# 7th Std Science Term 1 Solution in English | Lesson.2 Force and Motion

## Lesson 2. Force and Motion

### I . Choose the correct answer.

1. A particle is moving in a circular path of radius r. The displacement after half a circle would be

1. Zero
2. R
3. 2r
4. r/2

Ans : 2r

2. From the given v-t graph it can be inferred that the object is 1. in uniform motion
2. at rest
3. in non uniform motion
4. moving with uniform acceleration

Ans : in uniform motion

3. Which of the following figures represent uniform motion of a moving object correctly? Ans : d

4. Suppose a boy is enjoying a ride on a marry go round which is moving with a constant speed of 10 m/s. It implies that the boy is

1. at rest
2. moving with no acceleration
3. in accelerated motion
4. moving with uniform velocity

Ans : moving with no acceleration

5. What is one way you might increase the stability of an object?

1. lower the centre of gravity
2. raise the centre of gravity
3. increase the height of the object
4. shorten the base of the object

Ans : lower the centre of gravity

### II. Fill in the blanks:

1. The shortest distance between the two places is ___________.

Ans : Straight Path

2. The rate of change of velocity is_________.

Ans : Acceleration

3. If the velocity of an object increases with respect to time, then the object is said to be in__________ acceleration

Ans : Positive

4. The slope of the speed–time graph gives ___________.

Ans : Positive

5. In ___________ equilibrium its centre of gravity remains at the same height when it is displaced.

Ans : Positive

### III. Match the items

 1. Displacement Knot 2. Light travels through vacuum Geometric centre 3. Speed of ship Metre 4. Centre of gravity of the geometrical shaped object Larger base area 5. Stability Uniform velocity Ans : 1 – C, 2 – D, 3 – A, 4 – E, 5 – B

### IV. Use the analogy to fill in the blank:

1. velocity : metre/ second : : acceleration :  _________

Ans: metre/second2

2. length of scale : metre : : speed of aeroplane _________

Ans : Knot

3. displacement / time : velocity : : speed / time _______

Ans : Distance

### VII. Give very short answer

Mark the correct choice as

1. If both assertion and reason are true and reason is the correct explanation of assertion.
2. If both assertion and reason are true, but reason is not the correct explanation of assertion.
3. Assertion is true but reason is false.
4. Assertion is false but reason is true.

1. Assertion : Volume of a stone is found using a measuring cylinder.

Reason : Stone is an irregularly shaped object.

Ans : If both assertion and reason are true and reason is the correct explanation of assertion.

2. Assertion : Wood floats in water.

Reason : Water is a transparent liquid.

Ans : If both assertion and reason are true, but reason is not the correct explanation of assertion.

3. Assertion : Iron ball sinks in water.

Reason : water is denser than iron.

Ans : Assertion is true but reason is false.

### VI. Give very short answer:

1. Name some of the derived quantities.

Area, volume, speed, density.

2. Give the value of one light year.

9.46x 1015m

3. Write down the formula used to find the volume of a cylinder.

πr2h

4. Give the formula to find the density of objects.

Density (D) = Mass (M) / Volume(v)

5. Name the liquid in which an iron ball sinks.

Water

6. Name the units used to measure the distance between celestial objects.

Astronomical unit, Light year

7. What is the density of gold?

19,300 kg/m3

### V. Give short answer :

1. All objects having uniform speed need not have uniform velocity. Describe with the help of examples.

Velocity is the speed of the object in a particular direction.

Examples :

• A car moving on a straight road with uniform velocity.
• A ball rolling on floor.

2. She moves at a constant speed in a constant direction”. Rephrase the same sentence in fewer words using concepts related to motion.

She moves in a straight line path with uniform speed.

3. Correct your friend who says “The acceleration gives the idea of how fast the position changes “.

The acceleration gives the idea of how fast the velocity changes.

1. Show the shape of the distance – time graph for the motion in the following cases.

a. A bus moving with a constant speed.

b. A car parked on aroad side.

2. Distinguish between speed and velocity.

 Speed Velocity 1. Speed is the rate of change of distance Velocity is the rate of changein displacement.. 2. Speed = distance /time Velocity (V) = displacement/ time 3. Sl unit of speed is metre/second SI unit of velocity is metre / second

3. What do you mean by constant acceleration?

An object undergoes uniform acceleration when the change (increase or decrease ) in its velocity for every unit of time is the same.

4. What is centre of gravity?

Centre of gravity : The centre of gravity of an object is the point through which the entire weight of the object appears to act.

1. Explain the types of stability with suitable examples. Stability :

Stability is a measure of the body’s ability to maintain its original position.

The three types of stability are

1. Stable equilibrium
2. Unstable equilibrium
3. Neutral equilibrium

#### Stable Equilibrium • The frustum can be tilted through quite a big angle without toppling.
• Its centre of gravity is raised when itis displaced.
• The vertical line through its centre of gravity still falls within its base.

#### Unstable Equilibrium : • The frustum will topple with the slightest tilting. Its centre of gravity is lowered when it is displaced.
• The vertical line through its, centre of gravity falls outside its base.

#### Neutral Equilibrium : • It causes frustum to topple
• The frustum will rolls about but does not topple.
• Its centre of gravity remains at the same height when itis displaced.
• The body will stay in any position to which it has been displaced.

2. Write about the experiment to find the centre of gravity of the irregularly shaped plate. To find the centre of gravity of the irregularly shaped plate.

Apparatus :

Irregularly shaped card, string, pendulum bob, stand

1. Make three holes in the lamina.
2. Suspend the lamina from the optical pin through one of the holes as shown.
3. Suspend the plumbline from the pin and mark the position of the plumbline on the lamina.
4. Draw lines o  the lamina representing the positions of the plumbline.
5. Repeat the above steps for the holes.
6. Label the intersection of the three lines as X, the position of the centre of gravity of the lamina.

### VIII. Numerical problems.

1. Geetha takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/s, calculate the distance between her house and the school.

 Speed = Distance / time Distance = speed x time Speed of bicycle = 2 m/s time =15 minutes =15 x 60=900 s Distance = 2 x 900 Distance = 1800 m

2. A car started from rest and travelling with velocity of 20 m/s in 10 s. What is its acceleration ?

 Initial velocity of a car, u = 0 Final velocity of a car, v = 20m/s time, t =10 s acceleration a = V-u / t acceleration of a car, a = 20 – 0 / 10 = 2 acceleration of a car, a =2ms-2

3. A bus can accelerate with an acceleration 1 m/s2. Find the minimum time for the bus to reach the speed of 100km/s from 50 km/s.

 Acceleration of a bus, a = 1 mis2 Initial velocity , u = 50 km/s = 50000 /s Final velocity , v = 100 km/s = 100000 /s time, t = v-u / a 100000 – 50000 / 1 = 50000 t = 50000 s

### IX. Fill in the boxes.

 First Move Second Move Distance (m) Displacement 1. Move 4 meters east Move 2 meters west 6 2 m east 2. Move 4 meters north Move 2 meters south 6 2 m north 3. Move 2 meters east Move 4 meters west 6 2 m west 4. Move 5 meters east Move 5 meters west 10 0 5. Move 5 meters south Move 2 meters north 4 0 6. Move 10 meters west Move 3 meters east 13 7 m west

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