## Newton’s First Law:

If the resultant force acting on an object is zero and:

• the object is stationary, the object remains stationary
• the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.

So, when a vehicle travels at a steady speed the resistive forces balance the driving force.

So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.

The tendency of objects to continue in their state of rest or of uniform motion is called inertia.

Question: What is the tendency for objects to continue at the same speed in the same direction called?

Question: A car is moving at a constant velocity. Describe the relationship between the forces acting on the car.

## 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.

resultant force = mass × acceleration

Inertial mass is a measure of how difficult it is to change the

Inertial mass is defined as the ratio of force over acceleration.

Question: The acceleration of a trolley was 2.4 m/s2
The resultant force on the trolley was 1.2 N
Calculate the mass of the trolley.

Question: A fisherman pulls a boat towards land.
The fisherman exerts a force of 300 N on the boat to the right.
The sea exerts a resistive force of 250 N on the boat to the left.
Describe the motion of the boat.

## Newton’s Third Law:

Whenever two objects interact, the forces they exert on each other are equal and opposite.

The two forces must be of the same type acting on each individual object.

Question: shows a cyclist with a trailer attached to his bike.
Figure 1

Describe how Newton’s Third Law applies to the forces between the bike and the trailer. (2 marks)