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Physics Lesson 4.2.4 - What is Weight?
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Welcome to our Physics lesson on What is Weight?, this is the fourth lesson of our suite of physics lessons covering the topic of Types of Forces I. Gravitational Force and Weight, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
What is Weight?
By definition, Weight, W⃗ is the pushing force exerted by an object on the place it lies (or the pulling force when hanged on a string), caused by the gravity.
From the above definition, it is obvious that weight depends on the gravity, so many people often confuse Weight and Gravitational Force with each other. Despite the common things (and the same result obtained in most cases when calculating these two forces), there are some very important differences between them. They are:
- Weight is a contact force while gravitational force is a field one. This means when an object is flying in the air, it has no weight, as it is not exerting any pushing force on the ground (remember the example in the Introduction). However, there is a mutual gravitational force exerted by the object and the Earth, which causes the object to fall on the ground if no lifting force is available.
- Even when the object is on the ground, sometimes weight and gravitational force may not be numerically equal. This occurs for example when the object lies on a slope as shown in the figure.
As you can see, Weight is always normal to the surface in which the object lies while gravitational force is always vertical (downward). If the slope is inclined at an angle θ to the horizontal direction, we have for the magnitude of weight.
Equation 7
|W⃗| = |F⃗g| × cos θ = |m × g⃗| × cos θ(From geometry, it is known that two angles with perpendicular sides are equal. Therefore, the angle formed by the slope and the horizontal direction is equal to the angle formed by the weight vector and the vertical direction because horizontal direction is perpendicular to the vertical one and weight vector is perpendicular to the slope).
Remark! Unlike the gravitational force vector, the weight vector does not begin at the centre of the object because its application point is at the place of contact between the two objects, i.e. at the bottom face of the object (and as a result, at the upper face of the slope).
Example 3
A 20 kg object rests on a slope as shown in the figure.
Calculate:
- Gravitational force exerted by the Earth on the object
- Weight of the object
Take g⃗ = 9.81 m/s2, cos 200 = 0.940 and sin 200 = 0.342
Solution 3
First, we draw the vectors of weight and gravitational force acting on the object.
Gravitational force is:
= 20 kg × 9.81 m/s2
= 196.2 N
Weight of the object (in magnitude) is:
= |m × g⃗| × cos θ
= 196.2 N × 0.940
= 184.4 N
As you see, the results are different.
You have reached the end of Physics lesson 4.2.4 What is Weight?. There are 5 lessons in this physics tutorial covering Types of Forces I. Gravitational Force and Weight, you can access all the lessons from this tutorial below.
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