Understanding Newton's Laws of Motion and Work Done in Physics

WORKDONE Work done amount of work done in moving object Wd (Nm or j) = F(N) X Distance moved (m) 1 Nm = 1 joules (j) 1000j = 1 kilojoules (kj)

Work was described as taking place when a force acts upon an object to cause a displacement When a force acts to cause an object to be displaced, three quantities must be known in order to calculate the work.

Those three quantities are force, displacement and the angle between the force and the displacement. The work is subsequently calculated as force displacement cosine(theta) where theta is the angle between the force and the displacement vectors.

Newton's Law of Motion Newton's laws of motion are three physical laws that, together, laid the foundation for classical mechanics. They describe the relationship between a body and the forces acting upon it, and its motion in response to those forces.

First Law In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force i.e. a body with zero net force acting upon it does not accelerate; that is, such a body is at rest or moving at a constant velocity.

Second Law In an inertial frame of reference, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. (It is assumed here that the mass m is constant

Third Law When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

Gravity acceleration of an object in vacuum without any drag. This is the steady gain in speed caused exclusively by the force of gravitational attraction. Gravitational acceleration is the free fall

Example: Suppose that a force of 80N is used to move an object distance of 5m. 5m 80N Wd (j) = F (N) X dm (m) = 80 N X 5m = 400j

Example 2: The force at the point of shaping tool when it is cutting is 1500N and the length of the cut is 200mm. How much work done during one cutting stroke? Given: F = 1500 N Dm = 200mm = 200mm/1000mm /m = 0.2m Wd(j) = F (N) X dm (m) = 1500N X 0.2m = 300j

Work done by lifting an Object: Wd (j) = Weight of an object X Vertical height (m) Example: A casting has a mass of 20kg. Calculate the work done in lifting it through a height of 8m. Given: Weight of casting = 20kg X 9.81 = 196.2 Height = 8 m Wd (j) = 196.2N x 8m = 1570j

Work done by rotating force: Wd(j) = 2 X N X t N = # of revolution Torque (t) = F(N) x r (radius) Example: In order to tighten a nut force of 50N is applied at a distance of 150mm from the axis of the nut. If the spanner is rotated through 8 revolution how much work has been done in tightening the nut. Given: T = F X r = 50 N X 150mm /1000mm/m = 7.5Nm or j N = 8 rev Wd (j) = 2 X N X t = 2 X 8 X 7.5j = 377j

Work done by cutting a metal: Wd(j) = 2 X N x t Example: The vertical cutting resistance at a lathe tool is 2000N and the work is 50mm diameter. If the work makes 80 revolution find the work done. Given: N = 80 F= 2000N, r = 50mm/2 = 25mm T = F X r = 2000N X 25mm /1000mm/m = 50Nm Wd (j) = 2 X N X t = 2 X 80 X 50Nm = 25140j or 25.14kj

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