# NCERT Class 11 Physics Solutions: Chapter 4 – Motion in a Plane Part 7 (For CBSE, ICSE, IAS, NET, NRA 2022)

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**Question 4.16**:

A cricketer can throw a ball to a maximum horizontal distance of 100 m. How much high above the ground can the cricketer throw the same ball?

**Answer**:

Maximum horizontal distance = R

The cricketer will only be able to throw the ball to the maximum horizontal distance, when the angle of projection is 45°.

i.e.. , θ = 45°.

The relation gives the horizontal range for a projection velocity v:

The ball will achieve the maximum height when it is thrown vertically upward. For such motion, the final velocity v is zero at the maximum height H.

Acceleration, a = – g

Using the third equation of motion:

**Question 4.17**:

A stone tied to the end of a string 80 cm long is whirled in a horizontal circle with a constant speed. If the stone makes 14 revolutions in 25 s, what is the magnitude and direction of acceleration of the stone?

**Answer**:

Length of the string,

Number of revolutions = 14

Time taken = 25 s

Frequency,

Angular frequency, ω = 2πν

Centripetal acceleration,

The direction of centripetal acceleration is always directed along the string, toward the centre, at all points.

**Question 4.18**

An aircraft executes a horizontal loop of radius 1.00 km with a steady speed of 900 km/h. Compare its centripetal acceleration with the acceleration due to gravity.

**Answer**:

Radius of the loop, r = 1 km = 1000 m

Speed of the aircraft,

Centripetal acceleration,

Acceleration due to gravity, g = 9.8

**Question 4.19**:

Read below each statement carefully and state, with reasons, if it is true or false:

(a) The net acceleration of a particle in circular motion is always along the radius of the circle towards the centre

(b) The velocity vector of a particle at a point is always along the tangent to the path of the particle at that point

(c) The acceleration vector of a particle in uniform circular motion averaged over one cycle is a null vector

**Answer**:

**(a)** The net acceleration of a particle in circular motion is always along the radius of the circle towards the centre: **False**

**Explanation**:

The net acceleration of a particle in circular motion is not always directed along the radius of the circle toward the centre. It happens only in the case of uniform circular motion.

**(b)** The velocity vector of a particle at a point is always along the tangent to the path of the particle at that point: **True**

**Explanation**:

At a point on a circular path, a particle appears to move tangentially to the circular path. Hence, the velocity vector of the particle is always along the tangent at a point.

**(c)** The acceleration vector of a particle in uniform circular motion averaged over one cycle is a null vector: **True**

**Explanation**:

In uniform circular motion (UCM) , the direction of the acceleration vector points toward the centre of the circle. However, it constantly changes with time. The average of these vectors over one cycle is a null vector.