“Effects of force” is one of the most important sub-topics in Force and Motion. Under effects of force, there are 4 main subtopics: Newton’s second law, Newton’s third law, impulse and safety features of vehicles. In this lesson, we will only discuss Newton’s 1^{st} and 2^{nd} law. We will discuss “impulse” and “safety features of vehicles” in the very next lesson.

Well, in Newton’s second law, we will discuss the formula that deduced from Newton’s second law and some calculations related. We’ll also have a little discussion on friction.

After finishing Newton’s second law, we will proceed to Newton’s 3^{rd} law, where we will discuss some important forces that you need to know such as the normal reaction force and the tension.

When you are in primary 5, you should have learned that a force is push or pull exerted on an object and the unit of force is Newton, denoted by a symbol N.

In previous chapter, we have learned that if a physical quantity has direction, it is vector quantity. Force has direction, hence it is a vector quantity.

In short, force is push or pull exerts on an object. Its SI unit is Newton, and it is a vector quantity.

Let’s see this example. 2 forces are acting on a car; both are of the same magnitude but in the opposite direction. In such a case, the net force acting on the car is zero, and we say this are balanced forces or the forces are in equilibrium.

Let’s see another example. Again, 2 forces are acting on a car. The forward thrust that acting forward is 400N whereas the friction acting backward is 100N. In this case, the net force is 300N, which is not equal to zero. We say the 2 forces are not balanced or not in equilibrium.

In brief, forces are balanced or in equilibrium if the net force is equal to zero. Forces are unbalanced or not in equilibrium if the net force is not equal to zero.

In previous lesson, we have already learned Newton’s 1^{st} Law. Newton’s 1^{st} Law states that if the forces acting on an object is balanced, an object at rest **remains at rest** and an object in motion **continues in motion with a constant velocity** (that is, with a constant speed in a straight line).

For example, if there are 2 forces acting on a block, both are of same magnitude but in opposite direction. The net force is equal to zero or the 2 forces are in equilibrium. If the block is initially at rest, it will remain at rest or if it is initially moving, it will move with constant speed in a straight line.

When you are in form 2, you should have learned that force can change the size and shape of an object. It can also make a stationary object move, change the speed of a moving object, or change the direction of the object. All of these are called the effects of force.

We should take notes that changing stationary state, speed or direction of an object means acceleration. In other word, if there are unbalanced forces or non-zero net force acting on an object, the object will accelerate.