(a) Define work done by a constant force. Write its SI unit and define this unit.
(b) A 3000kg truck at a speed of `72kmh^(-1)` stops after covering some distance. The force applied by brakes is 2400 N. Compute the distance covered and work done by this force.

Explanation

(a) The work done by a constant force acting on a body is the product of the force and the distance moved by its point of application in the direction of the force.
The SI unit of work is the joule.
Definition of the joule : The amount of work done on a body when a force of 1 N displaces it by 1 m along the line of action of the force.
(b) Here, `m=3000kg,`
`u=72kmh^(-1)=72xx(5)/(18)ms^(-1)=20ms^(-1)`,
F (applied by the brakes) = 2400 N, `s=?`, `W=?`
The force exerted by the brakers,
`F=ma`
`therefore a=(F)/(m)=(2400)/(3000)=0.8ms^(-2)`
Here, the force is braking force, so, the value of a is `-0.8ms^(-2)`.
Now, `v^(2)-u^(2)=2as`
`therefore (0)^(2)-(20)^(2)=2xx(-0.8)xxs` (`v=0`, because the truck stops)
`therefore -(400)=2xx(-0.8)xxs`
`therefore s=(400)/(2xx0.8)=250m`
So, the truck will stop after covering a distance of 250 m.
Work done by the brakes,
`W=Fs=2400Nxx250m=6xx10^(5)J`
Work done by the force applied by the brakes is `6xx10^(5)J`.