Group Activity Newton's 2nd Law

Using Newton's 2nd law $$a=\frac{\Sigma F}{m}$$

If m is doubled $$a\ =\frac{\Sigma F}{2m}=\frac{1}{2}a$$

Equilibrium is when the sum of forces is equated to zero. All forces balance out.

Libra-balance (ln)

Equi (same)

The net force is the sum of the forces acting together. Friction opposes the motion so this is indicated by a negative sign therefore $$\Sigma F=300\ N-80\ N\ =\ 220\ N$$

Using Newton's 2nd Law $$a=\frac{\Sigma F}{m}=\frac{220\ N}{220\ kg}=1\ \frac{m}{s^2}$$

$$\Sigma F=F-f=\ 300\ N\ -300\ N\ =\ 0\ N.\$$ Remember that the negative sign for horizontal motion represents the force opposing the body's motion at the point of contact.

The sum of forces is always in the same direction as the acceleration. This comes from the very definition of Newton's 2nd Law: Acceleration of a body is directly proportional to the sum of forces and inversely proportional to its mass.