Newton’s Laws

a. Newton’s 1st Law

All bodies resist being set into motion, especially ones with large mass. In fact when an object is stationary, it needs a force to make it move.

From the objects below, the object which has the largest mass (the block of wood) would be the most difficult to start moving.

All bodies tend to stay at rest, especially ones with large mass. In fact when an object is stationary it needs a force to make it moving. The bigger the mass, the bigger the force needed to make it moving, the bigger the inertia.

Inertia makes an object:

 difficult to start or stop moving,
 difficult to change its direction of motion or
 difficult to accelerate.

The Inertia of a body can be seen during an accident.

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

Newton’s first law of motion states that:

If the forces on a mass are balanced (no resultant force),

– An object in motion, keeps moving.
– An object at rest, will remain at rest.
b. Newton’s 2nd Law.

Newton’ s second law is a link between the resultant force, the mass and the acceleration of an object. Consider the following example:

A block of wood is accelerated by a resultant force – the greater the resultant force, the acceleration.

If the mass of the block increase the acceleration decrease – the greater the acceleration the smaller the mass.

This relation can be found out using the following equipment
We can notice that:

– Resultant force is directly proportional to the acceleration
– Mass is inversely proportional to the acceleration

Newton’s second law can be summarised in the following equation:

Resultant Force = Mass x Acceleration

Example: A supermarket trolley has a mass of 20kg. When pushed by a force of 15N it accelerates at 0.5m/s2.

a. Calculate the resultant force on the trolley that gives it this acceleration.

F = m.a
= 20 x 0.5m/s2 = 10 N

b. What is the frictional force on the trolley?

15N – 10N = 5N

Example: The engine of a car of mass 800kg produces a force of 1500N. The frictional force on the car is 500N.

a. What is the resultant force on the car?

1500N – 500N = 1000N

b. What is the acceleration of the car?

F = m.a but F/ m = a = 1000N/800kg
Newton’s 3rd Law

When two spring balances are connected to each other, and if two students pull the spring balances with an equal force, the spring balances give equal readings.

When only one of the spring balances is pulled and the other remains fixed, the two spring balances continue to give equal readings.

In each case we have a pair of forces which are known as the action and reaction. Here are some more examples of action and reaction pairs.

Sir Isaac Newton was the first to point out that every force has an equal but opposite partner acting in the different direction. This ideas is summed up by Newton’ s third law of motion:

If object A exerts a force on object B, then object B will exert an equal but opposite force on object A.

OR

Action and reaction are equal and opposite.

The sky diver

Every object when falling under gravity falls with the acceleration due to gravity which is 9.8m/s2.

When a skydiver jumps from her aircraft, the air resistance on her increases as her speed rises.

Eventually, the air resistance is enough to balance her weight, and she gains no more speed. She is at her terminal velocity. Typically this is at 60m/s, though the actual values depend on the air conditions, as well as the size, shape and weight of the skydiver.

When the skydiver opens her parachute, the extra area of material increases the air resistance.

She loses speed rapidly until the forces are again in balance, at a greatly reduced terminal velocity.

The sky diver keeps this constant speed till she reaches the ground.