Chapter: Forces and Changes in Motion

1. Table of Contents Chapter: Forces and Changes in Motion Section 1: Motion Section 2: Forces and Motion Section 3: The Laws of Motion

2. Forces and Motion 2 Force • A force is a push or a pull that one object exerts on another object. • Objects like floors, chairs, and Earth also exert forces on other objects, including people.

3. Forces and Motion 2 Force Has Direction and Size • Just like velocity and acceleration, a force has both size and direction. • The direction of a force is the direction of the push or pull. • Pushing or pulling harder increases the size of the force you exert. The size of a force is measured in newtons (N).

4. Forces and Motion 2 How Forces Combine • When more than one force acts on an object, the forces combine. • The combination of all the forces acting on an object is the net force.

5. Forces and Motion 2 Combining Forces in the Same Direction • When two forces act in the same direction on an object, like a box, the net force is equal to the sum of the two forces.

6. Forces and Motion 2 Combining Forces in Opposite Directions • If two forces of equal strength act on the box in opposite directions, the forces will cancel, resulting in a net force of zero.

7. Forces and Motion 2 Combining Forces in Opposite Directions • When two unequal forces act in opposite directions on the box, the net force is the difference of the two forces.

8. Forces and Motion 2 Balanced and Unbalanced Forces • When the net force on an object is zero, the motion of the object doesn’t change. • The forces acting on an object are balanced forces if the net force is zero. • The forces acting on an object are unbalanced forces if the net force is not zero.

9. Forces and Motion 2 Contact and Non-contact Forces • A force is exerted when one object pushes or pulls on another. • A force that is exerted only when two objects are touching is a contact force. • Non-contact forces are forces that can be exerted by one object on another even when the objects aren’t touching.

10. Forces and Motion 2 Gravity • Gravity is a non-contact force that every object exerts on every other object due to their masses. • The gravitational force between two objects is an attractive force that tends to pull the two objects closer together.

11. Forces and Motion 2 Gravity • The size of the gravitational force between two objects depends on the masses of the objects and the distance between them. • The gravitational force between two objects increases if the mass of one or both of the objects increases. • If two objects move closer together, the gravitational force between them increases.

12. Forces and Motion 2 Mass and Weight • Because weight is a force, it is measured in newtons. • Weight is not the same as mass. Mass is the amount of matter an object contains, and is measured in kilograms.

13. Forces and Motion 2 Friction • Friction is a contact force that resists the sliding motion of two surfaces that are touching. • Friction causes a sliding object to slow down and stop. • Friction also can prevent surfaces from sliding past each other.

14. Forces and Motion 2 Static Friction • Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.

15. Forces and Motion • Sliding friction is the force that opposes the motion of two sliding surfaces in contact. 2 Sliding Friction • Sliding friction exists between all sliding surfaces that are touching.

16. Forces and Motion 2 What causes friction? • When two surfaces are in contact, the surfaces stick to each other where the dips and bumps on one surface touch the dips and bumps on the other surface. • Friction is caused by the sticking of the two surfaces at these bumps and dips.

17. Forces and Motion 2 The Buoyant Force • The buoyant force is a force exerted by a fluid on an object that is in the fluid. • The buoyant force is always upward. • If you are floating in water, the buoyant force is large enough to balance your weight.

18. Forces and Motion 2 Air Resistance • Air resistance is a contact force that opposes the motion of objects moving in air. • Just like friction, air resistance acts in the direction opposite to an object’s motion. • Air resistance is less for a narrow, pointed object than for a wide, flat object.

19. Section Check 2 Question 1 A force is a _______ or a _______. Answer Force is a push or a pull. FL: SC.C.2.3.2

20. Section Check 2 Question 2 A force exerted between two objects that are not touching is known as _______. Answer The answer is a long-range force. Gravity is a long-range force; although we are not touching the Sun, our planet is being acted on by the Sun to keep it in orbit. FL: SC.C.2.3.1

21. Section Check 2 Question 3 You try to push a desk across the floor one direction but a friend is trying to push it the opposite direction. As a result, the desk doesn’t move. This is an example of what kind of forces? A. balanced forces B. continuing forces C. extended forces D. unbalanced forces FL: SC.C.2.3.6

22. Section Check 2 Answer The answer is A. When forces are balanced, the net force is zero. FL: SC.C.2.3.6