Forming the N2L equation is the first step to solving many dynamics problems.

- Choose your mass.
- Include each and every force acting on that mass.
- Exclude each and every force not acting on that mass.

Example

Block A, B and C of mass 4.0 kg, 6.0 kg and 2.0 kg respectively are pushed along a smooth horizontal surface by an external force of 24 N.

Calculate

- a) the contact force between block A and B
- b) the contact force between block B and C

Solution

Using *F*_{xy} to denote the contact force that block X exerts on Y,

–

Considering all three blocks as a combined 12 kg mass,

a)

Let’s consider blocks B and C as a combined 8 kg mass,

–

Alternatively, we can consider block A by itself as a 4 kg mass.

b)

Considering block C by itself as a 2 kg mass,

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Alternatively, we can consider blocks A and B as a combined 10 kg mass.

Fully solved,

–

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