Unit 2 Lesson 3 Machines. Copyright Houghton Mifflin Harcourt Publishing Company

Transcription

1

2 Work It Out What do simple machines do? A machine is any device that helps people do work by changing the way work is done. The machines that make up other machines are called simple machines. The six types of simple machines are levers, wheel and axles, pulleys, inclined planes, wedges, and screws.

3 What do simple machines do? Work is done when a force is applied to an object and makes it move. The work that you do on a machine is called work input. The force you apply to a machine through a distance is called the input force. The work done by the machine on an object is called work output. The output force is the force a machine exerts on an object.

4 What do simple machines do? Bottle openers and hammers are examples of simple machines.

5 What do simple machines do? Work equals force times distance. If you apply less force with a machine, you apply that force through a longer distance. So the amount of work done remains the same. Some machines decrease the magnitude, or size, of the force needed to move an object. However, you apply the force through a longer distance. Other machines increase the amount of force needed, but you apply the force over a shorter distance.

6 What do simple machines do? Some machines change the way you do work by changing the direction of a force. Some machines can change the direction of a force, the magnitude of the force, and the distance through which the force is applied.

7 In and Out What is mechanical advantage? A machine s mechanical advantage is the number of times the machine multiplies the input force. Mechanical advantage is a way of comparing the input force with the output force. You can calculate the mechanical advantage (MA) of any machine by dividing the output force by the input force.

8 What is mechanical advantage? A machine that has a mechanical advantage greater than one makes a task seem easier because the output force is greater than the input force. A machine that has a mechanical advantage equal to one changes only the direction of the input force. A machine that has a mechanical advantage less than one allows you to apply a greater force over a shorter distance.

9 What is mechanical efficiency? Mechanical efficiency is a comparison of a machine s work output with the work input. Mechanical efficiency (ME) equals the work output divided by the work input, expressed as a percentage. Ideally, the work a machine does on an object is the same as the work that you put into it when you apply a force. But in the real world, the work input is greater than the work output because some work is done to overcome friction.

10 Gaining Leverage What are the classes of levers? A lever is a simple machine that has a bar that pivots at a fixed point. This fixed point is called a fulcrum. Levers are used to apply a force to move an object. The force of the object is called the load.

11 What are the three classes of levers? Ideal mechanical advantage is the mechanical advantage of a simple machine that does not take friction into account. The ideal mechanical advantage of a lever is equal to the distance from input force to fulcrum (d input ) divided by the distance from output force to fulcrum (d output ).

12 What are the three classes of levers? There are three classes of levers that differ based on the positions of the fulcrum, the load, and the input force. In a first-class lever, the fulcrum is between the input force and the load. In a second-class lever, the load is between the fulcrum and the input force. In a third-class lever, the input force is between the fulcrum and the load.

13 What are the three classes of levers? Which is a first-class lever, which is a second-class lever, and which is a third-class lever?

14 Turn, Turn, Turn What is a wheel and axle? A wheel and axle is a simple machine that is made of a wheel connected to a smaller cylindrical object, the axle. The ideal mechanical advantage of a wheel and axle equals the radius corresponding to the input force (radius input ) divided by the radius corresponding to the output force (radius output ).

15 What is a wheel and axle? The radius of the wheel is always larger than the radius of the axle. The mechanical advantage is greater than one when the input force is applied to the wheel. The mechanical advantage is less than one when the input force is applied to the axle.

16 What are the types of pulleys? A pulley is a simple machine that has a grooved wheel that holds a rope or a cable. A load is attached to one end of the rope, and an input force is applied to the other end. There are three different types of pulleys.

17 What are the types of pulleys? A fixed pulley is attached to something that does not move. It allows you to pull down on the rope to lift the load up. The wheel of a movable pulley is attached to the object being moved. One end of the rope is fixed. You can pull on the other end of the rope to make the wheel and load move along the rope. A block and tackle pulley is a pulley system made by combining a fixed pulley and a movable pulley.

18 So Inclined What are inclined planes? An inclined plane is a simple machine that is a straight, slanted surface. A smaller input force is needed to move an object using an inclined plane than is needed to lift the object. However, the force must be applied through a longer distance. So, the amount of work done on the object is the same. The ideal mechanical advantage of an inclined plane can be calculated by dividing the length of the incline by the height that the load is lifted.

19 What are inclined planes? When you push something up a ramp, you are using an inclined plane.

20 What are wedges? A wedge is a pair of inclined planes that move. They have one thick end and one thin end. Wedges are used to cut and split objects. The output force of the wedge is greater than the input force, but the output force is applied through a shorter distance. The longer and thinner the wedge is, the greater its ideal mechanical advantage.

21 What are wedges? Chisels, ax heads, and knife blades are wedges.

22 What are screws? A screw is an inclined plane that is wrapped in a spiral around a cylinder. When a screw is turned, a small force is applied through the distance along the inclined plane of the screw. The screw applies a large force through the short distance it is pushed.

23 What are screws? The longer the spiral on a screw is and the closer together the threads are, the greater the screw s mechanical advantage.