1. Force Friction

2.  What is a force? A force is a push or pull exerted on an object to change the motion of an object.  What 2 things do all forces have? A force has both size and direction. Force #1 Force #2 Total Force Force #1 Force #2 Total Force = 0

3.  What unit is used for forces? Forces are measured in Newtons.  Does motion always occur when a force acts on an object? No! Even though you are exerting a force on the chair you are sitting in, the chair does not move, because the floor is also exerting a force on the chair!

4.  What is net force? Net Force is the combination of all forces acting on an object.  How do you find net force if the forces are in the same direction? If the forces are in the same direction, you add the forces together to find the net force, and use the same direction. 25 N20 N Net force 25 N + 20 N = 45 N to the right

5.  How do you find net force if the forces are in different directions? If the forces are in different directions, you subtract the smaller force from the larger force and use the direction of the larger force. 10 N12 N Net force 12 N – 10 N = 2 N to the right

6.  What are balanced forces? When the forces acting on an object produce a net force of 0 Newtons, the forces are balanced.  Will balanced forces change the motion of a moving object? Balanced forces do not change the motion of a moving object, and will not cause a nonmoving object to start moving.  What are examples of balanced forces? a poster hanging on the wall, a bird’s nest in a tree Net force 5 N – 5 N = 0 N, no movement 5 N

7.  What are unbalanced forces? When the net forces on an object is not 0 N, unbalanced forces do cause a change in an object’s motion.  How do unbalanced forces change the motion of an object? Unbalanced forces either change the speed or direction of a moving object.  Will unbalanced forces make a nonmoving object start moving? Yes! Unbalanced forces are needed to start an object moving, but not to keep it moving. When you kick a soccer ball, the ball will continue to roll, until other forces stop it from rolling.

8.  What is an example of unbalanced forces? When one player passes the soccer ball to another, their kick is an unbalanced force. The second player’s kick is also an unbalanced force, which changes the speed or direction of the ball.

9.  Do balanced and unbalanced forces work together? Many times, both balanced and unbalanced forces work together.  What is an example of balanced and unbalanced forces working together in sports? Balanced forces keep a skaterboarder on the skateboard, but unbalanced forces let him roll down the ramp.  In transportation? Airplanes stay up when gravity is balanced by lift. To land, the pilot has to decrease lift so gravity is stronger, and the plane descends.  In the human body? Muscles exert unbalanced forces to move your body, but to hold your body still, forces have to be balanced.

10.  1) What is friction? Friction is a certain kind of force that opposes motion between two surfaces that are in contact.  2) How does friction happen? Friction happens because the surface of every object is rough. The “hills and valleys” of one object stick to the “hills and valleys” of another object.

11.  What two things does the amount of friction depend on? The amount of friction depends on:  1) The force pushing the surfaces together (i.e. the weight of the object)  2) The roughness of the surfaces (for example: a hockey puck sliding on concrete will experience more friction, so won’t slide as far as a hockey puck sliding on ice)

12.  There are two kinds of friction: kinetic and static.  1) Kinetic friction happens between moving surfaces. The three kinds of kinetic friction are: sliding, rolling, and fluid. For example: an earthquake can be caused by sliding friction in a strike-slip fault. For example: a wheel in contact with the road experiences rolling friction.

13.  2) Static friction happens when a force is applied to an object, but the force is equal to or less than the force of static friction. This means the object doesn’t move.  When does static friction stop? Once the object moves, static friction disappears, and kinetic friction happens.

14.  Is friction always harmful? We do need some friction, although friction can also cause problems.  Without friction, our cars and bikes can’t move, we couldn’t walk, write, or even scratch an itch!  However, the same friction that makes our cars move also wears down our tires, and causes the engine parts to eventually wear down.

15.  What are three ways to reduce friction?  To reduce friction, you can use a lubricant. A lubricant is something applied to the surface of an object to reduce friction. Many times lubricants are liquids, but they can be solids or gases, too.  Examples of lubricants are: wax, oil, grease, air, or soap.

16.  A second way to reduce friction is to switch to rolling friction from sliding friction.  A third way to reduce friction is to make the surface of the object smoother.

17.  Sometimes you need to increase friction. To increase friction, you can make the surfaces rougher. Examples are: sand on an icy road or textured batting gloves for better grip on the baseball bat.  A second way is to increase the force. An example is: scrubbing harder on a dirty dish.