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There may be several forces acting on an object, and when you add up all of those forces, the result is what we call the net force acting on the object. If the net force adds up to zero, then the object is not accelerating, therefore it moves with a constant speed. If the net force adds up to a non-zero value, then the object is accelerating.

http://study.com/academy/lesson/what-is-net-force-definition-magnitude-equations.html

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Average speed is a rate. In kinematics, a rate is always a quantity divided by the time taken to get that quantity (the elapsed time). Since average speed is the rate position changes, average speed = distance traveled/time taken.

http://www.batesville.k12.in.us/physics/PhyNet/Mechanics/Kinematics/AveSpeed.html
a car has an average speed of 25 miles per hour. Its average velocity might be 25 miles per hour due east.

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The friction experienced when individuals try to move a stationary object on a surface is Static friction, without actually triggering any relative motion between the body and the surface which it is on. It can be explained as the force of friction which precisely balances the applied force for the duration of the stationary state of the body. Static frictional force is a self-regulating force. i.e., static friction will at all times be equivalent and opposite to the force applied.
https://www.reference.com/science/examples-static-friction-542e4190fffb7aed#
An example of static friction is a wooden block kept on a table. Unless force is accelerated, the block would not slide down the table because static friction holds it's place, resisting from sliding.

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Kinetic friction, also known as sliding friction or moving friction, is the amount of retarding force between two objects that are moving relative to each other. This is important in understanding the coefficient of kinetic friction, which is the proportion you get when you take the pulling force needed to keep an object moving and divide that by the force holding the two sliding surfaces together. This coefficient is specific to the materials involved in the contacting surfaces. A rougher surface will have a higher coefficient.
http://study.com/academy/lesson/coefficient-of-kinetic-friction-definition-formula-examples.html
The tires are moving which causes the whole car to roll which causes kinetic friction.

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Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between charged particles in two touching surfaces.
https://www.livescience.com/37161-what-is-friction.html
The box is rolling on the balls which causes friction.