Force and Motion | Chapter 6 | Grade-7 | Science and Technology Notes - New Course 2079-2081
Test Your Concept!
1. Fill in the blanks.
a. The distance travelled in unit time is called speed.
b. The formula which is used to calculate work is w = F × d.
c. To lift a body is a type of work against gravity.
d. The SI unit of weight is newton (N).
e. The rate of doing work is called power.
Also Check:
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Scientific Studies | Chapter 1 | Grade-7 | Science and Technology Notes - New Course 2079-2081
2. Multiple choice questions (MCQs):
a. Work is calculated by using the formula:
A) w = F∕d
B) w = F × d
C) w = d∕F
D) w = F × v
Answer: B) w = F × d
b. Which one of the following is the work against friction?
A) pulling water from well
B) throwing stone upward
C) rolling a drum
D) flying a kite
Answer: C) rolling a drum
c. Which of the following is the SI unit of velocity?
A) m
B) m/s
C) m/s²
D) N
Answer: B) m/s
d. One hp is equal to:
A) 1000 Watt
B) 746 W
C) 600 W
D) 500 W
Answer: B) 746 W
e. Which one of the following is a vector quantity?
A) velocity
B) speed
C) length
D) volume
Answer: A) velocity
f. The shortest path between two points is:
A) distance
B) displacement
C) length
D) breadth
Answer: B) displacement
g. What is indicated by AC in the diagram?
A) distance
B) displacement
C) vector
D) scalar
Answer: B) displacement
3. Differentiate between:
a. distance and displacement
Answer: The differences between distance and displacement are - (choose as per your need😊)
| Distance | Displacement |
|---|---|
| 1. The length of the path travelled by a body in any direction is called distance. | 1. The shortest distance between two points towards a particular direction is called displacement. |
| 2. Distance is a scalar quantity. | 2. Displacement is a vector quantity. |
| 3. Distance can never be negative, always positive. | 3. Displacement can be negative, positive or even zero. |
| 4. Distance does not deacrease with time. | 4. Displacement can increase or decrease with time. |
| 5. Distance is not indicated with an arrow. | 5. Displacement is always indicated with an arrow. |
| 6. Distance depends upon the path i.e. it changes according to the path taken. | 6. Displacement does not depend upon the path and it only depends upon the initial and final position of the body. |
| 7. Distance is denoted by “d”. | 7. Displacement is denoted by “s”. |
| 8. The formula for distance is Distance(d) = Speed(s) × Time (t). | 8. The formula for displacent is Displacement(s) = Velocity(v) ×Time(t). |
b. speed and velocity
Answer: The differences between speed and velocity are - (choose as per your need😊)
| Speed | Velocity |
|---|---|
| 1. The distance traveled by a body in a unit of time is called speed. | 1. The distance traveled by a body per unit time on a straight line is called velocity. |
| 2. Speed defines how fast an object is moving. | 2. Velocity defines how fast an object is moving but in which direction. |
| 3. Speed is a scalar quantity. | 3. Velocity is a vector quantity. |
| 4. Speed can never be negative or zero. | 4. Velocity can be zero, negative, or positive. |
| 5. Speed may or may not be equal to velocity. | 5. An object may possess different velocities but the same speed. |
| 6. Speed is the rate of change of distance. | 6. Velocity is the rate of change of displacement. |
| 7. The formula for Speed is: Speed = Distance (d)/Time (t). | 7. The formula for velocity is: Velocity = Displacement (s)/Time (t). |
| 8. For example: A person making a roundabout journey at a speed of 30 kmph. | 8. For example: A woman plans to drive her car at a speed of 56 kmph in East Pokhara. |
c. work and power
Answer: The differences between work and power are - (choose as per your need😊)
| Work | Power |
|---|---|
| 1. Work is defined as the amount of energy required to displace an object from one place to another. | 1. Power is defined as the rate of doing work. |
| 2. Work is determined by factors like force and displacement. | 2. Power is determined by work done and time. |
| 3. The work done does not depend on the time. | 3. Power spent depends on the time. |
| 4. Work is the most frequently used concept in our regular living. | 4. Power is a comparatively less used concept but is of the same importance. |
| 5. The SI unit of work is Joules and it is denoted by J. | 5. The SI unit of power is the Watt and it is denoted by W. |
| 6. The formula for work is: Work = Force x Distance (W = F x d). Or, W = m.g.h. [∴ F = mg and d = h] | 6. The formula of power is: Power (P) = Work Done (W)/Time taken (t) |
4. Give reasons:
a. Sliding a box is the work against friction.
Answer: Sliding a box is work against friction because force applied to move the box is in oppose of friction.
Or,
Sliding a box is a work done against friction because frictional force resists any form of motion of an object and works against the kinetic energy of the body.
Or,
The frictional force acts in the opposing direction of motion in all situations. So, sliding a box is the work against friction.
b. Jumping up is the work against gravity.
Answer: Jumping up is the work against gravity because gravity is pulling us downward and jumping goes against downward.
Or,
The earth attracts every objects, including us towards its centre. When we jump, the gravity is still acting on us. So, jumping up is the work against gravity.
c. If two persons do the same amount of work, they may have a different power.
Answer: If two persons do the same amount of work, they may have different power because the time taken by two person to complete the same task differs from each other. And we know that power is affected by time taken.
Or,
If two persons do the same amount of work, they may have a different power because the amount of time taken by them to do the same amount of work might be different.
d. A speed is a scalar physical quantity.
Answer: A speed is a scalar physical quantity because it has only magnitude and no defined direction.
Or,
A speed is a scalar physical quantity because it has no defined direction and only magnitude.
e. The weight is dependent upon the mass of a body at a given place.
Answer: The weight is dependent upon the mass of a body at a given place because acceleration due to gravity is different for different places.
f. A person slips over banana peels.
Answer: A person slips over banana peels because the inner side of banana peel being smooth and slippery reduces the friction between the sole of our shoe and the surface of road.
Or,
When we step on a banana peel, it reduces the friction between the feet and the ground surface. So, a person slips over banana peels.
Or,
The banana peel makes the surface of the ground smooth and the friction between a person's feet and ground decreases. So, a person slips over banana peels.
Or,
A person slips over banana peels because the friction between banana peel and ground is very small.
g. Football players' boots have studs (spikes) at their soles.
Answer: Football players' boots have studs at their soles because the smaller area of the studs creates larger pressure which gives a better grip with ground. So, the players can run faster, turn around easily without causing any slipping issues.
Or,
Studs provide better grip with ground which helps them to move easily and run faster without slipping. So, football players' boots have studs at their soles.
Or,
Football players' boots have studs (spikes) at their soles because studs (spikes) increase frictional force between the ground and the shoes which makes players feel comfortable while playing and running.
h. The wheels of a car slip in mud.
Answer: The wheels of a car slip in mud because of very less friction between the wheels and the mud.
Or,
The mud is soft. When the wheel is rotating, the car will be deeper and deeper in mud. This happens because there is no grip (friction). Car needs grip in order to start from the mud. So, because of the less friction the wheels of a car slip in mud.
i. Fruits drop down from trees.
Answer: Fruits drop down from trees because of the force of gravity. As the earth attracts all the objects towards its centre.
Or,
Due to the force of gravity, fruits drop down from trees.
j. Lubricants are used in machines.
Answer: Lubricants are used in machine to reduce the friction between the parts of the machines which are connected together.
Or,
Lubricant reduces friction and so reduces wear and tear of the machine components and increases its life. So, lubricants are used in machines.
5. Short answer type questions:
a. What is force? Write its SI unit.
Answer: Force is an external agency that changes or tends to change the state of rest or of uniform motion of a body on a straight line. SI unit of force is Newton (N).
Or,
Force is a push or a pull that changes or tends to change the state of rest or motion of an object or changes the direction or shape of an object. Its SI unit is Newton (N).
b. Write the relationship between displacement, velocity and time.
Answer: The relationship between displacement, velocity and time is given below -
Velocity (V) = Displacement (s)/Time taken (t).
c. What is gravitation?
Answer: The force of attraction between any two bodies in the universe is called gravitation.
d. What is gravity?
Answer: The force with which a body is attracted towards the centre of the earth is called gravity.
e. What is acceleration due to gravity? Write its value on the earth.
Answer: The acceleration produced on a body due to the gravity of the earth is called acceleration due to gravity. Its value on the earth is 9.8 m/s².
f. Write one application of gravity on the earth.
Answer: One application of gravity on the earth is - the sun's gravity keeps Earth in orbit around it.
g. What can we say about friction when Ram cannot move a box by applying force?
Answer: We can say there is a static friction when Ram cannot move a box by applying force.
h. What are the two factors that affect friction?
Answer: The two factors that affect friction are adhesion and surface roughness.
i. Will force of friction comes into play when a rain drop rolls down a glass window pane?
Answer: Yes, the force of friction will come into play when a rain drop rolls down a glass window pane. But as the glass window pane surface is smooth, the frictional force is not so strong to hold the rain drop and thus rain drop rolls down.
j. What is meant by one newton force (work)?
Answer: One newton force (work) is defined as the force that is required to accelerate a mass of 1 kg by 1 m/s² in the direction of applied force.
k. Define one watt power.
Answer: One watt power is defined as that power in which one joule work is done in one second.
∴1 W = 1J/1S
6. Long answer type questions:
a. Define work and power with their units. Write the relationship of work with the distance and force. What are the factors of work?
Answer: Work is defined as that activity in which a forces covers some distance in the direction of the applied force. The SI unit of work is Joule (J).
Power is the rate of doing work. The SI unit of power is watt (W).
The relationship of work with the distance and force is -
Work (W) = Force (F) × Distance (d)
The factors of work are - Force, Displacement and Angle between Force and Displacement.
b. What is frictional force? Write any three advantages and two disadvantages of friction.
Answer: Frictional force is the force that always acts to resist the motion of one body sliding on another.
The three advantages and two disadvantages of friction are - (Choose as per your needs 😊)
Advantages of Friction:
(i) Friction helps us to walk, turn and stop.
(ii) Friction helps us to write on a paper or on a board.
(iii) Friction helps in stopping a vehicle on applying the brakes.
(iv) Objects can be piled up without slipping.
(v) Friction helps us to hold on to objects
Disadvantages of Friction:
(i) Friction reduces efficiency of machines.
(ii) Friction opposes motion, so more energy is needed to overcome it.
(iii) Friction produces unwanted noise.
(iv) Forest fires are caused due to friction between branches of trees.
(v) Friction causes wear and tear on the material of the moving body.
c. What is pushing force? Write with an example.
Answer: Pushing force is defined as a force that causes an object to move away from the person who is applying the force from its state of rest. When an object is pushed, it tends to move away. An example of pushing force is - kicking a ball, pushing a trolley, closing a door, inserting a plug into the socket, opening a door, the rocks on the hills to fall down, etc.
Or,
The force that pushes or tends to push a body is called pushing force. An example of pushing force is - pushing a trolley.
d. Write any three ways to reduce friction.
Answer: The three ways to reduce friction are -
(i) By polishing the surfaces so as to make them smooth.
(ii) By applying lubricants (like oil or grease) to the rubbing surface.
(iii) By using ball bearing between the moving parts of machine.
e. Why is the handle of a badminton racquet usually rough? Explain with reason.
Answer: The handle of a badminton racquet is usually rough so that badminton racquet does not slip out of the player’s hands during playing. The frictional force between the handle of a badminton racquet and the hands is caused by roughness, and gripping is impossible without it. So, to have a better grip on the racquet it should be rough because the rough surface decreases the friction and allow you to have a firm grip on the racquet.
Or,
The handle of a badminton racquet is usually rough, so that while playing, a badminton racquet does not slip away from the hands of the player. Roughness is responsible for the frictional force between handle of a badminton racquet and hands, without which firm gripping is not possible.
7. Diagrammatic questions:
Study the given figure and answer the following questions:
Here,
In context of Ram -
Mass (m) = 40 kg
Distance (d) = 200 m
Time taken (t) = 30 s
Acceleration due to gravity (g) = 9.8 m/s²
Force (F) = ?
Work (W) = ?
Power (P) = ?
We know that,
Force (F) = mass of object × acceleration due to gravity = m × g = 40 × 9.8 = 392 N
∴ Force (F) = 392 N.
And,
Work (W) = Force (F) × Distance (d) = 392 × 200 = 78400 J
∴ Work (W) = 78400 J.
Again,
Power (P) = Work done/Time taken = W/t = 78400 J/30 s = 2, 613.33 W
∴ Power (P) = 2,613.33 W.
In context of Rita -
Mass (m) = 40 kg
Distance (d) = 200 m
Time taken (t) = 20 s
Acceleration due to gravity (g) = 9.8 m/s²
Force (F) = ?
Work (W) = ?
Power (P) = ?
We know that,
Force (F) = mass of object × acceleration due to gravity = m × g = 40 × 9.8 = 392 N
∴ Force (F) = 392 N.
And,
Work (W) = Force (F) × Distance (d) = 392 × 200 = 78400 J
∴ Work (W) = 78400 J.
Again,
Power (P) = Work done/Time taken = W/t = 78400 J/20 s = 3,920 W
∴ Power (P) = 3,920 W.
a. Who does more work?
Answer: Ram and Rita (both of them) do the same work (W = 78400 J).
b. Who has more power? Why?
Answer: Rita has more power because she has finished her work in less time than Ram for the same work. (∵ Rita's power P = 3,920 W and Ram's power P = 2,613.33 W)
c. What is Ram's power?
Answer: Ram's power is 2,613.33 W.
d. What is Rita's power?
Answer: Rita's power is 3,920 W.
8. The diagram shows two boys applying a force on a box. If the magnitude of the force applied by each is equal, will the box experience any force of friction?
9. Numerical problems:
a. A car covers a distance of 200 m. If its velocity is 20 m/s, calculate the time taken.
Answer:
Here,
Distance travelled (d) = 200 m
Velocity (v) = 20 m/s
Time taken (t) = ?
We know that,
Velocity (v) = Distance travelled (d)/Time taken (t)
or, 20 = 200/t
or, t = 200/20
or, t = 10
∴ Time taken (t) = 10 s.
b. How much is the velocity of a body when it travels 600 m in 5 minutes?
Answer:
Here,
Distance travelled (d) = 600 m
Time taken (t) = 5 minutes = 5 × 60 s = 300 s
Velocity (v) = ?
We know that,
Velocity (v) = Distance travelled (d)/Time taken (t) = 600 m/300 s = 2 m/s.
∴ Velocity (v) = 2 m/s.
c. Hari drags a load of 60 kg along a distance of 12 m. What amount of work does he do? Also mention the type of work.
Answer:
Here,
Mass of an object (m) = 60 kg
Acceleration due to gravity (g) = 9.8 m/s²
Distance (d) = 12 m
Force (F) = ?
Work (W) = ?
We know that,
Force (F) = m × g = 60 kg × 9.8 m/s² = 588 N
∴ Force (F) = 588 N.
Now,
Work (W) = Force (F) × Distance (d) = 588 N × 12 m = 7056 J.
∴ Work (W) = 7056 J.
So, the amount of work done by Hari is 7056 J.
The type of work done is against friction - sliding friction.
d. Rahul is carrying a load of 80 kg along a distance of 200 metres in 5 minutes. But the same load is carried by Reena to the same distance in 4 minutes. Who has more power? Show it by calculation.
Answer:
Here,
In context of Rahul -
Mass (m) = 80 kg
Distance (d) = 200 m
Time taken (t) = 5 minutes = 5 × 60 s = 300 s
Acceleration due to gravity (g) = 9.8 m/s²
Force (F) = ?
Work (W) = ?
Power (P) = ?
We know that,
Force (F) = mass of object × acceleration due to gravity = m × g = 80 × 9.8 = 784 N
∴ Force (F) = 784 N.
And,
Work (W) = Force (F) × Distance (d) = 784 × 200 = 156,800 J
∴ Work (W) = 156,800 J.
Again,
Power (P) = Work done/Time taken = W/t = 156,800 J/300 s = 522.67 W
∴ Power (P) = 522.66 W.
In context of Reena -
Mass (m) = 80 kg
Distance (d) = 200 m
Time taken (t) = 4 minutes = 4 × 60 s = 240 s
Acceleration due to gravity (g) = 9.8 m/s²
Force (F) = ?
Work (W) = ?
Power (P) = ?
We know that,
Force (F) = mass of object × acceleration due to gravity = m × g = 80 × 9.8 = 784 N
∴ Force (F) = 784 N.
And,
Work (W) = Force (F) × Distance (d) = 784 × 200 = 156,800 J
∴ Work (W) = 156,800 J.
Again,
Power (P) = Work done/Time taken = W/t = 156,800 J/240 s = 653.33 W
∴ Power (P) = 653.33 W.
So,
Rahul's power is 522.67 W.
Reena's power is 653.33 W.
Hence, Reena has more power than Rahul.
e. Sarita has power of 480 watt. Calculate the amount of mass lifted by her up to 10 metres in 12 seconds.
Answer:
Here,
Power (P) = 480 W (watt)
Mass (m) = ?
Distance (d) = 10 m (metres)
Time taken (t) = 12 s (seconds)
Work (W) = ?
Force (F) = ?
We know that,
Power (P) = Work done (W)/Time taken (t)
or, 480 W = Work done (W)/12 s
or, 480 W × 12 s = Work done (W)
or, Work done (W) = 480 W × 12 s
or, Work done (W) = 5,760 J
∴ Work done (W) = 5,760 J.
Again,
Work done (W) = Force (F) × Distance (d)
or, 5,760 J = Force (F) × 10 m
or, 5,760 J/10 m = Force (F)
or, Force (F) = 5,760 J/10 m
or, Force (F) = 576 N
∴ Force (F) = 576 N.
Now,
Force (F) = Mass (m) × Acceleration due to gravity (g)
or, 576 N = Mass (m) × 9.8 m/s²
or, 576 N/9.8 m/s² = Mass (m)
or, Mass (m) = 576 N/9.8 m/s²
or, Mass (m) = 58.78 Kg
∴ The amount of mass lifted by Sarita is 58.78 Kg.
10. Extra Questions and Answers:
a. What is the standard unit of measurement for work?
Answer: The standard unit of measurement for work is Joule.
b. When a body falls freely under gravity, the work done by gravity is positive or negative?
Answer: If a force acting on a body has a component in the direction of displacement, then the work done by the force is positive. Hence, when a body falls freely under the influence of gravity the work done by the gravity is positive.
c. When a body slides against a rough surface, the work done by friction is positive or negative?
Answer: If a force acting on a body has a component in the direction opposite to displacement, then the work done by the force is negative. Hence, the work done by the frictional force on the body is negative.
d. Superman stops a truck from moving downhill by applying force in it. Is this an example of work being done?
Answer: No. As there is no displacement, no work is done. He is merely holding the car to prevent its descent down the hill.
e. A string is tied to an eraser, a person holds the string and applies a tension force to the string as it made to move in a circle at a constant speed. Is this scenario an example of work being done?
Answer: For uniform circular motion, the force acts perpendicular to the direction of the motion, so the force never does any work upon the object.
#From above grade/class-7 science notes of Chapter-6 "Force and Motion" New Course 2079-2081, we hope students will be able to:
- Define distance, displacement, speed and velocity; and differentiate between: i) distance and displacement, ii) speed and velocity.
- Solve simple numerical problems related to distance, displacement, speed and velocity. Demonstrate the fact that the earth attracts all the bodies towards it's centre.
- Measure the weight of different bodies using a spring balance and calculate weight according to their mass.
- Define friction and demonstrate that friction depends on weight of the body, nature of the surface in contact and shape of the body.
- Tell the measures of reducing friction as well as its use and advantages.
- Introduce work and power and mention their interrelation.
- Solve simple numerical problems related to work and power.
#Thanks for visiting us. We are really happy providing the notes of Grade/Class-7 Science, 'Force and Motion' Notes based on new course of year 2079-2081 under Science and Technology. Please leave comments or suggestions so that we can improve.😊
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