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Grade 7 Science Unit 4 Topic 4
by

## Ray Martens

on 4 May 2011

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#### Transcript of Forces, Loads and Stresses

Forces, Loads and Stresses Have you ever seen a tennis ball split open? How did this happen? Did the force of the racquet split the ball? Was it run over by a big truck? What happened inside the material for the ball to split? In previous lessons you looked at forces. You saw that an external force acts on an object in a particular direction. Such a push or pull can change the object’s speed or direction of motion. The change of a ball’s speed or direction of motion is certainly
clear when you hit the ball with a racquet. But an external force can also change the shape or size of an object or even break it. The design of a structure must consider external forces. The structure must be able to withstand the forces that are expected to occur. What happens when a structure is hit by a very large external force? Very large internal forces develop and something may break. Read the introductory paragraphs of “Topic 4: Forces, Loads, and Stresses” on page 305 of the textbook. It gives a description of external and internal forces and
how they affect objects. Question 1 Have you ever had to climb a hill? How about walking home in a big windstorm? Have you seen a water balloon hit the ground? If you have experienced any of these, you know something about external forces. Now it’s time to get an even better idea about them Turn to page 306 of the textbook and read “External Forces.” Question 2 As you saw in the last reading in the textbook, external forces can cause internal forces. You experience these every day. Is it easy to jump up in the gym and touch the basketball net? Think
of the forces your muscles are putting on your bones when you try. Do you chew your food? Think about the forces the roots of your teeth have to contend with. To find out more about internal forces, turn to pages 307 to 309
of the textbook and read “Internal Forces.” Question 3 You started this module discussing the particles that make up objects. How these particles act together explains a lot about the properties of the object. Turn to page 314 of the textbook and read “Resisting Stress—The Inside View.” Questions 4 and 5 Looking Back
In this lesson you identified and classified external and internal forces. You also studied the effect they have on structures.
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