would have said g = 10 N/kg but at this higher level we usually say that g = 9.81 N kg^-1 (or 9.81 ms^-2).

Newton's Third Law of Motion

"All forces act in pairs."  (Sometimes phrased as 'every action has an equal and opposite reaction'.)

If you push on something, it pushes back... this is how ice skaters move forward.

The pairs of forces mentioned in this law have to meet some conditions:

they are the same size (equal)
they act in opposite directions (opposite)
they are the same type (e.g. gravitational, electrical, frictional...)
they act on different objects

For example, a common mistake is to think that your weight and the support (reaction) force from your chair make up a Third Law Pair. They don't, because they both act on you! (Think... what are the pairs to these forces?)

Air resistance is a good example here: the air pushes back on you when you push through it.

Calculating Momentum

Momentum is the product of mass and velocity - heavy things moving fast have a lot of momentum.

momentum (kg ms^-1) = mass (kg) x velocity (ms^-1)

Momentum is a vector quantity which means that the direction of the movement matters too - and that momenta in opposite directions can cancel out.

Conservation of Momentum

In collisions (when objects collide) the total momentum afterwards is equal to the total momentum beforehand.

A specific example of this is an explosion, where the TOTAL momentum beforehand is zero... and so the TOTAL momentum of all of the fragments afterwards adds up to zero. This is because momentum is a VECTOR quantity and so opposites cancel out.

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