Chapter 10 - Work And Energy | Exam Idea Class 9 - Page 2 Of 6

A car of mass 900 kg is travelling at a steady speed of 30 m/s against a resistive force of 2000 N, as illustrated in figure.

(i) Calculate the kinetic energy of the car. (ii) Calculate the energy used in 1.0 s against the resistive force. (iii) What is the minimum power that the car engine has to deliver to the wheels? (iv) What form of energy is in the fuel, used by the engine to drive the car?

(𝐢) Kinetic energy = 𝑚𝑣2 =× 900 × (30)2 = × 900 × 900 = 4,05,000 J (𝐢𝐢) Energy used = Work done against resistive force = Force × Distance…

Four men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it. (a) How much work is done by the men in lifting the box? (b) How much work do they do in just holding it? (c) Why do they get tired while holding it? (g = 10 ms-2 )

(a) F = 250 kg × 10 ms-2 (g = 10 ms-2 ) = 2500 N s = 1 m W = F. s = 2500…

A light and a heavy object have the same momentum, find out the ratio of their kinetic energies. Which one has a larger kinetic energy?

Linear momentum of first object, 𝑝1 = 𝑚1𝑣1 and of second object, 𝑝2= 𝑚2𝑣2 But, 𝑝1 = 𝑝2 or, 𝑚1𝑣1 = 𝑚2𝑣2 If 𝑚1 < 𝑚2 then 𝑣1 > 𝑣2…

Show that when a body is dropped from a certain height, the sum of its kinetic energy at any instant during its fall is constant.

The mechanical energy (kinetic energy + potential energy) of a freely falling object remains constant. It may be shown by calculation as follows: suppose a body of mass m falls…

Briefly describing the gravitational potential energy, deduce an expression for the gravitational potential energy of a body of mass m placed at a height h, above the ground.

When an object is raised through a certain height above the ground, its energy increases. This is because the work is done on it, against gravity. The energy present in…

A body thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object? (b) You lift a heavily packed carton of mass m in vertically upward direction through a height h. What is the work done (i) by you on the carton, (ii) by force of gravity on the carton? (c) Anil is doing work at a rapid rate but works for only one hour. Ashok does work at a somewhat slower rate but continues to work for six hours. Who has greater power? Who has more energy?

(a) Work done is zero. This is because equal and opposite work is done in the two paths. (b) (i) Work done by me is positive and having a value…

Two workmen are employed on a building project, as shown in figure. (i) Workman 1 drops a hammer, which falls to the ground. The hammer has a mass of 2.0 kg, and is dropped from a height of 4.8 m above the ground. (a) Calculate the change in gravitational potential energy of the hammer when it is dropped. (b) Describe the energy changes from the time the hammer leaves the hand of workman 1 until it is at rest on the ground. (ii) Workman 2 picks up the hammer and takes it back, up the ladder to workman 1. He climbs the first 3.0 m in 5.0 s. His total weight, including the hammer, is 520 N. Calculate the useful power which his legs are producing.

(i) (a) Mass, m = 2.0 kg, height, h = 4.8 m Change in gravitational potential energy = mgh =2 × 10 × 4.8 = 96 J (b) Potential energy…

Figure shows, in order, five stages of an athlete successfully performing a pole-vault. Describe the energy changes which take place during the performance of the pole-vault, from the original stationary position of the pole-vaulter before the run-up, to the final stationary position after the vault.

Standing : Chemical energy Run-up : Kinetic energy Pole bent : Elastic energy Rise : Potential energy gained Fall : Kinetic energy gained On mat : Heat or sound energy

Give one example of each of the following: (i) Small mass but high kinetic energy (ii) Large mass but low kinetic energy (b) Prove mathematically that the total mechanical energy of a freely falling body in air is conserved.

(a) (i) A cricket/hockey ball which has been hit hard and is travelling fast. (ii) A shot put thrown by an athlete (b) Let the body of mass m at…

The velocity of a body moving in a straight line is increased by applying a constant force F, for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.

v2 – u2 = 2as This gives s = F = ma We can write work done (W) by this force F W = Fs W = ma() = =…

Flash Education