Mammoth Memory

Newton's first law – examples

Example 1

Objects in motion stay in motion and objects at rest stay at rest unless acted upon by an outside force (unbalanced force).

A stationary ball will stay stationary unless acted upon by a force.

A stationary object with no outside force will not move.

With no outside forces a moving ball will continue moving in the same speed and direction.

With no outside forces, a moving object will not stop.

Although Sir Isaac Newton did not have access to space stations, this first law can clearly be seen:

A screwdriver dropped in space will continue in the same direction and speed forever.

An astronaut who has their screwdriver knocked into space will see the screwdriver continue on at the same speed and direction forever. There are no forces acting on the screwdriver from friction. With no forces this object will never stop.


Example 2

Unless I am forced I do the same thing.

A box will continue what its doing unless a force is applied.

An object at rest stays at rest.

A box with balanced forces applied continues to do what it is doing.

An object acted upon by balanced forces stays at rest.

A box with unbalanced forces applied will accelerate.

An object acted upon by unbalanced forces changes speed and can change direction.


Example 3

Newton's first law is often referred to as the law of inertia – inertia is an object's tendency to resist changes in motion.

A passenger in a car is thrown forward under braking.

When a car brakes quickly, the passenger will be thrown forward because inertia (the tendency to remain unchanged) tries to keep the passenger moving.


Example 4

The first law explains why you go flying over the handlebars if your bike stops suddenly:

A rider flys over the bikes handlebars when it suddenly stops.

Things tend to keep doing whatever they were doing before.


Example 5

If an object is stationary, it will remain stationary. If the object is moving it will continue to move at the same speed and in the same direction.

A hockey puck on an ice rink.

An object at rest stays at rest.

A puck accelerates across the ice when a force is applied to it with the hockey stick.

An object acted upon by an unbalanced force changes speed and can change direction.

A football heading towards a goal.

An object in motion stays in motion.

The football is stopped in the goal when the net applies an unbalanced force to it.

An object acted upon by an unbalanced force changes speed and direction.


Example 6

The second part of Newton's first law is sometimes forgotten – you have to remember that it applies to bodies that are moving at a constant velocity, not just those at rest.

A stationary car will not move unless a force is applied to it.

This car will never move unless pushed to start moving.

A moving car will continue to move in the same direction and speed unless a force is applied to it.

This car would never stop unless a force acts upon it. These forces that reduce its speed are friction and drag.


Example 7

Bodies at equilibrium are balanced and therefore there is no acceleration. If all of the forces acting on an object are balanced then the object will continue at the same speed or remain stationary.

If the weight and reaction force on an object are balanced then the object will remain at rest.



The box is at equilibrium – the sum of all the forces must equal zero.

NOTE: `w\e\i\g\h\t=m\a\s\s\xx\g\r\a\v\i\t\y`


Example 8

The motion of an aircraft flying through the air can be described by Newton.

An aircraft travelling at constant velocity has balanced thrust and drag and lift and weight.

If thrust and drag are equal, and lift and weight are also equal, then the aircraft has constant speed and altitude. This means that:





Example 9

Newton's first law can describe a roller coaster

The track of a roller coaster applies a force to the carriage to change its direction.

A roller coaster has inertia. When it starts a drop it wants to continue moving in the same direction at a constant speed. It doesn't, however, because the tracks act as an outside force that changes the roller coaster's direction.


Example 10

The law of inertia – the tendency for an object to resist changes in motion. 

An egg is sat on a roll of cardboard placed on a sheet of card above a glass of water.


The egg falls into the water when the sheet of card is pulled away.

The energy of your movement is passed on to the card making it fly out of the way quickly, but the card moves too quickly and there is not enough friction to affect the egg. The egg ends up in the water with nothing holding it up and gravity takes over.

NOTE: This is a similar experiment to pulling a table cloth out from under plates on a table.


Example 11

The movement of a ball in a box can be explained by Newton's first law of inertia.

A ball is stationary inside a moving box.

Place a ball in a box and slowly push the box.

When the box stops the ball will roll forwards inside the box.

Abruptly stop the box. The ball will keep moving. According to Newton's first law, an object in motion tends to stay in motion unless acted upon by an unbalanced outside force, so the ball keeps rolling even though the box has stopped.


Example 12

We put this in for fun and to help you remember the words of Newton's first law of motion.

An object at rest will remain at rest unless acted upon by another force.

An object at rest will remain at rest unless acted upon by another force.


 Example 13

Galileo made observations about friction and Newton built on these observations.

Galileo observed balls rolling down different curves.

Galileo rolled balls down curves, the smoother the curved surface the closer the final height was to the initial height.

The ball will run down one side of the curve then up the other. Galileo noticed that if smooth surfaces were used, the ball got closer to its original height (its height at the starting point). Galileo reasoned that the ball would get to the original height if there were no friction. The ball would get to the original height if there were no outside force (unbalanced force).

Galileo found the angle of the curved surface made no difference.

Regardless of the angles, the ball will always move towards its original height. If the incline of the second curve is less than that of the first, then the ball will travel a greater distance but still will not reach its original height due to friction – ie, the ball encountering resistance from the surface it is running along. Without friction, the ball would roll up the opposite slope to its original height even if the two ramps had different slopes.

With no friction a rolling ball will carry on forever.

Galileo concluded that if the curve ended with no incline then the ball would carry on forever were it not for friction eventually stopping it.

Newton used these observations to conclude that a force is not needed to keep an object in motion. It is in fact an external force that prevents the ball continuing.

What made Newton so radical was that before his first law of motion, scientists had said an object came to a stop because there was no force acting on it.


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