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.

Newton’s First Law Flowchart

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:

Newton’s 3rd 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)