Work and Energy Worksheet-4

Work and Energy Worksheet-4

 

  1. A car weighing 1000 kg and traveling at 30 m/s stops at a distance of 50 m decelerating uniformly. What is the force exerted on it by the brakes? What is the work done by the brakes?

 

  1. How much work is done when a body of mass m is raised to a height h above the ground?

 

  1. State the SI unit of work.

 

  1. Is work a scalar or a vector quantity?

 

  1. Define 1 joule of work.

 

  1. What is the condition for a force to do work on a body?

 

  1. Is energy a vector quantity?

 

  1. What are the units of (a) work, and (b) energy?

 

  1. What is the work done against gravity when a body is moved horizontally along a frictionless surface?

 

  1. By how much will the kinetic energy of a body increase if its speed is doubled?

 

Answer:

  1. In order to calculate the force, we have to find out the acceleration or retardation.

Initial speed, u = 30 m/s

Final speed, v = 0      (Since the car stops)

Acceleration  a = ?                

Distance s = 50 m

Now, we know that:

          v2 = u2 + 2as

 So,   (0)2 = (30)2 + 2 × a × 50

          100 a = –900

          a =–(900/100)

      Acceleration, a =9 m/s2

The force exerted by the brakes can now be calculated by using the formula :

          F = m × a

Here, Mass, m = 1000 kg (Given)

Acceleration, a = –9 m/s2

Force, F = 1000 × (–9)

          F = –9000 N

Thus, the force exerted by the brakes on the car is of 9000 newtons. The negative sign shows that it is a retarding force.

The work done by the brakes :

          W = F × s

 Here, Force, F = 9000 N

          Distance, s = 50 m

So, Work done, W = 9000 × 50 J

                   = 450000 J

                   = 4.5 × 105 J

Thus, the work done by the brakes is 4.5 × 105 joules.

 

  1. m × g × h

 

  1. Joule

 

  1. vector

 

  1. When a force of 1 Newton moves a body through a distance of 1 meter in its own direction, then the work done is known as 1 joule. This is

          1 joule = 1 Newton ´ 1 meter

Or     1 J = 1 Nm

 

  1. The force should produce motion in the body.

 

  1. No it is a scalar quantity

 

  1. Joule

 

  1. Zero

 

  1. Becomes four times