Question

b) An object of mass 40 kg is been acted upon by a force that changes its velocity from 30 km/h to 25 km/h
How much work is done by the force? Is the work done negative or positive?​

Answer 1

Explanation,

Given,

• Mass of an object (m) = 40 kg
• Initial velocity (u) = 30 km/h
• Final velocity (v) = 25 km/h

To Find,

• Work done by the force? is the work done negative or positive?

Solution,

Firstly, convert km/h into m/s,

Initial velocity (u) = 30 km/h

⇒ u = 30 × 5/18

⇒ u = 1.6 × 5

⇒ u = 8 m/s

Final velocity (v) = 25 km/h

⇒ v = 25 × 5/18

⇒ v = 1.3 × 5

⇒ v = 6.5 m/s

According to the work - Energy theorem,

Work done = Change in kinetic energy

.°. W = ΔK.E

⇒ W = ¹/2 × m × (v² - u²)

⇒ W = ¹/2 × 40 × (6.5² - 8²)

⇒ W = 20 × (42.25 - 64)

⇒ W = 20 × -21.75

⇒ W = -435 J

Therefore,

The work done by the force is -435 J and is the work done negative.