Question

Using a free body diagram, show that it is easy to pull an object than to push it.

Answer 1

When a body is pushed at an arbitrary angle θ [0 to `π/2`], the applied force F can be resolved into two components as F sin 0 parallel to the surface and F cos 0 perpendicular to the surface as shown in the figure. The total downward force acting on the body is mg + F cos θ. It implies that the normal force acting on the body increases. Since there is no acceleration along the vertical direction the normal force N is equal to

N_push = mg + F cos θ …………(1)

As a result the maximal static friction also increases and is equal to

`f_S^max = mu_rN_{push} = mu_s("mg" + F cos theta)` .................(2)

Equation (2) shows that a greater force needs to be applied to push the object into motion.

An object is pushed at an angle θ

When an object is pulled at an angle θ, the applied force is resolved into two components as shown in the figure. The total downward force acting on the object is –
N_pull = mg – F cos θ ………….(3)

An object is pulled at an angle θ

Equation (3) shows that the normal force is less than – N_push. From equations (1) and (3), it is easier to pull an object than to push to make it move.