Question

what is the result of unbalanced forces? not all forces in opposite directions balance each other. think about two students pushing on the opposite sides of a box. what happens if one student pushes harder than the other as in the picture? the forces are not balanced, and the box will move in the direction of the larger force. the student who is pushing harder will move the box in the direction of the force. the net force that moves the box is the difference between the two forces. suppose both students push on the same side of the box as in the second figure. the students are both exerting force in the same direction. again the forces are unbalanced. the forces combine, or add together, because they are exerted in the same direction. again, net force is equal to both forces added together. what is static friction? suppose you fill a cardboard box with books, but it is too heavy to lift. you try to push it, but you cannot get it to move. this means there must be a force that is working against you. the force is friction caused by microwelds between the bottom of the box and the floor. this is called static friction. static friction is the force that keeps two surfaces at rest from sliding across each other. in this case, your push is not large enough to break the microwelds between the box and floor. so, the box will not start moving. what is sliding friction? suppose you ask your friend to help move the box. as you and your friend push together, the box starts to move. the two of you have applied enough force to break the microwelds between the box and the floor. if you stop pushing, the box will stop moving. another force, sliding friction, is opposing the motion of the box. sliding friction is the force that works against the motion of two surfaces that are sliding across each other. sliding friction is caused by the microwelds breaking and then forming again as the box slides along the floor. what is rolling friction? a wheel digs into the surface that it is rolling over. this causes both the wheel and the surface to be deformed. there is static friction in the deformed area where the wheel and surface are in contact. this makes a frictional force called rolling friction. rolling friction is the frictional force between a rolling object and the surface it rolls on. have you ever seen a cars wheels spinning in the mud or snow? there is sliding friction between the spinning wheels and the surface. sand or gravel on the surface increases the sliding friction, so the wheels will stop slipping. 2. why are the forces in the figure said to be balanced? a. when the 2 students pushes on the opposite sides of the box, the student on the left pushes harder. this cancels out the force and the box move to the right. b. when the 2 students push with the same amount of force but in opposite directions, the force cancels out. the box does not move. c. when the 2 students pushes on the opposite sides of the box, the student on the right pushes with more force. this causes the box to move to the left. but the force is not cancelled out. d. both student pushes with little force causing the box to have a net force of zero. 5. define static friction. a. the force that works against the motion of 2 surfaces that are sliding across each other. b. the frictional force between a rolling object and the surface it rolls on. c. the force that keeps two surfaces at rest from sliding across each other. d. the measure of gravity pull. 6. pushing a heavy couch across the living room floor from one side of the room to another side would be an example of a. static friction b. sliding friction c. rolling friction d. no friction

Explanation:

1. Static friction is defined as the force that keeps two surfaces at rest from sliding across each other. This is exactly what option c states.
2. When pushing a heavy couch across the floor, the couch is sliding, so the friction involved is sliding friction.

Answer:

1. c. The force that keeps two surfaces at rest from sliding across each other.
2. b. sliding friction