Question

1. consider the following physical situations. identify whether the indicated force (in boldface type) does positive work, negative work or no work.

a. positive work \tb. negative work \tc. no work
description of physical situation \t+, -, or no work
a. a cable is attached to a bucket and the force of tension is used to pull the bucket out of a well. \t+
b. rusty nales uses a hammer to exert an applied force upon a stubborn nail to drive it into the wall. \t+
c. near the end of the shockwave ride, a braking system exerts an applied force upon the coaster car to bring it to a stop. \t-
d. the force of friction acts upon a baseball player as he slides into third base. \tno work
e. a busy spider hangs motionless from a silk thread, supported by the tension in the thread. \tno work
f. in baseball, the catcher exerts an abrupt applied force upon the ball to stop it in the catcher’s mitt. \t-
g. in a physics lab, an applied force is exerted parallel to a plane inclined at 30-degrees in order to displace a cart up the incline. \t+
h. a pendulum bob swings from its highest position to its lowest position under the influence of the force of gravity. \t+

Explanation:

Work is defined as $W = Fd\cos\theta$, where $\theta$ is the angle between force ($F$) and displacement ($d$). Positive work: $\cos\theta > 0$ (force and displacement in same direction). Negative work: $\cos\theta < 0$ (opposite directions). No work: $d=0$ or $\cos\theta=0$ (perpendicular).

a. Tension pulls bucket upward (same as displacement): positive.
b. Hammer force drives nail into wall (same direction): positive.
c. Braking force opposes coaster motion: negative.
d. Friction opposes player's sliding: negative (correction: original mark wrong—friction acts opposite displacement).
e. Spider is motionless ($d=0$): no work.
f. Catcher's force stops ball (opposite motion): negative.
g. Applied force is parallel to incline (same as displacement): positive.
h. Gravity component pulls bob downward (same as motion direction): positive.

Answer:

a. Positive Work
b. Positive Work
c. Negative Work
d. Negative Work
e. No Work
f. Negative Work
g. Positive Work
h. Positive Work