Question

A truck is traveling horizontally to the right (Fig. 4–38). When the truck starts to slow

down, the crate on the (frictionless) truck bed starts to slide. In what direction could the net force

be on the crate?

(a)       No direction. The net force is zero.

(b)       Straight down (because of gravity).

(c)        Straight up (the normal force).

(d)       Horizontal and to the right.

(e)        Horizontal and to the left.

A satellite in circular orbit around the Earth moves at constant speed. This orbit is

maintained by the force of gravity between the Earth and the satellite, yet no work is done on the

satellite. How is this possible?

(a)       No work is done if there is no contact between objects.

(b)       No work is done because there is no gravity in space.

(c)        No work is done if the direction of motion is perpendicular to the force.

(d)       No work is done if objects move in a circle.

Answer 1

First question -

Option (d) is the correct answer.

Explanation -

When the truck slows down, suppose it deaccelerates with a m/s^2

So, net stooping force, F = ma (where m is the mass of the truck with crate)

Now, from Newton's third law of motion -

for every action, there is an equal and opposite reaction.

So, this stopping force will move the crate in opposite to its direction, that is, in the RIGHT direction. Of-course the force shall be horizontal.

Second question -

Option (c) is the correct answer.

Explanation -

Work done, W = F*d cos theta

where theta is the angle between force and displacement.

here theta = 90 deg.

so, cos theta = 0

So, work done, W = 0