# First Year Physics Work and Energy vi. Which one of the following converts light energy into electrical energy?(a) electric bulb(b) electric generator(c) Photocell(d) Electric cell

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##### vi. Which one of the following converts light energy into electrical energy?(a) electric bulb(b) electric generator(c) Photocell(d) Electric cell

v. The kinetic energy of a body of mass 2 \mathrm{~kg} is 25 \mathrm{~J} . Its speed is(a) 5 \mathrm{~ms}^{-1} (b) 12.5 \mathrm{~ms}^{-1} (c) 25 \mathrm{~ms}^{-1} (d) 50 \mathrm{~ms}^{-1}

6.1 A man has pulled a cart through 35 \mathrm{~m} applying a force of 300 \mathrm{~N} . Find the work done by the man.(10500 J)

How large a force is required to accelerate an electron \left(\mathrm{m}=9.1 \times 10^{-31} \mathrm{~kg}\right) from rest to a speed of 2.0 \times 10^{7} \mathrm{~ms}^{-1} through a distance of 5.0 \mathrm{~cm} ?

6.5 Define energy give two types of mechanical energy.

When a rocket re-enters the atmosphere its nose cone becomes very hot. Where does this heat energy come from?

What sort of energy is in the following:(b) Water in a high dam

A diver weighing 750 \mathrm{~N} dives from a board 10 \mathrm{~m} above the surface of a pool of water. Use the conservation of mechanical energy to find his speed at a point 5.0 \mathrm{~m} above the water surface neglecting air friction.

A rain drop \left(\mathrm{m}=3.35 \times 10^{-5} \mathrm{~kg}\right) falls vertically at a constant speed under the influence of the forces of gravity and friction. In falling through 100 \mathrm{~m} how much work is done by.(a) Gravity and (b) Friction

Ten bricks each 6.0 \mathrm{~cm} thick and mass 1.5 \mathrm{~kg} lie flat on a table. How much work is required to stack them one on the top of another?

ii. If the direction of motion of the force is perpendicular to the direction of motion of the body then work done will be(a) Maximum(b) Minimum(c) zero(d) None of the above

6.6 A motor boat moves at a steady speed of 4 \mathrm{~ms}^{-1} . Water resistance acting on it is 4000 \mathrm{~N} . Calculate the power of its engine. (16 kW)

6.7 A man pulls a block with a force of 300 \mathrm{~N} through 50 \mathrm{~m} in 60 \mathrm{~s} . Find the power used by him to

A force F acting on an object varies with distance x as shown in the figure. Calculate the work done by the force as the object moves from x=0 to x=6 \mathrm{~m} .

vii. When a body is lifted through a height \mathrm{h} the work done on it appears in the form of its:(a) kinetic energy(b) potential energy(c) elastic potential energy(d) geothermal energy

ix. The energy stored in a dam is(a) electric energy(b) potential energy(c) kinetic energy(d) thermal energy

EXAMPLE 6.7Calculate the power of a pump which can lift 70 \mathrm{~kg} of water through a vertical height of 16 metres in 10 seconds. Also find the power in horse power.

An object has 1 \mathrm{~J} of potential energy. Explain what does it mean?

What sort of energy is in the following:(a) Compressed spring

A force F acts through a distance L . The force is then increased to 3 \mathrm{~F} and then acts through a further distance of 2 L . Draw the work diagram to scale.

6.8 Why fossils fuels are called nonrenewable form of energy?

A ball of mass m is held at a height h_{1} above a table. The table top is at a height h_{2} above the floor. One student says that the ball has potential energy mgh but another says that it _{1} is m g\left(h_{1}+h_{2}\right) . Who is correct?

In which case is more work done? When a 50 \mathrm{~kg} bag of books is lifted through 30 \mathrm{~cm} or when a 50 \mathrm{~kg} create is pushed through 2 \mathrm{~m} across the floor with a force of 50 \mathrm{~N} ?

6.10 An electric motor of 1 \mathrm{hp} is used to run water pump. The water pump takes 10 minutes to fill an overhead tank. The tank has a capacity of 800 litres and height of 15 \mathrm{~m} . Find the actual work done by the electric motor to fill the tank. Also find the efficiency of the system.

x. In Einsteins mass-energy equation c is the(a) speed of sound(b) speed of light(c) speed of electron(d) speed of Earth

A boy uses a catapult to throw a stone which accidentally smashes a green house window. List the possible energy changes.

6.2 A block weighing 20 \mathrm{~N} is lifted 6 \mathrm{~m} vertically upward. Calculate the potential energy stored in it.(120 J)

A car mass 800 \mathrm{~kg} traveling at 54 \mathrm{kmh}^{-1} is brought to rest in 60 meters. Find the average retarding force on the car. What has happened to original kinetic energy?