Module 3 - Section 5 - Newtons Laws Of Motion

Question 1

What is Newton’s first law of Motion ?

Answer 1

It is the law of inertia
A body will to remain at rest or will continue to move in a straight line at a constant speed - unless it gets acted upon by a resultant force

Question 2

Whats happens to a body if the forces aren’t balanced ?

Answer 2

If the forces aren’t balanced then the overall resultant force will cause the body to accelerate

Acceleration can mean a change of speed or direction or both

Question 3

What happens if an apple is sitting on a table and its forces are balanced ?

Answer 3

If an apple has balanced forces then it will continue to sit on the table and will not move and this is because of Newton’s first law - a resultant force is not acting on the apple as it has balanced forces therefore the apple will continue to remain at rest

The forces acting on the apple:
- Weight (mg) - force of gravity pulling the apple to the ground
- Reaction force (R) - force of table pushing apple up

These forces are equal - R=mg

Question 4

Whats Newtons Second law ?

Answer 4

Newton’s second law states that:

The rate of change of momentum is directly proportional to the resultant force which acts on the object

Or

F = change in P / change in t

Question 5

What happens to Newton’s second law if the mass is constant ?

Answer 5

If the mass is constant then F = P / t
Can be written as:

F = ma

Question 6

What’s Newtons Third Law of Motion ?

Answer 6

It is when for e.g. Object A exerts a force on object B, then object B exerts an equal but opposite force on object A

For e.g. if you push against a wall, the wall will push back against you just as hard - as soon as you stop pushing the wall stops pushing as well

Question 7

Why is the book on the table not an example of Newton’s third law ?

Answer 7

Because when there is a book on a table then there is 2 forces that are equal and opposite however they are not the same type of forces therefore the second law doesn’t apply to them - the gravity is a non-contact force and the reaction force is a contact force

For this example - Newton’s first law is used as the forces are equal and opposite therefore there is no resultant force acting on them therefore other body remains at rest and doesn’t accelerate

Question 8

In what situations does Newtons Third Law apply ?

Answer 8

Newton’s third law applies to all situations and to all types of force - but the pairs of forces always have to be of the SAME TYPE - e.g. both gravitational or both electrical forces

Question 9

How is Newtons 3rd law a consequence of the conservation of momentum ?

Answer 9

This is because when a resultant force acts on an object - it means a change in acceleration ( f=ma ) - which means a change in momentum - and if momentum is always conserved so whenever one object exerts a force on another - and changes it momentum ( cs a resultant force means a change in acceleration which means a change in momentum ) the second object must exert an equal sized force back onto the first object so that the overall change in momentum is zero - because if the second object exerted a lesser more bigger force back to the first object then the overall change in momentum would not be zero and momentum would not be conserved as the total momentum after the interaction would not be the same as before the interaction so this wouldn’t work