What is force? A force is a push or pull.

Transcription

1 2.3 Motion & Forces What is force? A force is a push or pull. force can cause the motion of an object to change. The force of the moving ball causes the ball at rest to move in the direction of the force

2 2.3 Motion & Forces Inertia and Mass Inertia (ih NUR shuh) is the tendency of an object to resist any change in its motion. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.

3 3.0 Forces Inertia and Mass The velocity of the object remains constant unless a force changes it. If an object is at rest, it tends to remain at rest. The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

4 3.0 Forces Newton's Laws of Motion The British scientist Sir Isaac Newton ( ) was able to state rules that describe the effects of forces on the motion of objects.

5 2.3 Motion and Forces Newton's First Law of Motion Newton's first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. If an object is at rest, it stays at rest unless an unbalanced net force acts on it. This law is sometimes called the law of inertia.

6 2.3 Motion and Forces What happens in a crash? How can a car crash explain the law of inertia?

7 2.3 Motion and Forces What happens in a crash? Any passenger not wearing a safety belt continues to move forward at the same speed the car was traveling. Within about 0.02 s (1/50 of a second) after the car stops, unbelted passengers slam into the dashboard, steering wheel, windshield, or the backs of the front seats.

8 2.3 Safety Belts Motion and Forces This reduces the force exerted on the person. The safety belt also prevents the person from being thrown out of the car.

9 2.3 Air Bags Motion and Forces Air bags also reduce injuries in car crashes by providing a cushion that reduces the force on the car's occupants. When impact occurs, a chemical reaction occurs in the air bag that produces nitrogen gas. The air bag expands rapidly and then deflates just as quickly as the nitrogen gas escapes out of tiny holes in the bag.

10 2.3 Motion & Forces Balanced Forces When two or more forces act on an object at the same time, the forces combine to form the net force. Forces on an object that are equal in size and opposite in direction are called balanced forces.

11 2.3 Motion & Forces Unbalanced Forces When two students are pushing with unequal forces in opposite directions,or pushing on the box in the same direction.

12 2.3 Section Check Question 1 A force is a. Answer A force is a push or pull. Forces, such as the force of the atmosphere against a person s body, are not always noticeable.

13 2.3 Section Check Question 2 When are forces on an object balanced? Answer When forces are equal in size and opposite in direction, they are balanced forces, and the net force is zero.

14 2.3 Section Check Question 3 Inertia is. A. the tendency of an object to resist any change in its motion B. the tendency of an object to have a positive acceleration

15 2.3 Section Check C. The tendency of an object to have a net force of zero. D. The tendency of an object to change in speed or direction.

16 2.3 Section Check Answer Inertia is the tendency of an object to resist any change in its motion. An unbalanced force must act upon the object in order for its motion to change.

17 3.1 Newton s Second Law Newton s Second Law of Motion Newton s second law of motion describes how the forces exerted on an object, its mass, and its acceleration are related.

18 3.1 Newton s Second Law Calculating Net Force with the Second Law Newton s second law also can be used to calculate the net force if mass and acceleration are known.

19 3.1 Newton s Second Law Calculating Net Force with the Second Law F=Force M= Mass A= acceleration

20 3.1 Friction Newton s Second Law The amount of friction between two surfaces depends on two factors the kinds of surfaces and the force pressing the surfaces together.

21 3.1 Newton s Second Law Three Types of Friction Sliding Friction Rolling Friction Static Friction

22 3.1 Newton s Second Law Air Resistance A friction-like force called air resistance opposes the motion of objects that move through the air. Air resistance causes objects to fall with different accelerations and different speeds.

23 3.1 Newton s Second Law Air Resistance The amount of air resistance on an object depends on the speed, size, and shape of the object. Air resistance, not the object s mass, is why feathers, leaves, and pieces of paper fall more slowly than pennies, acorns, and apples.

24 3.2 Gravity What is gravity? Gravity is an attractive force between any two objects that depends on the masses of the objects and the distance between them.

25 3.2 Gravity Earth s Gravitational Acceleration When all forces except gravity acting on a falling object can be ignored, the object is said to be in free fall. Close to Earth s surface, the acceleration of a falling object in free fall is about 9.8 m/s 2. This acceleration is given the symbol g and is sometimes called the acceleration of gravity.

26 3.2 Gravity Weight and Mass Weight and mass are not the same. Weight is a force and mass is a measure of the amount of matter an object contains. Weight and mass are related. Weight is the force that gravity exerts on a mass.

27 3.2 Gravity Weight and Mass As the mass of an object goes up, its weight (force due to gravity) goes up, proportional to the acceleration due to gravity. The weight of an object can change, depending on the gravitational force on the object.

28 3.2 Gravity Weight and Mass The table shows how various weights on Earth would be different on the Moon and some of the planets.

29 3.3 The Third Law of Motion Newton s Third Law Newton s third law of motion describes action-reaction pairs this way. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.

30 3.3 The Third Law of Motion Action and Reaction When a force is applied in nature, a reaction force occurs at the same time. Example: When you jump on a trampoline, for example, you exert a downward force on the trampoline.