Exemplo 4.1 de College Physics, 4.3 Newton’s Second Law of Motion: Concept of a System

What Acceleration Can a Person Produce when Pushing a Lawn Mower?

Problema e solução em inglês, como no livro original.

Suppose that the net external force (push minus friction) exerted on a lawn mower is 51 N (about 11 lb) parallel to the ground. The mass of the mower is 24 kg. What is its acceleration?

A man pushing a lawnmower to the right. A red vector above the lawnmower is pointing to the right and labeled F sub net.

Figure 4.7 The net force on a lawn mower is 51 N to the right. At what rate does the lawn mower accelerate to the right?

Resolva no papel primeiro. Depois abra a solução um passo de cada vez e pare assim que conseguir terminar sozinho.

Estratégia
Since $F_{net}$ and $m$ are given, the acceleration can be calculated directly from Newton’s second law as stated in $F_{net} = m a$ .
Solução
The magnitude of the acceleration $a$ is $a = \frac{F_{net}}{m}$ . Entering known values gives

$a = \frac{51 N}{24 kg}$
4.9

Substituting the units $kg \cdot {m/s}^{2}$ for N yields

$a = \frac{51 kg \cdot {m/s}^{2}}{24 kg} = 2.1 m {/s}^{2} .$
4.10
Discussão
The direction of the acceleration is the same direction as that of the net force, which is parallel to the ground. There is no information given in this example about the individual external forces acting on the system, but we can say something about their relative magnitudes. For example, the force exerted by the person pushing the mower must be greater than the friction opposing the motion (since we know the mower moves forward), and the vertical forces must cancel if there is to be no acceleration in the vertical direction (the mower is moving only horizontally). The acceleration found is small enough to be reasonable for a person pushing a mower. Such an effort would not last too long because the person’s top speed would soon be reached.

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