How Forces Interact

I) Forces Act Together

A) Only a force can change the motion of an object

1) It must be stronger than the inertia of the object

B) Whenever the motion of an object changes, more than one force is acting

1) Example: a runner’s leg and foot pushes on the ground and the ground reaction pushes the runner forward

C) All forces are measured in newtons

1) At Earth’s surface, gravity pulls on a 0 1) At Earth’s surface, gravity pulls on a 0.1-kg with the force of 1 newton

2) Fun Fact – a quarter-pound burger weighs about 1 newton

II) Friction

A) Friction is a force that opposes motion whenever two surface rub against each other

1) Friction happens only when one force, such as gravity or muscle power acts to move an object against something else

B) Overcoming Friction to put an object into motion requires energy

1) It takes less energy to keep a object moving then it does to get it started.

C) It is possible to reduce friction

1) one way to reduce friction is to use rollers or wheels

2) another way is by using lubricants, such as oil and grease

D) Friction can be reduced, but can not be eliminated.

E) Friction is needed

1) Walking - friction between your foot and the ground offers the resistance necessary to push your body forward.

2) Tire treads, shoe soles, and cleats are all designed to increase friction and prevent slipping

III) Balanced Forces

A) Balanced forces acting on an object are equal in size but opposite in direction

1) the result is the two forces seem to cancel each other out

2) When forces are balanced the object does not move, or it continues it motion at a constant velocity

B) Every object that is at rest acted on by balanced forces.

1) the force of air pressure weighing down on your body is balanced by an equal force with you pressing out (also air pressure)

C) Objects that move at a constant velocity are also acted on by balanced Forces

1) a parachuter comes down under balanced forces at a constant velocity, because the parachute cancels out someof the force of gravity

IV) Unbalanced Forces

A) Objects often have more than one force acting on them

B) Net force is the combination of all of the forces acting on an object

C) Unbalanced forces can cause an object to speed up, slow down, or change direction

1) When two unequal forces push in opposite directions on an object, the object changes its speed in the direction of the stronger force

2) example: game of tug or war

D) A car stopped on a hill, the force of friction from the tires is balanced with the force of gravity

1) speeding up: break is released, which reduces friction on tires, and gravity is stronger

2) slowing down: the force of friction increases

3) changing direction: driver turns the wheel