Forces

Forces A force is a push or pull that causes changes in motion. It can speed things up, slow things down, or make things change direction. There are different kinds of forces: friction, gravity, and magnetic force. Nothing changes direction, speed, or position without force.

Friction A force that works against motion. It can make things slow down or stop, or it can keep things from moving at all. Whenever surfaces touch you have friction. Friction makes heat (rub your hands together; there is friction). Can cause you not to slide down a surface smoothly.

Gravity The attraction between you and the Earth. Pulls us to the Earth. Causes things to speed up as they fall. Gravitational force states that all objects pull on each other.

Magnetic Force An object that attracts iron is a magnet. Magnetic force is the force that is exerted by a magnet. Magnets can attract, repel, or push each other away. Compasses use magnetic force to help us navigate.

Balanced Forces Balanced forces act on an object but balance each other out (think about 2 teams playing tug of war, when neither team can pull the other team down you have a balanced force). Forces are balanced as long as they are not moving.

Unbalanced Force When unbalanced forces act on an object, motion changes. The object may start, change direction, speed up, or stop.

Example of Balanced and Unbalanced forces working together A seesaw shows balanced and unbalanced forces. If the weight on one end of the seesaw is heavier than the other, the heavier side goes down (they are unbalanced), if the weight is the same the forces are balanced and the seesaw does not move.

Net Force The combination of all forces acting on an object. When forces are balanced the net force is 0. If 2 children are pulling a toy and one pulls the toy away from the other then the net force is unbalanced.

Buoyant Force Upward force. Acts in the opposite direction then weight. Helps objects float. If an object’s weight is greater than the buoyant force than the object sinks. Ships float because they are not solid and they are filled with air.

Work Using force to move an object through a distance. Work=Force X Distance You know that work has been done when something moves a distance

Machines and Work Machines make work easier. A simple machine is a tool that makes a task easier by changing the strength or direction of a force. A lever is a bar that makes it easier to move things. They have two parts; the lever arm which moves and the fulcrum or the balance point. The fulcrum supports the arm but doesn’t move. A wheel-and-axle helps you open a door. A pulley is a wheel with a groove for a rope. An inclined plane is a ramp. A wedge is made up of 2 inclined planes.

Levers

Wheel-and-Axle A doorknob is a wheel-and-axle. The knob is the wheel and the rod that goes through the door is the axle. Includes a wheel and an axle. A potter’s wheel uses a wheel-and-axle.

Pulley A flagpole uses a pulley to raise and lower the flag.

Inclined Plane Ramps

Simple Machines Building a house involves using simple machines. Screws are inclined planes. Nails are wedges. Hammers are levers.

Compound Machines Made up of two or more simple machines. Make work easier. Sailors and movers use block and tackle machines to move heavy loads. Examples are a push lawn mower (wedge and wheel-and-axle) and hand operated can opener (wedge and wheel-and-axle).

Compound Machines Shovels ( the blade is a wedge and the handle acts as a lever).

Questions What forces affect objects on Earth every day? What are balanced and unbalanced forces? What is work, and how is it measured? What is the purpose of a machine? Name four types of simple machines.