JPO 152 Assignment 4 – Kinetic Energy & Work

Due date: 28th March 2014

Name:
Student No:
Field Of Study:
Group:
Total / _____/20

I am aware of the University of Pretoria’s plagiarism clause and steps that may be taken against me,should I be found guilty. This work is my original effort.

Signature: ______

Question 1[6]

a)Two forces which do zero work in most cases are static friction and the normal force. These forces however do zero work for two very different reasons. Explain when and why each of these forces does zero work. (3)

where is angle between and . does no work since in most cases it is perpendicular to motion. In all casesdoes no work since it only applied to stationary bodies and in that case

b)If an object moves from any point A to any point B at constant velocity then the net work done on this object is zero. If the net work done on the object between any two points A and B is zero then does this mean that the velocity the box travels at between these two points is a constant value? Explain or give a counter example (3)

No. The object just needs to have the same initial and final velocities in order for the net work to be zero as is stipulated by the Work-Kinetic energy theorem. The value of velocity between the two points may vary so long as this criteria is met

Question 2 [8]

In a foolhardy vendetta, the political sciences students kidnapped your physics teacher because he said political science wasn’t a real science. They locked him up in a crate (combined mass of 230 kg)and suspended him over a pit of lava from the end of a rope of length L=12.0 m. In an attempt to get him to safety you push horizontally on the crate with a varying force to move it a distance d=4.00 m to the side where there is a ledge where the crate is at rest. During the motion calculate:

(a)The work done on the crate by the gravitational force(2)

(b)The work done on the crate by the pull of the tension in the rope(2)

(tension is always perpendicular to motion of box)

(c)The net work done on the crate(2)

(box starts and ends at rest)

(d)The work done by the force (2)

Question 3 [6]

A ball at the end of a piece of string is capable of swinging like a pendulum about a fixed point as illustrated below.

The ball moves from points 1 through to point 5 and as it does so it encounters three forces, namely: gravity, tension and air resistance

a)Describe the work done on the ball due to gravity as it moves from point 1 to point 5. Justify all statements during your description (4)

From points 1 to 3 the work done is positive since and here is negative. Likewise from points 3 to 5 then is positive and thus work is negative hence the total work done by gravity is increasing from points 1 to 3 and then slowly decreases from point 3 to 5 and ends up at zero at point 5 (same final height as initial height)

b)Which force always does zero work on the ball? Explain(2)

Tension. It is always perpendicular to the motion of the ball, hence and thus