Friction

FRICTION

Friction is necessary for walking due to the following reason, As per Newton’s third law of motion, (every action has an equal and opposite reaction) we can walk if and only if the ground we are walking on push our feet back with a force. Now, as per the third law the ground would definitely push our feet back but if we are walking on a perfectly smooth ground which has no friction our force would simply cancel out the force reverted by the ground and we would fall.

If there was no friction, your foot would simply slide back as you tried to take steps, and you would go nowhere.
In order for something to move, it has to have a force moving it. That force has to have leverage, or friction. when you walk you push the ground backward . your force is given on a large mass so the ground acceleration is negligible . now according newton third law of motion there is a reaction force which acts on less mass so you accelerate in other way you move . no if there was no friction and when you push the ground backward you would slip . friction force gives you the grip to move forward .

If you rub your hands together for several seconds, you’ll notice that your hands feel warm. That warmth is caused by a force called friction. When objects like your hands come in contact and move against each other, they produce friction. Friction happens when you overcome the resistance of one object rubbing against the other. The force of the friction is opposite in direction to the direction of the motion. If you just put your hands together, there’s no resistance, so there’s no friction produced. Rub them together and there’s friction.

Friction force results from the interactions of surfaces. Irregularities in the structure of the matters causes friction force. These irregularities can be detected in micro dimensions. You may not see any irregularity on the surface of the material however, it does exist. Friction force is always opposite to the direction of motion and tends to decrease net force. All materials have their own friction constant in other words friction force depends on the type of materials. Another factor affecting friction force is the normal force. When you apply a force to an object, then friction force becomes active and resists with the force of having opposite direction to your net force.

GRAVITY

The center of gravity (CG) is the center of an object's weight distribution, where the force of gravity can be considered to act. It is the point in any object about which it is in perfect balance no matter how it is turned or rotated around that point. For a finite set of point masses, CG may be defined as the average of positions weighted by mass. That is, the (Sum of mass*position)/(Sum of mass).

There is a popular story that Newton was sitting under an apple tree, an apple fell on his head, and he suddenly thought of the Universal Law of Gravitation. As in all such legends, this is almost certainly not true in its details, but the story contains elements of what actually happened.
Probably the more correct version of the story is that Newton, upon observing an apple fall from a tree, began to think along the following lines: The apple is accelerated, since its velocity changes from zero as it is hanging on the tree and moves toward the ground. Thus, by Newton's 2nd Law there must be a force that acts on the apple to cause this acceleration. Let's call this force "gravity", and the associated acceleration the "accleration due to gravity". Then imagine the apple tree is twice as high. Again, we expect the apple to be accelerated toward the ground, so this suggests that this force that we call gravity reaches to the top of the tallest apple tree.

Gravity plays an essential role in terrestrial locomotion. Due to gravity we can hit the ground without losing contact with it. However, because of gravity, work has to be done each step to lift the body even when moving on a level surface. Gravity and the braking action of the ground interact during locomotion with two different energy conserving strategies: the pendular mechanism of walking, up to intermediate speeds, and the bouncing mechanism of running, up to the highest speeds attainable.

COLLISION

Collisions between objects are governed by laws of momentum and energy. When a collision occurs in an isolated system, the total momentum of the system of objects is conserved. Provided that there are no net external forces acting upon the objects, the momentum of all objects before the collision equals the momentum of all objects after the collision. If there are only two objects involved in the collision, then the momentum change of the individual objects are equal in magnitude and opposite in direction.

An inelastic collision is where some energy is lost in the collision so momentum is not conserved. when you bounce a tennis ball. Some energy is used up in compressing the rubber of the ball and so it doesn't bounce back to its original height.

In a head-on collision the forces on the two vehicles are constrained to be the same by Newton's third law. But from both Newton's second law and the work-energy principle it becomes evident that it is safer to be in the bigger truck.