Unit 15 Lesson 1 What Are Forces? Copyright © Houghton Mifflin Harcourt Publishing Company 1

Pushing and Pulling A force is a push or pull. Unit 15 Lesson 1 What Are Forces? Pushing and Pulling A force is a push or pull. Forces can cause an object at rest to move, speed up, slow down, change direction, or stop. Forces can change the shape of an object. Copyright © Houghton Mifflin Harcourt Publishing Company 2

Unit 15 Lesson 1 What Are Forces? Pushing and Pulling Spring scales are used to measure force in units called newtons (N). The larger the force, the greater the change it can cause to the motion of an object. Smaller forces cause smaller changes. For example, when rowers pull back on oars, the oars push against the water. The water pushes back against the oars, causing the boat to move. Copyright © Houghton Mifflin Harcourt Publishing Company 3

Two Common Forces Gravity Unit 15 Lesson 1 What Are Forces? Two Common Forces Gravity Gravity is a force of attraction between two objects. It acts even when objects are not touching. The size of the force increases as the mass of the objects increases. The size of the force decreases as the distance between the objects increases. Copyright © Houghton Mifflin Harcourt Publishing Company 4

Unit 15 Lesson 1 What Are Forces? Friction Friction is a force that opposes motion and acts between two objects that are touching. Friction can also exist between air and a moving object. This is called air resistance. Friction can also change the energy of motion into thermal energy. Copyright © Houghton Mifflin Harcourt Publishing Company 5

Balanced or Unbalanced? Unit 15 Lesson 1 What Are Forces? Balanced or Unbalanced? Balanced forces are forces on an object that are equal in size and opposite in direction. Unbalanced forces are forces that cause a change in motion. A force must overcome the force of friction before an object will move. Copyright © Houghton Mifflin Harcourt Publishing Company 6

Unit 15 Lesson 1 What Are Forces? Pull (or Push) Harder! A large force will cause a large change in the motion of an object. A small force will cause little change. The large force makes the object change its velocity, or accelerate, quickly. Changes in velocity include changes in speed and changes in direction. Copyright © Houghton Mifflin Harcourt Publishing Company 7

Unit 15 Lesson 1 What Are Forces? Pull (or Push) Harder! What should the boy do differently to make the bell ring? Copyright © Houghton Mifflin Harcourt Publishing Company 8

Unit 15 Lesson 1 What Are Forces? I’m Not Moving! When the same force is applied, objects with greater mass accelerate more slowly than objects with less mass. How can the steel ball be made to go higher? Copyright © Houghton Mifflin Harcourt Publishing Company 9

Unit 15 Lesson 1 What Are Forces? I’m Not Moving! The larger the force applied to an object, the greater the acceleration. The less an object’s mass is, the less force is needed to change its motion. Copyright © Houghton Mifflin Harcourt Publishing Company 10

Unit 15 Lesson 1 What Are Forces? I’m Not Moving! Why does it take the full truck longer to stop than the empty truck? Copyright © Houghton Mifflin Harcourt Publishing Company 11

Unit 15 Lesson 1 What Are Forces? Let’s Go to Mars! An understanding of forces helped to safely send a spacecraft to Mars. Unbalanced forces allowed rockets to oppose Earth’s gravity. Small booster rockets changed the direction of the vehicle’s motion. Copyright © Houghton Mifflin Harcourt Publishing Company 12

Unit 15 Lesson 1 What Are Forces? Let’s Go to Mars! Rockets reached an “escape velocity” at which Earth’s gravity no longer pulled the vehicle back down. Balanced forces allowed a spacecraft to move in one direction at a constant velocity through space. Once the spacecraft reached Mars, it accelerated toward the surface because of gravity. Copyright © Houghton Mifflin Harcourt Publishing Company 13