Forces and Levers

In this lesson, students use simple machines to explore the relationship between forces and motion. They focus on how people can use levers to make work easier, investigating how the structure of the lever affects how much work it can do. 

• Simple machines are devices that help us do work by redirecting force. 
• Forces cause all motion, and they are needed to transfer energy into or out of objects or systems. A force is a push or pull that acts on an object, changing its speed, direction, or shape.
• Levers are a kind of simple machine. A lever is a stiff beam that pivots on a point called a fulcrum, and it can be used to lift or move loads.

• What do you think of when you think of a force?
• How can you apply a force to an object?
• How can a force cause the motion of an object to change by slowing down?
• Why is the fulcrum an important part of the lever?
• Why do we talk about the mass of people on seesaws rather than weight?
• What would happen to the seesaw if two people of identical mass get on the seesaw at an equal distance from the fulcrum? 
• How is a hammer’s claw an example of a lever? How do hammer claws make it easier to do work?
• How would you compare the structure of the hammer to a seesaw?  
• How do scissors use simple machines?  
• How do forces transfer energy to scissors to do work?

Misconception: Objects that aren’t moving don’t have energy.

Fact: All matter has energy. Energy can be either stored (called potential energy) or in motion (called kinetic energy). Objects that aren’t moving still have stored potential energy.

Misconception: Each form of energy is created to meet a certain need.

Fact: Energy is never created or destroyed. However, it can change form in an energy system. People can design energy systems such as catapults that change energy from one form to another to do work.

Effort force: the force you apply to a machine

Lever:  a stiff beam that rests on a support called a fulcrum that lifts or moves loads

Simple Machine:  any device that helps us do work by redirecting force; there are six kinds: an inclined plane, a lever, a pulley, a screw, a wedge, and a wheel and axle

Sewing for Football Players

Marge Switzer sews many football jerseys. Marge is the seamstress for the Green Bay Packers. The Packers are the football team of Green Bay, Wisconsin.

Marge has a sewing room across from the locker room. One of her jobs is to mend the players’ uniforms. She fixes rips or tears that occur during the game.

As a seamstress, Marge uses different tools to help her do her job. One tool is a good pair of scissors. Many people don’t think of scissors as machines, but they are. In science, a machine is any device that does work. Work is any change in position, speed, or state of matter due to force.

 Forces on Earth

Remember that a force is a push or pull that acts on an object. Force is always needed to transfer energy into or out of a system.

Gravity is a kind of pulling force. Gravity is the force of attraction between all matter. On Earth, the force of gravity pulls all objects near Earth’s surface down.

Gravity is why a football thrown by a player will fall back to the ground. It is also why we don’t all float off into space.

Friction is another force that slows motion whenever two objects rub against each other. Friction slows motion because it causes some of the energy of the moving object to change into heat.

Forces on Earth

Remember that a force is a push or pull that acts on an object. Force is always needed to transfer energy into or out of a system.

Gravity is a kind of pulling force. Gravity is the force of attraction between all matter. On Earth, the force of gravity pulls all objects near Earth’s surface down. Gravity is why a football thrown by a player will fall back to the ground. It is also why we don’t all float off into space.

Friction is another force that slows motion whenever two objects rub against each other. Friction slows motion because it causes some of the energy of the moving object to change into heat.

 Simple Machines Help Do Work

To understand how scissors use force to do work, we need to begin with their structure. Scissors are made up of two smaller simple machines. A simple machine is any device that helps us do work by redirecting force.

We’ll use a seesaw to explore this idea further because seesaws are similar to scissors. Both seesaws and scissors use levers to do work. A lever is a simple machine that is made up of a stiff beam that rests on a support called a fulcrum. The beam is called an arm.

Scissors are made up of two levers. Seesaws are one lever. They have a simple structure. A long, narrow board is supported by the fulcrum in the middle of the board.

For the hands-on activity in this lesson, students carry out several investigations to analyze the phenomena of how a lever’s fulcrum and the effort force affect the amount of force needed to move a load. Students analyze their protopye’s data to solve a problem using simple machines to redirect force and make work easier.