Skip to Content Go to sign in Skip to Primary Navigation Skip to Secondary Navigation Skip to Page Navigation Skip to Header Navigation Skip to Footer Navigation Read more about accessability options and our navigation

Physics Classroom is making strides to make our site accessible to everyone. Our site contains 6 navigation areas. The Primary, Secondary, and Page Level navigations have a screen reader version of their nav structure that allows using the left and right keys to navigate sibling navigation items, and up or down keys to navigate parent or child navigation items. The others can be navigated using tabs. The Primary Navigation handles the first 2 levels of site pages. The Secondary (which is not always available) handles the 3rd and 4th level of structure. The Page level navigation allows you to navigate the current page's headings quickly. The Header Navigation contains the Light/Dark Mode toggle, Search, Notifications and account login. The Breadcrumb Navigation contains the breadcrumb of the current page. If the current page has a breadcrumb, you can get to it by skipping to the content and tabbing in reverse (shift plus tab). The Footer Navigation contains links such as Privacy, Contact, about and terms. Some resources contain an Audio Player that can be activated by holding down the T key for 3 seconds, and then using K to pause and resume. While not every area of Physics Classroom is usable purely from keyboard and screen reader, we are committed to continue work on making this possible. If you have questions or need additional help, please use this link to contact us.

Return to screen reader navigation
Home Minds On Work and Energy Mission WE9 Work and Energy Conversions Help Help 8

Mission WE9 Work and Energy Conversions

The Question

A boy in a wagon is moving along a level sidewalk with 20 Joules of kinetic energy. The boy's older brother applies a 25-Newton horizontal force for a distance of 2 meters. The kinetic energy of the boy is now ____ Joules.

Physics Rules

Work - Mechanical Energy Relationships:
If non-conservative forces do net work upon an object, then the total mechanical energy of that object is changed. The sum of the kinetic and potential energies will change as work is done upon the object. The amount of work done on the object by non-conservative forces is equal to the amount of change in mechanical energy.

Formula Fix

Mathematically, work (W) is calculated from knowledge of the force (F) that acts upon an object, the displacement (d) that the force causes, and the angle (Θ) between the force and displacement vectors. The formula is
 
W = F • d • cosine(Θ).

Think About It

In the case of the boy and wagon being pulled along the sidewalk, there is an applied force doing work upon it. The applied force is a non-conservative force and serves to change the total mechanical energy of the boy and wagon (see Know the Law section). The amount of work done can be calculated (see Formula Frenzy section). Since the force is in the direction of the motion, positive work is done in order to add mechanical energy to the object. There will be an overall gain in energy - either due to a gain of kinetic energy, potential energy or both. The wagon is said to be pulled horizontally, so there is no gain in potential energy. The work done on the boy and wagon must be equal to its gain in kinetic energy. The final kinetic energy can be determined from the work done and the initial kinetic energy value.

Hot Link