Kerala Syllabus Class 8 Basic Science: Chapter 2 Motion and Force - Questions and Answers
Std 8: Physics: Chapter 2: Motion and Force: Questions and Answers
♦ How can we understand whether an object is moving? Let's analyze the picture given below. What are the situations are shown in the pictures?
2. Person riding a horse
3. Log of wood in a moving lorry
4. Twig in the beak of a flying bird
5. Motion of planets in the solar system
6. A book on a table
♦ Tabulate the situations that the objects change their position relative to the surroundings and those do not.
| Objects changing position relative to surroundings | Objects not changing position relative to surroundings |
|---|---|
| • Flying bird • Running horse • Running lorry | • Table • Book • Giant wheel, logs |
• Objects that change the position with respect to their surroundings are considered to be in motion, while those that do not change position are considered to be stationary.
♦ The above situations are given in the table below. Complete the table suitably by putting ✔marks.
We can state whether the object is in a state of rest or motion, only with reference to another object.
♦ What is a reference object?
The object taken to determine the state of motion or the state of rest of a body is called the reference object.
♦ What is meant by motion? How can we say an object is stationary?
• If an object changes its position relative to the reference object, it is said to be in motion, and if it does not change position, it is said to be stationary.
• Thus, moving objects undergo a change in position, while stationary objects remain fixed in place.
♦ Distance
Distance is the length of path travelled by an object. The SI unit of distance is metre.
♦ The picture of a 400 m track is given below.
When the athlete completes two rounds in a 400 m track, the distance travelled by him/her is 800 m.
♦ What is an odometer?
An odometer is a device used in vehicles to measure the distance travelled. It records the distance in kilometres.
♦ The figure shows a railway track from A to F. Note down the distance travelled by the train upon reaching each place.
♦ Speed
Speed is the distance travelled by an object in unit time. The SI unit of speed is m/s.
♦ Let's do an experiment.
Take a tall glass jar. Mark the top and bottom as A and B, respectively. Fill the glass jar with glycerin and drop a stone from the top. Start the stopwatch when the stone reaches A. Stop the watch when the stone reaches B.
• Time taken for the stone to reach B = 4 s
• Distance travelled = 20 cm
• Distance travelled by the stone in one second = 20 cm/4 s = 5cm/s
• The unit of speed is m/s.
Speed = Distance/Time taken
♦ The speed of vehicles is usually expressed in km/h.
• Observe the relation between km/h and m/s.
1 km/h = 5/18 m/s
• Can you explain how this was calculated?
1 km/h = 5/18 m/s
1 km/h = 1000m/60 x 60 s = 5/18 m/s
♦ Bus A travelled 75 m in 5 seconds. Bus B travelled 112 m in 7 seconds. Which bus was faster?
Distance travelled by bus A = 75 m,
Time Taken = 5 s
Speed of bus A = Distance/Time = 75 m/5 s = 15 m/s
Distance travelled by bus B = 112 m,
Time Taken = 7 s
Speed of bus B = Distance/Time = 112 m/7 s = 16 m/s
Bus B has more speed than Bus A.
♦ Speedometer
A speedometer is the device that shows the speed of a vehicle.
♦ The details of 3 children who participated in a 400 m running race in school sports meet are given in the table below. Can we find the fastest athlete among them?
♦ Look at the clock dial shown. Complete the table based on the movement of the tip of the second hand (P in the figure). You can measure the distance between different points using a thread, right? (Measure the distances and complete the table).
Distance travelled every 10 s = 2 cm
• What is the distance travelled in every 15 seconds?
Distance travelled every 15 s = 3 cm
♦ Uniform Speed, Non-uniform Speed
• If an object travels equal distance in equal intervals of time, it is in uniform speed.
e.g., for uniform speed - clock dial, movement of the Earth
• If an object travels unequal distances in equal intervals of time, the object is said to be in non-uniform speed.
e.g., for non-uniform speed- Birds flying, humans walking, a ball rolling.
♦ Road accidents due to excessive speed and carelessness are in the daily news, aren't they? What can we do to avoid such road accidents?
• Obey traffic rules
• Observe the signboards and signals
• Avoid overspeed
• While driving, avoid drugs and alcohol
• Pedestrians should walk along the footpath
• Walk along the right side of the rod
• Crossroads only at a zebra crossing
• While walking along the road during the night or early morning, avoid dark-coloured clothes.
♦ What do you see in the figure?
• Now, write the definition of force.
Force is that which changes or tends to change the shape, size, or state of motion or direction of a body.
♦ Examine the figures.
| Situation | Force | Through Contact / Non-Contact |
|---|---|---|
| Mango falling downward | Earth's Gravitational Force | Non-Contact force |
| Leaves swaying in the wind | Force of wind | Contact force |
| A magnet attracts a safety pin | Magnetic force | Non-Contact force |
| Hammering a nail | Force of hammer on nail | Contact force |
| Swimming | Force of water | Contact force |
♦ Contact Force, Non-contact Force
The force experienced when objects come into contact with each other is called contact force. The force effected when there is no contact with the object is called non-contact force.
♦ What is the standard unit of force? Why is it represented by a capital letter?
• The standard unit of force is called the newton.
• It is represented by the capital letter N.
• The capital letter is used because it is the first letter of the scientist Isaac Newton’s name, after whom the unit is named.
♦ What is Frictional Force?
When a surface moves or tries to move over another surface, a parallel force is produced between them against their relative motion. This is frictional force.
♦ Let's try an activity.
Take a wooden block and make one side smooth leaving the other rough. Slide its smooth surface down the inclined plane. Then try to slide its rough side down the inclined plane.
• What difference do you feel?
Inference:
• Frictional force is greater on the rough surface
• Speed decreases with an increase in friction.
♦ Sliding friction
It is the friction that occurs between the surfaces when the object is sliding.
♦ Rolling friction
It is the friction that occurs when the object is rolling. Rolling friction less than sliding friction.
♦ Write down the advantages of friction.
• striking a matchstick
• lighting a lighter.
• Helps to hold objects firmly.
• Helps us in walking and running.
• Helps vehicles move without slipping.
• Air resistance helps to slow down the falling of the parachute.
♦ Write down the disadvantages of friction.
• Surfaces in contact wear out.
• Obstructs smooth movement of machine parts.
• Energy loss in the form of heat
• Slow down the speed
♦ Ways to reduce friction
♦ Activity.
Take a wooden block, an ice block, and a rubber piece of the same mass. Allow them to slide down an inclined plane. The area in contact with the inclined surface should be equal in all cases.
The nature of the surfaces in contact influences the frictional force.
♦ Activity.
Take some water in a glass jar and drop a round stone and a sharp-edged stone of equal mass into the water. Which one falls down faster?
The shape of the moving object affects the frictional force.
♦ Lubricants
Substances that help to reduce friction between contacting surfaces are lubricants.
Eg, Oil (liquid), Grease (semi-liquid), graphite (solid)
♦ Why is graphite used as a lubricant in machines?
Graphite is a solid lubricant. It is commonly used as a lubricant between machine parts at high temperatures. Graphite reduces friction and wear, allowing smooth movement of machine parts.
♦ Ball bearings
Ball bearings are used to reduce friction with the axle. Its principle is that rolling friction is less than sliding friction.
♦ Streamlining
The method of reducing friction by changing the shape is called streamlining.
Eg: shape of boat, aeroplane, vehicles, etc.
♦ Write the various ways to reduce friction.
• Polishing surface
• Lubrication
• Using ball bearings.
• Streamlining.
♦ Let’s assess
1. A car starts from A and reaches B, which is 75 m away. The uniform speed of the car is 25 m/s. Another car with a uniform speed of 30 m/s starts from A and reaches B through C. Which car will reach the destination first?
Distance travelled by the second car = 50 m + 70 m = 120 km
Speed = 30 m/s
Time taken by the first car = distance / time = 75 m/25 m/s = 3s
Time taken by the second car = distance / time = 120 m/30 m/s = 4s
The first car reaches the destination first.
2. A bus starts from C and reaches D in 7 s. If the uniform speed of the bus is 50 m/s, find the distance from C to D.
Answer:
Time taken to travel = 7s
Speed of the bus = 50 m/s
Distance travelled by bus = speed x time = 50 m/s x 7s = 350 m
3. How long will it take to hear thunder from 12000 m away?
(The speed of sound is 340 m/s).
Answer:
Distance = 12000 m
Speed = 340 m/s
Time = distance / speed = 12000 m / 340 m/s = 35.29 s
4. Complete the puzzle given below.
• Can move a stationary object.
• Changes the speed of a moving object
• Changes the direction of a moving object
• Change the size of the object
• Changes the shape of an object
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