Grade 7 Q&A: Chapter 7: Motion, Force and Work

Concept Questions

Q1: What is motion?

Answer: Motion is defined as the change in the position of an object with respect to time and a reference point.

Q2: Differentiate between linear and non-linear motion. Give an example of each.

Answer:

• Linear Motion: Motion in a straight line. Example: A car moving on a straight road.
• Non-linear Motion: Motion that is not in a straight line. Example: A fan blade rotating (circular motion), a child on a swing (oscillatory motion).

Q3: Define speed and velocity. What is the SI unit for both?

Answer:

• Speed: The distance covered by an object in unit time. It is a scalar quantity (only magnitude).
• Velocity: The displacement of an object in unit time in a given direction. It is a vector quantity (magnitude and direction).
• The SI unit for both speed and velocity is metre per second (m/s).

Q4: What is a force? List three effects a force can have on an object.

Answer: A force is a push or a pull. Three effects a force can have on an object are: 1. It can change the state of motion of an object (e.g., start a stationary object moving or stop a moving object). 2. It can change the direction of motion of an object. 3. It can change the shape of an object.

Q5: Name and describe two types of forces with examples.

Answer: 1. Muscular Force: The force exerted by the muscles of living beings. Example: Lifting a bag, pushing a cart. 2. Gravitational Force: The force of attraction exerted by the Earth on all objects towards its center. Example: An apple falling from a tree.

Q6: What is friction force? Give an example of its effect.

Answer: Friction force is the force that opposes the relative motion between two surfaces in contact. An example of its effect is a rolling ball slowing down and eventually stopping on the ground.

Q7: In physics, when is work said to be done?

Answer: In physics, work is said to be done when a force causes a displacement of an object in the direction of the force (or has a component in that direction).

Q8: What are the two essential conditions for work to be done?

Answer: The two essential conditions for work to be done are: 1. A force must be applied to an object. 2. The object must move (be displaced) as a result of the force.

Q9: What is the SI unit of work? Define it.

Answer: The SI unit of work is the Joule (J). One Joule of work is done when a force of one Newton displaces an object by one metre in the direction of the force.

Q10: What is energy, and how is it related to work?

Answer: Energy is the capacity to do work. Work and energy are closely related because when work is done, energy is transferred from one object to another or transformed from one form to another.

Q11: Give an example of oscillatory motion.

Answer: An example of oscillatory motion is a swinging pendulum or a child on a swing.

Q12: Is the motion of the hands of a clock a periodic motion? Explain.

Answer: Yes, the motion of the hands of a clock is a periodic motion because it repeats itself at regular intervals of time (e.g., the minute hand completes a full circle every 60 minutes).

Q13: What is mechanical force? Give an example.

Answer: Mechanical force is the force applied by a machine. Examples include the force used by a mixer to blend ingredients or the force generated by a bicycle's gears to move it forward.

Q14: When is no work done in a scientific sense, even if a force is applied? Give an example.

Answer: No work is done in a scientific sense if there is no displacement of the object, even if a force is applied. An example is pushing a stationary wall that does not move, or standing still while holding a heavy bag (because there is no displacement in the direction of the force).

Q15: A ball is thrown upwards. What force causes it to eventually fall back down?

Answer: The **gravitational force** exerted by the Earth causes the ball to eventually fall back down.

Application-Based Questions

Q16: A student pushes a heavy box across the floor. Is work being done? Explain.

Answer: Yes, work is being done. Work is done because the student is applying a force to the box, and the box is being displaced (moving) in the direction of the applied force.

Q17: A satellite orbits the Earth in a circular path. What type of motion is this, and is it periodic?

Answer: This is an example of **circular motion**, which is a type of non-linear motion. Yes, it is also **periodic motion** because the satellite repeats its path around the Earth at regular intervals of time.

Q18: You are riding a bicycle uphill. What type of force are your leg muscles primarily exerting to move the bicycle forward?

Answer: Your leg muscles are primarily exerting **muscular force** to pedal the bicycle and move it forward against gravity and friction.

Q19: When you rub your hands together, you feel warmth. Which force is responsible for this heat generation?

Answer: **Friction force** is responsible for this heat generation. When you rub your hands together, the friction between the surfaces converts some of the mechanical energy into heat energy, making your hands feel warm.

Q20: A person walks 50 metres to the east in 10 seconds. Calculate their speed and velocity.

Answer: * **Speed:** Speed = Distance / Time = 50 m / 10 s = 5 m/s. * **Velocity:** Velocity = Displacement / Time = 50 m East / 10 s = 5 m/s East. (Note: Displacement is 50 m East as it's a straight line motion).

Q21: Why does a spring regain its original shape after being stretched or compressed? Which force is at play?

Answer: A spring regains its original shape due to **elastic force**. Elastic force is the restoring force exerted by an elastic material that tends to bring it back to its original configuration after deformation (stretching or compressing).

Q22: Give an example from daily life where gravitational force is clearly observed.

Answer: An example from daily life where gravitational force is clearly observed is when you drop an object, like a pen or a ball. It always falls downwards towards the Earth's surface due to the pull of gravity.

Q23: A scientist is observing the random movement of tiny dust particles in the air. What type of motion are these particles exhibiting?

Answer: These tiny dust particles are exhibiting **random motion**. This type of motion involves irregular changes in both direction and speed, often due to collisions with air molecules (Brownian motion).

Q24: If you push against a heavy cupboard, and it moves a short distance, have you done work? Explain using the scientific definition.

Answer: Yes, you have done work. According to the scientific definition, work is done when a force causes a displacement of an object in the direction of the force. Since the cupboard moved a short distance as a result of your push, both conditions for work being done are met.

Q25: How is the concept of 'energy' related to 'work' in simple terms?

Answer: In simple terms, energy is the ability or capacity to do work. You need to have energy to perform work. When work is done, energy is either transferred from one object to another or transformed from one form to another.

Textbook Exercise Solutions

A. Fill in the blanks:

1. The change in the position of an object with respect to time and a reference point is called motion.
2. Motion in a straight line is called linear motion.
3. The force exerted by the muscles of living beings is called muscular force.
4. The force that opposes the relative motion between two surfaces in contact is called friction force.
5. The SI unit of work is the Joule.

B. Match the following:

Column A

1. Linear motion
2. Circular motion
3. Oscillatory motion
4. Random motion
5. Periodic motion

Column B

1. A fan blade rotating
2. A ball rolling on a flat surface
3. A child on a swing
4. A butterfly flying
5. Hands of a clock

Answers:

1. Linear motion - A ball rolling on a flat surface
2. Circular motion - A fan blade rotating
3. Oscillatory motion - A child on a swing
4. Random motion - A butterfly flying
5. Periodic motion - Hands of a clock

C. Answer the following questions:

Q1: What is motion? Explain its different types with examples.

Answer: Motion is the change in the position of an object with respect to time and a reference point.

• Linear Motion: Motion in a straight line. Example: A car moving on a straight road.
• Non-linear Motion: Motion that is not in a straight line. This includes:
  • Circular Motion: Motion along a circular path. Example: A merry-go-round.
  • Oscillatory Motion: To and fro motion about a mean position. Example: A swinging pendulum.
  • Random Motion: Motion that changes direction and speed irregularly. Example: The movement of gas molecules.
• Periodic Motion: Motion that repeats itself at regular intervals of time. Example: The hands of a clock.

Q2: Differentiate between speed and velocity.

Answer:

• Speed: It is the distance covered by an object in unit time. It is a scalar quantity, meaning it only has magnitude (e.g., 50 km/h) but no direction. Formula: Speed = Distance / Time.
• Velocity: It is the displacement of an object in unit time in a given direction. It is a vector quantity, meaning it has both magnitude and direction (e.g., 50 km/h East). Formula: Velocity = Displacement / Time.

Q3: What is a force? Explain different types of forces with examples.

Answer: A force is a push or a pull that can change the state of motion of an object, change its direction, or change its shape.

• Muscular Force: Force exerted by muscles of living beings. Example: Kicking a football.
• Gravitational Force: Force of attraction exerted by Earth on all objects towards its center. Example: A ball falling to the ground.
• Elastic Force: Force exerted by an elastic material when stretched or compressed, tending to restore its original shape. Example: A stretched rubber band snapping back.
• Friction Force: Force that opposes relative motion between two surfaces in contact. Example: A bicycle slowing down when pedaling stops.
• Mechanical Force: Force applied by a machine. Example: Force exerted by a crane to lift heavy objects.

Q4: When is work said to be done in physics? Give an example where work is done and where it is not.

Answer: In physics, work is said to be done when a force causes a displacement of an object in the direction of the force.

• Example where work is done: Pushing a heavy box across the floor. Here, a force is applied, and the box moves (is displaced) in the direction of the push.
• Example where no work is done: A person pushing a wall that does not move. Even though a force is applied, there is no displacement, so no work is done in the scientific sense.

Q5: What is energy? How is it related to work?

Answer: Energy is the capacity or ability to do work. Work and energy are closely related because work is a measure of energy transfer. When work is done on an object, energy is transferred to that object. Conversely, an object that possesses energy has the ability to do work. For example, a moving car (possessing kinetic energy) can do work by pushing another object.

D. Give reasons:

Q1: A ball rolling on the ground eventually stops.

Answer: A ball rolling on the ground eventually stops due to **friction force**. Friction is a force that opposes the motion between the surface of the ball and the ground. This opposing force acts to slow down the ball until it comes to a complete stop.

Q2: It is difficult to walk on a slippery surface.

Answer: It is difficult to walk on a slippery surface because there is very **little friction** between your feet and the surface. Friction is necessary to provide the grip that allows you to push backwards and move forward. Without sufficient friction, your feet tend to slip, making it hard to gain traction and maintain balance.

Q3: Work is not done when a person stands still holding a heavy bag.

Answer: In a scientific sense, work is not done when a person stands still holding a heavy bag because there is **no displacement** of the bag. Although the person is applying an upward force to hold the bag against gravity, if the bag is not moving (i.e., its position is not changing), then no work is being performed on the bag.

Q4: All oscillatory motions are periodic.

Answer: All oscillatory motions are periodic because oscillatory motion is defined as a to-and-fro motion about a mean position that inherently repeats itself over a regular interval of time. For example, a pendulum swings back and forth, completing a full cycle in a fixed duration, thus being periodic.

Q5: A force can change the shape of an object.

Answer: A force can change the shape of an object because force is a push or a pull that can cause deformation in a material. When a sufficient force is applied to an object, it can cause its particles to rearrange or its structure to bend, stretch, compress, or break, leading to a change in its original shape. For example, squeezing a sponge changes its shape.

References

1. Maharashtra State Board 7th Standard Science Syllabus (Based on current curriculum for motion, force, and work)
2. Screenshot_2025_0527_235744.jpg (Provided content and exercise questions)