Work Energy and Simple Machines Class 9 Worksheet with Answers Science Chapter 7

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Class 9 Work Energy and Simple Machines Worksheet

Worksheet On Work Energy and Simple Machines Class 9

Work Energy and Simple Machines Worksheet Class 9

→ Work (W): Work is done when a force causes displacement, and the displacement has a component in the direction of the force.

→ Joule (J): t is the SI unit of work and energy. One joule of work is done on an object when a constant force of 1 newton is applied to it and it is displaced by one metre in the direction of the force. Thus, 1 J = 1 N × 1 m

→ Positive Work: When the displacement is in the same direction as the applied force, the work done by the force on the object is said to be positive.

→ Negative Work: When the displacement is in the direction opposite to that of the force, then the work done by the force on the object is negative.

→ Zero Work: When the applied force is zero, when there is no displacement, or when the force acts perpendicular to the direction of the displacement.

→ Energy: The capacity of an object to do work. SI unit is joule (J).

→ Work-Energy Theorem: The work-energy theorem states that the net work done on an object is equal to the change in its kinetic energy.

→ Kinetic Energy (KE): Energy possessed by an object due to its motion. KE = – mv².

→ Potential Energy (PE): The energy stored by an object as a result of its deformation or in a system of objects due to their relative positions is called the potential energy.

→ Gravitational PE: Potential energy of an object at height h near Earth’s surface. U = mgh, where m = mass, g = acceleration due to gravity, h = height.

→ Mechanical Energy (ME): Sum of kinetic energy and potential energy of an object.

→ Conservation of ME: In the absence of non-conservative external forces (like friction), the total mechanical energy of an object remains constant.

→ Power (P): Rate at which work is done. P = W/t.

→ Watt (W): SI unit of power. 1 W = 1 J s^-1.

→ Horsepower (hp): Another unit of power commonly used for engines or pumps used to lift water. 1 hp = 746 W.

→ Simple Machine: A device that makes work easier by changing the magnitude or direction of a force, without reducing total work done.

→ Effort: The force applied to a simple machine to do work.

→ Load: The force that a simple machine is used to overcome.

→ Mechanical Advantage: Ratio of load to effort (MA = Load / Effort). MA > 1 means the machine multiplies force.

→ Pulley: A wheel with a groove guiding a rope. A fixed pulley changes the force direction (MA = 1). A movable pulley increases MA.

→ Inclined Plane: A sloping surface used to raise objects. MA = Length of inclined plane / Height. The effort is reduced when the length of inclined plane is increased.

→ Lever: A rigid bar that rotates about a fulcrum. MA = Effort arm/Load arm. Levers are of three classes based on the relative positions of effort, fulcrum and load.

→ Fulcrum: The fixed pivot point about which a lever rotates.

→ Effort Arm: Perpendicular distance from the fulcrum to the point where effort is applied.

→ Load Arm: Perpendicular distance from the fulcrum to the point where load acts.

Class 9 Science Exploration Chapter 7 Worksheet

Class 9 Science Work Energy and Simple Machines Worksheet

A. Multiple-Choice Questions

Question 1.
A boy pushes a wall with a force of 50 N but the wall does not move. The work done by the boy on the wall is
(a) 50 J
(b) 0 J
(c) 20 J
(d) 40 J

Question 2.
A goalkeeper stops a ball by applying a force in the direction opposite to the motion of the ball. The work done by the goalkeeper on the ball is
(a) positive
(b) zero
(c) negative
(d) equal to the ball’s kinetic energy.

Question 3.
A girl carries a heavy box horizontally across a room at constant velocity. The work done by the upward force she applies on the box is
(a) equal to mgh
(b) positive
(c) zero
(d) negative.

Question 4.
If the velocity of a vehicle is doubled, its kinetic energy becomes
(a) 2 times
(b) 4 times
(c) 1/2 times
(d) 3 times.

Question 5.
A ball of mass m is thrown upward with velocity v. At its highest point, its kinetic energy is
(a) \(\frac{1}{2}\) mv²
(b) mgh
(c) zero
(d) \(\frac{1}{2}\) mv² + mgh

Question 6.
The mechanical advantage of a fixed pulley is
(a) greater than 1
(b) less than 1
(c) equal to 1
(d) dependent on the load.

Question 7.
A weightlifter holds a 100 kg barbell steady above her head for 10 seconds. The work done by her on the barbell during this time is
(a) 1000
(b) 10000
(c) 980
(d) 0

Question 8.
An inclined plane has a length of 5 m and a height of 1 m. Its mechanical advantage is
(a) 1
(b) 5
(c) 0.2
(d) 50

Question 9.
This question consists of an Assertion (A) and a Reason (R). Read the Assertion and Reason and choose the appropriate answer.
Assertion (A): The kinetic energy of a freely falling object increases as it falls.
Reason (R): As the object falls, its potential energy decreases and is converted to kinetic energy, keeping mechanical energy constant.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Question 10.
This question consists of an Assertion (A) and a Reason (R). Read the Assertion and Reason and choose the appropriate answer.
Assertion (A): Simple machines reduce the total amount of work to be done.
Reason (R): Simple machines change the magnitude or direction of the applied force.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

B. State True (T) or False (F).

Question 1.
Work is said to be done, when a force is applied even, if the object does not move.

Question 2.
Lifting a bucket vertically upward results in positive work done on the bucket.

Question 3.
The SI unit for both work and energy is joule (J).

Question 4.
A stretched rubber band at rest possesses kinetic energy.

Question 5.
Energy can change from one form into another.

C. Fill in the blanks.

Question 1.
Work done = ______ × ______(in the direction of force).

Question 2.
One joule of work is done when a force of ______ newton displaces an object by one metre in the direction of the force.

Question 3.
The expression for kinetic energy of a body of mass m and velocity v is ______.

Question 4.
The potential energy of an object of mass m at a small height h from Earth’s surface is _______.

Question 5.
Power is defined as the ________ at which work is done.

D. Assign one word to the following.

Question 1.
The energy possessed by an object due to its motion.

Question 2.
The energy stored by an object due to its position above the ground.

Question 3.
The fixed point about which a lever rotates.

Question 4.
The amount of work done per unit time.

Question 5.
The physical quantity whose SI unit is watt.

E. Match the Column 1 with Column II.

Question 1.

Column I	Column II
(i) W = F x s	(a) Effort arm / Load arm
(ii) KE = \(\frac{1}{2}\) mv2	(b) Gravitational energy
(iii) U = mgh	(c) Rate of doing work
(iv) p = W/t	(d) Work done by a constant force
(v) Mechanical advantage	(e) Kinetic energy

F. Very Short Answer Type Questions

Question 1.
Why is the efficiency of a real machine always less than 100%?

Question 2.
State and explain the law of conservation of mechanical energy. Give one example.

Question 3.
While saving a goal, a goalkeeper’s hand moved back by 15 cm as he stopped a ball while applying a force of 200 N. Calculate the work done by the goalkeeper on the ball.

G. Short Answer Type Questions

Question 1.
Explain the three classes of levers in detail with examples, and mention the mechanical advantage of each.

Question 2.
A fielder threw a cricket ball of mass 200 g to a height of 10 m above the ground How much potential energy does the ball have at its maximum height? (g = 10 m s^-2)

Question 3.
Define kinetic energy and potential energy. Using the example of a ball thrown vertically upward, explain how energy is transformed between these two forms. Also state the work-energy theorem.

H. Long Answer Type Questions

Question 1.
A 10.0 kg block is moving on a horizontal floor with negligible friction. As shown in the figure. A variable force is applied on the block in its direction of motion from its position at 0 m till 4 m. If the block had a kinetic energy of 180 J when it was at 0 m, find the block’s speed (i) at 0 m, and (ii) at 4 m. Does the block have negative acceleration in any portion of its motion?

Question 2.
On a seesaw with sliding seats, a child is sitting on one side and an adult on the other side. The adult weighs twice that of the chil(d) The seesaw however is balance(d) Draw a figure which depicts this situation showing the distances from the fulcrum where the child and the adult are seate(d)

Question 3.
The potential energy-displacement graph of a 0.5 kg ball moving along a frictionless track is shown in Fig. At O, the velocity of the ball is 30 j and potential energy is 30 J. Calculate the velocity of the ball at P, Qand R.

Wonder way

A. Give reasons for the following.
Question 1.
A weightlifter feels tired while holding a heavy barbell steady above her head, even though she is not doing any work on the barbell in the scientific sense.

Question 2.
The velocity of a child at the bottom of a slide depends only on the height of the slide, not on the child’s mass or the shape of the slide.

Question 3.
Roads in hilly areas are built with gentle winding slopes rather than going straight uphill.

Question 4.
Climbing an inclined ladder is easier than climbing a vertical ladder to reach the same height.

Question 5.
In a pendulum, the bob slows down over many oscillations and eventually stops, even though mechanical energy should be conserved.

Learn by Doing

A. Label the following diagrams. Idenriv
Question 1.
Label the diagram of the pendulum below, marking where the object has only potential energy, only kinetic energy, and both. Also, indicate the height h which denotes the height from which the bob is released.

Write your observations here.

Question 2.
The figures below show three different methods of raising a block to the same vertical height h. Label each figure as follows:
(i) The arrangement requiring the largest force.
(ii) The arrangement requiring the smallest force.
Give reason for your answer.

B. Observe and record.
The table below shows data about three simple machines. Study the data and answer the questions.

Machine	Load (N)	Effort (N)	Load arm (m)	Effort arm (m)
Lever A	300	100	0.5	1.5
Lever B	200	100	0.4	0.8
Inclined Plane C	500	250	–	–

Question 1.
Calculate the mechanical advantage of each machine. Which gives the greatest mechanical advantage?

Question 2.
If ‘Lever A’ is used to lift a load of 300 IN, how much effort is required? Does this match the table?

Question 3.
For ‘Inclined Plane C’, if its length is 2 m, what is its height?

C. Observe and calculate – Energy Roller Coaster

A ball is released from point A (height = 40 m) on a frictionless roller coaster track. Study the potential
energy displacement graph and answer the questions.

Question 1.
What is the ball’s kinetic energy at each labelled point if mass is 0.8 kg and g is 10 m s^-2 ?

Question 2.
At which point is the ball moving fastest? Justify your answer.

Explore with Cariosity

A. Study the speed-time graph of a car and answer the following questions.

Question 1.
Describe how the car moves between positions A and B (0 s to 1 s). What is its kinetic energy at A, if its mass is 1000 kg?

Question 2.
Calculate the work done by the brakes in bringing the car to rest between B and C.

Question 3.
What form of energy does the kinetic energy of the car transform into when it stops?

B. Analyse and answer.

Question 1.
A jet aircraft of mass 15000 kg approaches an aircraft carrier at 252 km h^-1. A wire exerts a constant backward force of 367500 N to stop it. Calculate: (a) the initial kinetic energy, and (b) the stopping distance.

Question 2.
A truck of mass 10000 kg moving at 72 km h^-1 has its brakes fail. The driver steers it onto an escaperamp inclined at 300. The sand exerts 50000 N opposite to its motion. Find the minimum ramp length needed to stop the truck. (g = 10 m s^-2, Hint: Truck rises 1 m vertically per 2 m along ramp)

Question 3.
A ball of mass 2 kg is thrown upward with a velocity of 20 m s^-1. (a) Identify the sign of work done by gravity during upward and downward motion, (b) If the ball reaches 19.4 m; how much work was done by air resistance? (g – 10 m s^-2)

C. Read the following passage and answer the following questions.

In the Himalayan region, water flowing downhill converts its potential energy into kinetic energy. Traditionally, this energy was harnessed in devices called gharat or panchakki, water mills used to grind grain. These mills still operate in hilly regions today. The water starts from the top with potential energy stored by its height. As it flows down through a pipe, potential energy converts to kinetic energy. The kinetic energy of moving water drives a wheel and sets it into rotational motion. The wheel is connected to a grinding stone at the top. In modern times, dams use the same principle to generate electricity. A dam of height 50 m stores water of mass 1000 kg at its top (g = 10 m s^-2).

Question 1.
Calculate the potential energy of 1000 kg of water stored at the top of the 50 m dam.

Question 2.
If all the potential energy converts to kinetic energy as the water reaches the bottom, what would be the velocity of the water?

Question 3.
Identify the energy transformations that occur in the gharat (water mill) from start to finish.

Question 4.
Why is the actual velocity of water at the bottom less than what you calculated in question 2? What principle explains this?

Suggested Activities

A. Remove both ends of a pen so that the refill can slide freely through the barrel. Attach a rubber band to the pen cap clipped to the side. Stretch the rubber band to different lengths and launch the refill each time. Measure the distance travelled by the refill and record your observations below. What relationship do you notice between the stretched length of the rubber band (elastic PE stored) and distance travelled by the refill (KE gained)?

B. Using cardboard, rulers, pencils (as fulcrum), thread and small weights, build a lever, a pulley system or an inclined plane. Use your model to lift a small load, Measure the effort and load, calculate the mechanical advantage, and compare it to the theoretical value. Describe your design, observations and findings below.

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