1st Law of Motion

Mass and inertia

If you took a physics textbook, with a weight of 10 newtons, into space, it would become weightless, but it would still have the same mass. 

The mass of an object is the amount of matter in it.  It is measured in kilograms.

When an object is stationary, it needs a force to make it move.  The bigger the mass, the bigger the force needed to make it move. We say masses have inertia, a reluctance to start moving.

 Activity 1

1.      Hang two tins from a long piece of string.

2.      Fill one tin with wet sand or stones and leave the other empty.

3.      Try pushing the cans.

Which can is harder to push?

Which can has the larger inertia?

Which can has the larger mass?

Since the experiment depends on the mass of the object and not the weight, you would feel the same effect on the Moon or out in space – if you push a large mass you would feel a large amount of inertia.   

In a similar way, moving objects need a force to stop them moving.  Their inertia tends to keep them moving. 

Activity 2

1.      Using the cans from activity 1 – push them to get them moving. 

2.      Try to stop them moving with your hand.

Which can is harder to stop?

Passengers in a car have a lot of inertia and so they need seatbelts.  If the car stops suddenly, the people will tend to keep moving (even through the windscreen) unless the seatbelts exert a large force to stop them.

What happens when the car you are travelling in turns a corner?  Your body, because of its inertia, will tend to travel straight on.  You can feel your body sway as the car turns, but fortunately your car seat exerts a force on you and this pulls your round the corner with the car.

 Newtons first law of motion:

Examples of objects with uniform motion

Newton's first law can be used to explain the movement of objects travelling with uniform motion (constant velocity). For example, when a car travels at a constant velocity, the driving force from the engine is balanced by the resistive forces such as air resistance and frictional forces in the car's moving parts. The resultant force on the car is zero.

Other examples include:

a runner at their top speed experiences the same air resistance as their thrust

an object falling at terminal velocity experiences the same air resistance as its weight

Examples of objects with non-uniform motion

Newton's first law can also be used to explain the movement of objects travelling with non-uniform motion. This includes situations when the speed changes, the direction changes, or both change. For example, when a car accelerates, the driving force from the engine is greater than the resistive forces. The resultant force is not zero.

Other examples include:

at the start of their run, a runner experiences less air resistance than their thrust, so they accelerate

an object that begins to fall experiences less air resistance than its weight, so it accelerates