Dynamics_phsics-NEWTON'S law and summary.pptx

Dynamics

1. Newton's first law Dynamics Newton's second law Newton's third law Summary activities

2. Forces and motion Aristotle thought that all objects tend to move towards their natural place in the universe, and that their velocity was determined by the strength of this force. If there is no resultant force on an object, can it be moving? Galileo later realised that the acceleration of an object, not its velocity, is proportional to the size of the total force acting on it, and in 1687, Sir Isaac Newton published his first law of motion , which confirmed this. For instance, heavier objects would move towards the ground more quickly than lighter ones.

3. Newton’s first law A moving object has kinetic energy . This energy will not be lost unless a force acts to slow the object down. This is Newton’s first law , also known as the law of inertia . Objects moving in free space with no forces acting on them will continue to move in a straight line at a constant velocity until a force causes them to change speed or direction. For instance, a space shuttle requires large forces from its engines during launch, but can move through space with little or no propulsion if far enough away from gravitational influences.

4. Newton’s first law Are these statements about motion true or false? An object will always be stationary when there are no forces acting on it. Kinetic energy of an object is maintained by applying a constant force. A resultant force must be applied to cause an object to change its speed or direction. The velocity of an object with constant mass is proportional to the resultant force acting on it. The size and direction of the force on an object determine its acceleration.

5. Balanced and Unbalanced Forces Force is a vector. The vector sum of all the forces on an object gives a net or resultant force . When more than one force acts on an object, the net force determines how the motion of an object changes. If the motion of an object changes, its velocity changes. A change in velocity means the object is accelerating. A force can act on an object without causing it to accelerate if other forces cancel the push or pull of the force. Two or more forces exerted on an object are balanced forces if their effects cancel each other and they do not change the object’s velocity. If the forces on an object are balanced , the net force is zero. If the net force is not zero, the forces are unbalanced forces . Then the effects of the forces don’t cancel, and the object’s velocity changes.

6. Balanced forces Look at the forces on this object: 3 N 3 N 4 N 4 N To see the vector sum of the forces, add the vectors together nose-to-tail. There is no ‘net’ or resultant force on the mass: the forces are balanced . 4 N 4 N 3 N 3 N

7. Unbalanced forces 4 N There is a resultant force of 5 N at a direction of 53° from the horizontal. The forces on this mass are unbalanced : 3 N 8 N What is the resultant force? Add the vectors together again, nose-to-tail. 8 N 3 N 4 N 4 N 3 N θ F F = √3 2 + 4 2 = 5 N θ = tan -1 (4/3) = 53 °

8. Applying Newton’s first law In everyday life, most things don’t move unless a force is applied. Why is this? What forces act on this car as it travels at a steady speed? Friction from the road pushes the car forward. This force is a reaction to the backwards push of the wheels on the road. The car is travelling at a steady speed, so these two forces must be equal. Air resistance acts against the direction of motion.

9. Newton's second law Dynamics Newton's first law Newton's third law Summary activities

10. Unbalanced forces What is Newton’s second law? If there is a resultant force on an object, it accelerates in the direction of that force. Its acceleration is directly proportional to the force, and inversely proportional to the object’s mass. This can be stated in a vector equation:

11. Unbalanced forces Units of Force Force is measured in newtons, abbreviated N. Because the SI unit for mass is the kilogram (kg) and acceleration has units of meters per second squared (m/s 2 ), 1 N also is equal to 1 kg•m /s 2 . In other words, to calculate a force in newtons from the equation shown on the prior page, the mass must be given in kg and the acceleration in m/s 2 . For example, the downward force on an object due to gravity (weight) at the Earth’s surface is: weight = m g Where g = 9.8ms -2 (standard acceleration due to gravity).

12. Newton’s second law example A car and caravan are accelerating at 0.5 ms -2 . 1. Find the driving force produced by the engine. The engine must accelerate both the car and the caravan: F = ma = (3000 kg + 5000 kg) × 0.5 ms -2 = 4000 N 3000 kg 5000 kg ?

13. Newton’s second law example A car and caravan are accelerating at 0.5 ms -2 . 2. Find the tension in the tow-bar. The tow-bar accelerates the caravan only: F = ma = 5000 kg × 0.5 ms-2 = 2500 N 3000 kg 5000 kg ? 4000 N

14. Weight The weight of an object is the size of the gravitational force exerted on an object. Your weight on Earth is the gravitational force between you and Earth. On Earth, weight is calculated from this equation: W = m • (9.8 m/s 2 ) W is the weight in N, and m is the mass in kg. Your weight would change if you were standing on a planet other than Earth.

15. Weight and Mass Weight and mass are different. Weight is a force, just like the push of your hand is a force, and is measured in newtons. When you stand on a bathroom scale, you are measuring the pull of Earth’s gravity—a force. However, mass is the amount of matter in an object, and doesn’t depend on location. Weight will vary with location, but mass will remain constant. A book with a mass of 1 kg has a mass of 1 kg on Earth or on Mars. However, the weight of the book would be different on Earth and Mars. The two planets would exert a different gravitational force on the book.

16. C enter of mass When you throw a stick, the motion of the stick might seem to be complicated. However, there is one point on the stick, called the center of mass , that moves in a smooth path. The center of mass is the point in an object that moves as if all the object’s mass were concentrated at that point. For a symmetrical object, such as a ball, the center of mass is at the object’s center. However, for any object the center of mass moves as if the net force is being applied there.

17. Summary Force and Acceleration According to Newton’s second law, the net force on an object, its mass, and its acceleration are related by F net = ma Gravity The force of gravity between any two objects is always attractive and depends on the masses of the objects and the distance between them Using Newton’s Second Law A moving object speeds up if the net force is in the direction of the motion. A moving object slows down if the net force is in the direction opposite to the motion. A moving object turns if the net force is at an angle to the direction of motion