Newton's Laws

Question 1

What are Newton’s laws used to describe?

Answer 1

They are used to a high degree of accuracy how the motion of a body depends on the resultant force acting on the body. These are defined as ‘classical mechanics’

Question 2

When can you not use Newton’s laws of motion on?

Answer 2

• Speeds close to the speed of light
• Very small bodies (atoms and smaller)

Question 3

What is Newton’s 1st law of motion?

Answer 3

A body will remain at rest or move with a constant velocity unless it is acted on by a net external resultant force.

Question 4

What is constant velocity?

Answer 4

It means a constant speed along a straight line

Question 5

What is inertia?

Answer 5

The reluctance of a body to having its velocity changed

Question 6

Example of Newton’s first law of motion with a stationary box

Answer 6

The box will only move if the push force is greater than friction.

Question 7

Example of Newton’s first law of motion with a box moving

Answer 7

If the push force equals friction there will be no net force on the box and it will move with a constant velocity.

Question 8

Example of Newton’s first law in the inertia trick?

Answer 8

When the card is flicked, the coin drops into the glasses because the force of friction on it due to the moving card is too small to shift it sideways.

Question 9

Newton’s second law of motion

Answer 9

The rate of change of momentum of an object is directly proportional to the resultant force acting on it.

Question 10

If the mass of an object remains constant we can restate the second law as: (talk about acceleration)

Answer 10

The acceleration of a body of a constant mass is directly proportional to the net external resultant force acting on it.

Question 11

Resultant force equation

Answer 11

Resultant force = mass x acceleration

Question 12

Contact force def

Answer 12

Two bodies touch when their repulsive molecular forces (due to electrons) equal the force that is trying to bring them together. The thrust exerted by a rocket is a form of contact force.

Question 13

Friction def (also air resistance and drag forces)

Answer 13

When two bodies are in contact their attractive molecular forces (due to electrons and protons) try to prevent their common surfaces moving relative to each other.

Question 14

Tension def

Answer 14

The force exerted by a body when it is compressed. It is due to repulsive molecular forces.

Question 15

Compression def

Answer 15

The force exerted by a body when it is compressed. It is due to repulsive molecular forces.

Question 16

Fluid upthrust def

Answer 16

The force exerted by a fluid on a body because of the weight of the fluid that has been displaced by the body. Archimedes’s Principle states that the upthrust force is equal to the weight of fluid displaced.

Question 17

Electrostatic force def

Answer 17

Attractive and repulsive forces due to bodies being charged.

Question 18

Magnetic force def

Answer 18

Attractive and repulsive forces due to moving electric charges.

Question 19

Electromagnetic force def

Answer 19

Attractive and repulsive forces due to bodies being charged. Contact, friction, tension, compression, fluid upthrust, electrostatic and magnetic forces are all forms of electromagnetic force.

Question 20

Electro-weak force def

Answer 20

It is now thought that both the electromagnetic and weak nuclear forces are both forms of this fundamental force.

Question 21

Strong nuclear force def

Answer 21

This is a the force responsible for holding protons and neutrons together within the nucleus. It is one of the FUNDAMENTAL forces.

Question 22

What is the gravitational force?

Answer 22

The force exerted on a body due to its mass.
The weight of a body is equal to the gravitational force acting on the body.
Remember W=mg

Question 23

Formula of Thrust and tension

Answer 23

T -mg = ma
or
T =ma + mg

Question 24

Newton’s third law of motion

Answer 24

When body A exerts a force on body B, body B exerts an equal and opposite force along the same line on body A, and both forces are of the same type.

The important thing to note here is that although both forces are the same magnitude, they are acting on different bodies and are therefore having a different effect.

Question 25

Example of Newton’s third law of motion with the Earth and Moon

Answer 25

The gravity pull of the Earth on the moon is responsible for the Moon’s orbital motion.
The Gravity pull of the moon on the Earth causes the ocean tides.

Question 26

Example of Newton’s third law of motion with a rocket

Answer 26

A = THRUST CONTACT push
of the ROCKET ENGINES
DOWN
on the EJECTED GASES

B = CONTACT push
of the EJECTED GASES
UP
on the ROCKET ENGINES

Question 27

Example of Newton’s third law of motion with a person standing on a floor

Answer 27

There are a pair of gravity forces:
A = GRAVITY pull of the EARTH
DOWN on the PERSON
B = GRAVITY pull of the PERSON
UP on the EARTH

And there are a pair of contact forces:
C = CONTACT push of the FLOOR
UP on the PERSON
D = CONTACT push of the PERSON
DOWN on the FLOOR

Note: Neither forces A & C nor forces D & B are
Newton 3rd law force pairs as the are
NOT OF THE SAME TYPE
although all four forces will usually have the
same magnitude.

Question 28

How does an object reach terminal speed?

Answer 28

• When the body is initially released the only significant force acting on the body is due to its weight, the downward force of gravity.
• The body will fall with an initial acceleration = g
• As the body accelerates downwards the drag force exerted by the fluid increases.
• Therefore the resultant downward force on the body decreases causing the acceleration of the body to decrease.
  FRES = (weight – drag) = ma
• Eventually the upward drag force equals the downward gravity force acting on the body.
• Therefore there is no longer any resultant force acting on the body.
• FRES = 0 = ma and so: a = 0
• The body now falls with a constant velocity.
• This is also known as ‘terminal speed’