Motion, Force and Work | Class 7 | Science | Chapter 7 | Maharashtra State Board
Join us as we embark on a captivating journey into the fundamental concepts of motion, force, and work. In this engaging lesson, you’ll dive deep into the intriguing distinctions between distance and displacement, master the art of calculating speed and velocity, and gain a profound understanding of the concept of acceleration.
Whether you’re a diligent student seeking to excel in your physics class or simply driven by curiosity about the forces that shape our world, this video is tailored just for you. Get ready to unlock the secrets of the physical world and gain a deeper understanding of the forces at play in our everyday lives.
🔬 Topics Covered: Distance vs. Displacement: In the first segment, we’ll unravel the essential disparities between distance and displacement. Understanding these concepts is fundamental to grasping the intricacies of motion. Speed and Velocity: Join us as we explore the crucial aspects of speed and velocity. You’ll learn how to calculate these parameters and appreciate their significance in the world of physics. Average vs. Instantaneous Velocity: Delve into the nuances of velocity, both average and instantaneous, to gain a comprehensive understanding of their implications in real-world scenarios. Acceleration and Its Causes: Explore the intriguing concept of acceleration. We’ll delve into what causes objects to change their speed and direction, shedding light on this fundamental aspect of motion. Newton’s First Law of Motion: In this segment, we’ll introduce you to the groundbreaking insights of Sir Isaac Newton’s First Law of Motion. This law lays the foundation for our understanding of inertia and how objects behave when no force acts upon them. Force and Its Measurement: Discover the essence of force and how it’s quantified in the world of physics. Gain a deeper insight into the push and pull that governs the motion of objects. The Work-Energy Relationship: Delight in the fascinating relationship between work and energy, a cornerstone in comprehending the physical world. Understand how work done on an object is directly related to its change in energy.
Questions & Answers
1. Fill in the blanks with the proper words from the brackets.
(stationary, zero, changing, constant, displacement, velocity, speed, acceleration, stationary but not zero, increases)
1. If a body traverses a distance in direct proportion to the time, the speed of the body is constant.
2. If a body is moving with a constant velocity, its acceleration is zero.
3. Distance is a scalar quantity.
4. Displacement is the distance traversed by a body in a particular direction in unit time.
2. Observe the figure and answer the questions.
Sachin and Sameer started on a motorbike from place A, took the turn at B, did a task at C, travelled by the route CD to D and then went on to E. Altogether, they took one hour for this journey. Find out the actual distance traversed by them and the displacement from A to E. From this, deduce their speed. What was their velocity from A to E in the direction AE? Can this velocity be called average velocity?
Ans. 1. Actual distance:
Actual distance = AB + BC + CD + DE
Actual distance = 3 km + 4 km + 5 km + 3 km
Actual distance = 15 km
2. Displacement:
Displacement = AB + BD + DE
Displacement = 3 km + 3 km + 3 km
Displacement = 9 km
3. Speed:
Speed = Total distance travelled / Total time taken
Distance = 15 km = 15,000 meters
Time = 1 hour = 3,600 seconds
Speed = 15,000 meters / 3,600 seconds = 4.17 m/s (approximately) or 15 km/hour
4. Velocity:
Velocity = Displacement / Total time taken
Displacement = 9 km = 9,000 meters
Time = 1 hour = 3,600 seconds
Velocity = 9,000 meters / 3,600 seconds = 2.5 m/s (approximately) or 9 km/hour
5. Yes, this velocity can be called the average velocity because it is the total displacement divided by the total time, and it indicates the overall motion from A to E, taking direction into account. Average velocity considers both the magnitude and direction of motion.
3. From the groups B and C, choose the proper words, for each of the words in group.
A | B | C |
Work | Newton | erg |
Force | Metre | cm |
Displacement | Joule | dyne |
Ans.
A | B | C |
Work | Joule | erg |
Force | Newton | dyne |
Displacement | Metre | cm |
4. A bird sitting on a wire, flies, circles around and comes back to its perch. Explain the total distance it traversed during its flight and its eventual displacement.
Ans. While the bird’s total distance covered during its flight can be relatively long due to its circling around, its displacement is zero because it ends up back at its initial position. Displacement considers the overall change in position, while total distance accounts for the sum of all distances travelled.
5. Explain the following concepts in your own words with everyday examples: force, work, displacement, velocity, acceleration, distance.
Ans. Explanations of these physics’ concepts with everyday examples:
Force:
Definition: Force is a push or pull applied to an object that can cause it to move, accelerate, or change direction.
Everyday Example: When you push a car to get it moving, you’re applying a force. Similarly, when you pull a door to open it, you exert force.
Work:
Definition: Work is done when a force causes an object to move in the direction of the force applied.
Everyday Example: If you lift a heavy box off the ground and place it on a shelf, you’ve done work against gravity because you’ve moved the box upward.
Displacement:
Definition: Displacement is the change in an object’s position from its initial point to its final point, measured in a straight line.
Everyday Example: If you walk from your home to a nearby park, your displacement is the straight-line distance from your home to the park, not the path you took while walking.
Velocity:
Definition: Velocity is the rate of change of displacement per unit of time. It includes both speed and direction.
Everyday Example: Driving at 60 kilometres per hour (kph) northward is an example of velocity because it specifies both the speed (60 kph) and the direction (northward).
Acceleration:
Definition: Acceleration is the rate of change of velocity per unit of time. It means an object can speed up, slow down, or change direction.
Everyday Example: When a car accelerates from a complete stop to a certain speed, it’s experiencing positive acceleration. When it applies the brakes to slow down, it’s experiencing negative acceleration (deceleration).
Distance:
Definition: Distance is the length of the path traveled by an object. It doesn’t depend on direction and doesn’t consider the starting or ending points.
Everyday Example: If you go for a jog around a track, the distance you’ve covered is the total length of the path you ran, regardless of how many laps you did.
These concepts help describe and quantify the motion and interactions of objects in our everyday lives and in the field of physics.
6. A ball is rolling from A to D on a flat and smooth surface. Its speed is 2 cm/s. On reaching B, it was pushed continuously up to C. On reaching D from C, its speed had become 4 cm/s. It took 2 seconds for it to go from B to C. What is the acceleration of the ball as it goes from B to C.
Ans. The acceleration of the ball as it goes from B to C is 2 cm/s².
We know that acceleration is the rate of change of velocity. In this case, the velocity of the ball is increasing from 2 cm/s to 4 cm/s. The acceleration is calculated by dividing the change in velocity by the time taken.
acceleration = change in velocity / time
acceleration = (4 cm/s - 2 cm/s) / 2 s
acceleration = 1 cm/s²
Therefore, the acceleration of the ball as it goes from B to C is 1 cm/s².
I hope this helps! Let me know if you have any other questions.
7. Solve the following problems.
(a) A force of 1000 N was applied to stop a car that was moving with a constant velocity. The car stopped after moving through 10m. How much is the work done? Ans. The work done by the force is calculated by multiplying the force by the distance moved. In this case, the force is 1000 N and the distance moved is 10 m. Therefore, the work done is:
Work = Force x Distance
Work = 1000 N x 10 m
Work = 10000 J
Therefore, the work done is 10000 J.
(b) A cart with mass 20 kg went 50 m in a straight line on a plain and smooth road when a force of 2 N was applied to it. How much work was done by the force? Ans. The work done by the force is calculated by multiplying the force by the distance moved. In this case, the force is 2 N and the distance moved is 50 m. Therefore, the work done is:
work = force * distance
work = 2 N * 50 m
work = 100 J
Therefore, the work done is 100 J
E X T R A
1. Fill in blanks:
1. Displacement is a vector quantity.
2. The velocity of an object can change even while it is moving along a straight line. 3. The instantaneous velocity can be different at different times. 4. Change in velocity per second is called acceleration. 5. The interaction that brings about the acceleration is called force. 6. The scientist Sir Isaac Newton was the first to study force and the resulting acceleration. 7. Ability to do work is called Energy. 8. W = F × S. 9. Force is a vector quantity. 10. The velocity at a particular time is called instantaneous velocity. 11. The speed of a body is the distance traversed per unit time. 12. Unit of acceleration is m/s2 and cm/s2.
13.Force is measured by the acceleration that it produces. 14. Work done by a body with no displacement will be zero.
2. Say whether True or False, correct the false 1 statement:
1. Velocity is distance travelled per unit of time. Ans. False. Speed is distance travelled per unit of time
2. In displacement, both distance and direction are taken into account. Ans. True.
3. Speed = Distance/time. Ans. True
4. Change in speed per second is acceleration. Ans. False. Change in velocity per second is acceleration
5. Work done depends on the force and the displacement. Ans. True
6. C.G.S. unit of acceleration is m/s2. Ans. False. C.G.S. unit of acceleration is cm/s2.
7. M.K.S. unit of force is dyne. Ans. False. M.K.S. unit of force is Newton
8. Force is measured by the acceleration that it produces. Ans. True
3. Write the difference between the following:
1. Speed and Velocity Ans.
Speed | Velocity |
Speed is a scalar quantity that measures how fast an object is moving. It only considers the magnitude of motion and doesn't specify the direction. | Velocity is a vector quantity that not only measures the magnitude of motion (speed) but also includes the direction of motion. It specifies both how fast and in which direction an object is moving. |
Speed is represented as a positive value (e.g. 60km/h or 30m/s) without indicating direction. | Velocity is represented as a vector, including both magnitude (speed) and direction. (e.g. 60 km/h north or 30 m/s east) |
Speed is a scalar because it only has magnitude. | Velocity is a vector because it has both magnitude and direction. |
Speed is a measure of how quickly an object is moving at a particular moment in time. | Velocity is a measure of how quickly an object us moving at a particular moment in time. |
Speed is always positive or zero (it can't be negative). | Velocity can be negative if the object is moving in the opposite direction to the chosen positive direction. |
Formula: Speed = Distance traversed/ Total time | Formula: Velocity = Displacement/ Total time |
2. Distance and Displacement Ans.
Distance | Displacement |
Distance is a scalar quantity that measures the length of the actual path traveled by an object. It is always positive and does not consider direction. | Displacement is a vector quantity that measures the change in position of an object. It includes both the magnitude (distance) and the direction of the change in position. |
Distance is represented as a postive value (e.g. 5 kilometers or 10 meters) without specifying direction. | Displacement is represented as a vector, including both magnitude (distance) direction. (e.g. 5 kilometers east or 10 meters north.) |
Distance is a scalar because it only has magnitude. | Displacement is a vector because it has both magnitude and direction. |
The distance traveled by an object is the total length of its path, regardless of changes in direction. | Displacement is the change in position of an object from its initial point to its final point, considering both the distance and the direction of that change. |
Distance is always positive or zero. | Displacement can be positive (if the object moves in the positive direction), negative (if it moves in the negative direction), or zero (if there is no change in position.) |
4. Solve the following problems!
1. A bus travelled 200 km in the first 3 hours and then 100 kms for the next one and a half hours and then 120 kms for the next one and a half hours. What is the average velocity of the bus if it has moved in a straight line for the whole journey. Ans. The total distance travelled is 200 km + 100 km + 120 km = 420 km.
The total time taken is 3 hours + 1.5 hours + 1.5 hours = 6 hours.
Therefore, the average velocity of the bus is 420 km / 6 hours = 70 km/h.
So, the average velocity of the bus for the entire journey is 70 kilometres per hour.
5. Use your brainpower:
1. Acceleration is a vector quantity. Is force a vector quantity too? Ans. Yes, acceleration is a vector quantity, meaning it has both magnitude and direction. Acceleration is the rate of change of velocity, which is also a vector quantity.
Force is a vector quantity as well, meaning it has both magnitude and direction. Force is the cause of acceleration, so it makes sense that force is also a vector quantity.
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