# NCERT Class 11 Physics Solutions: Chapter 5 – Laws of Motion Part 2

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**Question 5.4:**

One end of a string of length ( *l* ) is connected to a particle of mass m and the other to a small peg on a smooth horizontal table. If the particle moves in a circle with speed v the net force on the particle (directed towards the centre) is:

T is the tension in the string. [Choose the correct alternative].

**Question 5.5:**

A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of . How long does the body take to stop?

**Answer:**

Retarding force,

Mass of the body,

Initial velocity of the body,

Final velocity of the body,

Using Newton’s second law of motion, the acceleration(a) produced in the body can be calculated as:

Using the first equation of motion, the time (t) taken by the body to come to rest can be calculated as:

v = u + at

**Question 5.6:**

A constant force acting on a body of mass 3.0 kg changes its speed from to in . The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force?

**Answer:** 0.18 N; in the direction of motion of the body.

**Explanation:**

Mass of the body,

Initial speed of the body,

Final speed of the body,

Time,

Using the first equation of motion, the acceleration (a) produced in the body can be calculated as:

As per Newton’s second law of motion, force is given as:

Since the application of force does not change the direction of the body, the net force acting on the body is in the direction of its motion.

**Question 5.7:**

A body of mass 5 kg is acted upon by two perpendicular forces 8 N and 6 N. Given the magnitude and direction of the acceleration of the body.

**Answer:**, at an angle of 37° with a force of 8 N

**Explanation:**

Mass of the body,

The given situation can be represented as follows:

The resultant of two forces is given as:

is the angle made by R with the force of 8 N.

The negative sign indicates that θ is in the clockwise direction with respect to the force of magnitude 8 N.

As per Newton’s second law of motion, the acceleration (a) of the body is given as: