# The Force of Gravity

In gravity, we will discuss the gravitational field, free falling, lift and pulley. Under gravitational field, we will discuss what is gravitational field, gravitational field strength, gravitational acceleration and weight are.

In free falling, we will discuss dropping object from a high place and launching object upward. We will also discuss the graphs of motion of these 2 cases.

Under lift, we will discuss the apparent weight of an object inside a lift when the lift moving up or down.

In pulley, we will discuss pulley with 1 load. Let’s start with the gravitational field in the very next slide.

We have learned that gravitational field strength at a point in the gravitational field is the gravitational force acting on a mass of 1 kg placed at that point. In equation, write this as g=F/m, where g is the gravitational field strength, F is the gravitational force acting on an object and m is the mass of the object.

For example, if there is an object of mass 2kg placed at a point A near to the earth and the gravitational force exerted on the object is 10N, the gravitational field strength will be 10N over 2 kg, equal to 5N/kg.

One thing that you need to know in SPM physics is the difference between the gravitational field strength and the gravitational acceleration.

Both the gravitational field strength and gravitational acceleration have the symbol, g and the same value (10ms-2) on the surface of the earth.

However, when we considering a body falling freely in a gravitational field, the g is referring to the gravitational acceleration. If the object is not moving, the g is referring as the gravitational field strength.

Another important thing that you need to know in SPM is the difference between mass and weight. Mass is defined as the quantity of matter whereas weight is the gravitational force acing on an object.

Mass is independent of the gravitational field strength, which means it is not affected by the gravitational field strength. For example, if you put an object on the surface of the earth or on the surface of the moon, the mass will be the same. In opposite, weight depends on the gravitational field strength. It will be affected by the gravitational field strength.

Mass is a base quantity and also a scalar quantity. Weight is actually gravitational force. A force is a derive quantity and also a vector quantity. It has magnitude and direction. The unit of mass is kilogram and the unit of weight is Newton, because weight is actually a force.

In SPM, there are a few things that you need to know about free falling. First of all, of course you need to know the definition of free falling. Free falling is a motion under the action of the gravitational force as the only force acting on the falling object.

Second, you need to know about the acceleration of the objects undergo free falling. An object undergoes free falling will move at a uniform acceleration. The acceleration is equal to the gravitational acceleration, which is 10ms-2.

Third, you need to know the 2 cases of free falling, namely

1. Dropping object from a high place
2. Launching an object vertically upward.

We are going to discuss this in next page.

Before solving a free falling problem, you need to determine which direction we want to take as positive.  You can take any direction as positive. However, I’ll always advise you to take the direction of the initial movement as positive. For example, in this case, the ball is moving downward, hence we take downward as the positive direction of the motion.

There are a few important things that you need to know regarding this motion. When an object is released from a high place, the initial velocity is zero. In SPM, we always ignore the effect of air resistance, therefore the motion is a free falling and the acceleration is equal the gravitational acceleration, which is 10ms-2. When the ball hit the ground, the displacement of the ball is equal to the high of the building.

This is what you should know when solving problems related to free falling.