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Newton’s Laws of Motion




Introducing balanced forces


What is Newton’s first law?

If the forces on an object are balanced, the object

will continue to do what it is already doing:

if the object is stationary, it will remain stationary

if the object is moving, it will continue to move at

the same velocity (at the same speed and in the

same direction).

If the resultant force acting on an object is zero, all the

forces are said to be balanced.

This forms the basis of Newton’s first law of motion,

which states:

The tendency of an object to continue in its state of rest

or uniform motion is called inertia.


Balanced and unbalanced forces

Newton’s first law tells us that the forces acting on an object that is travelling at a constant velocity are balanced.

For example, when a car travels at a constant speed in a

straight line, the resistive forces acting on the car balance

the driving force.

Newton’s first law also tells us that an object that is changing speed or

direction must have unbalanced

forces acting on it.

If the forces acting on the

object are unbalanced,

there is a resultant force

acting on the object.


Examples of the first law?


The effect of resultant forces




Introducing unbalanced forces


If the resultant force acting on an object is not zero, the

forces on the object are said to be unbalanced.

This forms the basis of Newton’s second law of motion,

which states:

What is Newton’s second law?

The acceleration of an object is proportional to the

resultant force acting on the object, and inversely

proportional to the mass of the object.

The object will therefore change velocity. This may be a

change in speed, direction or both.


How is acceleration calculated from force?

Newton’s second law of motion can be written as an equation:

Force is the resultant force of all the forces acting

on an object and is measured in newtons (N).

Mass is measured in kilograms (kg).

Acceleration is measured in metres per second per

second (m/s2).

force = mass × acceleration


How do we use the equation?

A car has a mass of

1,000kg. What force

must the car’s engine supply to cause an

acceleration of 2m/s2?


= 1,000 × 2

= 2,000N


Using a formula triangle

A formula triangle helps you to rearrange a formula.

The formula triangle for force (F), mass (m) and

acceleration (a) is shown below.

×

Cover the quantity that you are trying to work out to find

the rearranged formula needed for the calculation.

So to find

acceleration (a),

cover up a……which gives the formula…

a =F

m


How do we find acceleration?

A lorry has a mass of 12,000kg. What acceleration is caused

by a force of 10,000N?


= 0.83m/s2

force

massacceleration =

10,000

12,000=


Inertial mass

Inertia is the tendency of an object to continue in its state of

rest or uniform motion.

Newton’s second law tells us that an object with a higher mass will require a larger force in order to change its speed

or direction.

This means that the mass of an object is a measure of

its inertia.

The inertial mass of an object tells you how difficult it

is to change the velocity of the object. It is defined as

the ratio of force over acceleration.


Investigating force and acceleration


Investigating mass and acceleration


F = ma calculations




A force cannot exist on its own – a force always causes a

reaction in the form of a second force.

This forms the basis of Newton’s third law of motion,

which states:

What is Newton’s third law?

These pairs of forces that act between two objects are

sometimes called action–reaction pairs or interaction pairs.

If object A exerts a force on object B, then object B

exerts an equal but opposite force on object A.

Forces arise from interactions between two objects.


Action–reaction pairs


What forces are acting between Mel’s computer and the table it is resting on?

The computer pushes

down on the table

because it is attracted by

the Earth’s gravity.

What forces support objects?

contact

force

reaction

force

The table exerts an equal

and opposite force

pushing upwards on the

computer. This is called the

reaction of the surface.


Reaction of the surface

If the downward force exerted by a stationary object

increases (for example, if it gets heavier), the reaction of

the surface will also increase to match it.

This remains true up to a

certain limit, when the

downward force overcomes

the reaction of the surface.

At this point, the object will

move downwards through

the surface.


Reaction forces and thrust




Glossary


How many pairs of balanced, unbalanced and

action–reaction forces can you spot?

Examples of Newton’s laws


Multiple-choice quiz

Documents

Newton's Laws of Motion - todhigh.comtodhigh.com/.../wp-content/uploads/2018/03/Newtons_Laws_of_Motion.pdf · This forms the basis of Newton’s third law of motion, which states: