In this video, you can see a rigid body, formed by joining two balls, being thrown across the screen. Tracing the trajectory of either ball shows a complicated path. On the other hand, the trajectory of the CM is the familiar parabolic arc of a projectile motion. This shows that an object’s motion can be treated as a translational motion of the CM juxtaposed with a rotational motion about its CM.
Category: 04 Forces
4.2 Upthrust
Why is upthrust up?
3.4.1 Frictional Force
Friction does not oppose motion. Friction opposes relative motion. In fact, when walking, we use friction both to initiate and stop motion.
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One useful thing to know about friction is this: the friction between two surfaces increases if the two surfaces are pressed more strongly into each other.
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When friction is multiplied thousands of times…
3.4.1.1 Laws of Friction

 The frictional force between two stationary surfaces is called the static friction, f_{s}.
 The frictional force between two sliding surfaces is called the kinetic friction, f_{k}.
 It can be shown empirically that between two particular surfaces
 static friction f_{s} ranges from zero to a maximum value.
0 < f_{s} < μ_{s}N

 kinetic friction f_{k} is constant and independent of speed.
f_{k} = μ_{k}N

 maximum static friction is larger than kinetic friction.
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This video shows clearly how the static friction increases until reaching its maximum value, then drops to the constant kinetic friction.
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Unlike drag force, friction is unaffected by the sliding speed. It is also unaffected by the area of the surfaces in contact. All that matters is the material of the two surfaces and how hard they are pressed against each other.
3.4.2 Drag Force
How to win the paper race? Watch this video to check your understanding of air resistance and gravity.
3.4.2.1 Laws of Drag Force
The details of drag force is not required by the H2 syllabus. But this is a really nice poster summarizing the basics of the drag force. Probably easier to view if you right click and open it in new tab.
Photo Credit: Physics Review.
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Derivation of the Inertia Drag Force formula
4.3.7 Why must Three Forces Intersect at the Same Point?
A video explanation of the origin of this “rule”.
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A visual “proof” of this rule.
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The classic “ladder against wall” problem:
4.3.6 Static Equilibrium
Here is the “proof” for the summation of moments is constant regardless of pivot point when net force is zero. This is not required by H2 syllabus. So watch it only if you’re interested.
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Below are a few worked examples
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4.3.2 Moments
Just a video explanation on how to calculate moments.
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4.3.1 Translational Equilibrium
How can a 50 g mass lift a 100 g mass?
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A few worked examples on translational equilibrium:
Picture Frame
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Block on Slope
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Submerged Buoy