Physics when is work done

A lift moves her 40 m to the top floor of a hospital. Calculate the work done on the doctor by the lift. In a scrum, a rugby team pushes the other team backwards 5 m using a force of N. Calculate the work done moving the other team.

How to calculate work done Work is done when energy is transferred from one store to another. The work done on a system by a constant force is the product of the component of the force in the direction of motion times the distance through which the force acts. Figure 1. No energy is transferred to or from the briefcase.

Energy is transferred to the briefcase and could in turn be used to do work. Here the work done on the briefcase by the generator is negative, removing energy from the briefcase, because F and d are in opposite directions.

To examine what the definition of work means, let us consider the other situations shown in Figure 1. The person holding the briefcase in Figure 1b does no work, for example. Why is it you get tired just holding a load? There must be motion for work to be done, and there must be a component of the force in the direction of the motion.

For example, the person carrying the briefcase on level ground in Figure 1c does no work on it, because the force is perpendicular to the motion. In contrast, when a force exerted on the system has a component in the direction of motion, such as in Figure 1d, work is done—energy is transferred to the briefcase. Finally, in Figure 1e, energy is transferred from the briefcase to a generator. There are two good ways to interpret this energy transfer.

The other interpretation is that the generator does negative work on the briefcase, thus removing energy from it. The drawing shows the latter, with the force from the generator upward on the briefcase, and the displacement downward.

Work and energy have the same units. From the definition of work, we see that those units are force times distance. Thus, in SI units, work and energy are measured in newton-meters. One joule is not a large amount of energy; it would lift a small gram apple a distance of about 1 meter. How much work is done on the lawn mower by the person in Figure 1a if he exerts a constant force of But once up to speed , the tray will stay in its straight-line motion at a constant speed without a forward force.

And if the only force exerted upon the tray during the constant speed stage of its motion is upward, then no work is done upon the tray. Again, a vertical force does not do work on a horizontally displaced object. The equation for work lists three variables - each variable is associated with one of the three key words mentioned in the definition of work force, displacement, and cause.

The angle theta in the equation is associated with the amount of force that causes a displacement. As mentioned in a previous unit , when a force is exerted on an object at an angle to the horizontal, only a part of the force contributes to or causes a horizontal displacement.

Let's consider the force of a chain pulling upwards and rightwards upon Fido in order to drag Fido to the right. It is only the horizontal component of the tension force in the chain that causes Fido to be displaced to the right. The horizontal component is found by multiplying the force F by the cosine of the angle between F and d.

In this sense, the cosine theta in the work equation relates to the cause factor - it selects the portion of the force that actually causes a displacement. When determining the measure of the angle in the work equation, it is important to recognize that the angle has a precise definition - it is the angle between the force and the displacement vector.

Be sure to avoid mindlessly using any 'ole angle in the equation. A common physics lab involves applying a force to displace a cart up a ramp to the top of a chair or box. A force is applied to a cart to displace it up the incline at constant speed.

Several incline angles are typically used; yet, the force is always applied parallel to the incline. The displacement of the cart is also parallel to the incline. Since F and d are in the same direction, the angle theta in the work equation is 0 degrees. Nevertheless, most students experienced the strong temptation to measure the angle of incline and use it in the equation.

Don't forget: the angle in the equation is not just any 'ole angle. It is defined as the angle between the force and the displacement vector. On occasion, a force acts upon a moving object to hinder a displacement. Examples might include a car skidding to a stop on a roadway surface or a baseball runner sliding to a stop on the infield dirt. In such instances, the force acts in the direction opposite the objects motion in order to slow it down.

The force doesn't cause the displacement but rather hinders it. These situations involve what is commonly called negative work. The negative of negative work refers to the numerical value that results when values of F, d and theta are substituted into the work equation. Since the force vector is directly opposite the displacement vector, theta is degrees. The cosine degrees is -1 and so a negative value results for the amount of work done upon the object. Negative work will become important and more meaningful in Lesson 2 as we begin to discuss the relationship between work and energy.

Whenever a new quantity is introduced in physics, the standard metric units associated with that quantity are discussed.