1. © SSER Ltd. Physics (14 - 16) Newton’s Laws

2. Force Force is a vector quantity, so it has magnitude and direction. A force is a pulling or pushing effect.

3. Force A force will cause a free body to change velocity. Press the accelerator to apply the driving force, so that the velocity increases; press the brake to apply a force in the opposite direction, so that the velocity decreases...

4. Force A force will cause a free body to change velocity. Press the accelerator to apply the driving force, so that the velocity increases; press the brake to apply a force in the opposite direction, so that the velocity decreases...

5. Resultant Force When two or more forces are applied to a body, they combine to produce a single resultant force. The resultant force is the overall effect of all the forces acting together on a body.

6. Resultant Force Use the diagram to see what happens to the resultant force when... When two or more forces are applied to a body, they combine to produce a single resultant force. The resultant force is the overall effect of all the forces acting together on a body. 1. Two forces have the same direction? 2. Two forces have the opposite direction? 3. Two forces are applied at an angle?

7. Resultant Force When two forces act in the same direction on a body, the resultant force is in the same direction as the individual forces, and the magnitude is the sum of the two individual forces. When two forces act in the opposite direction, the resultant force is in the direction of the largest force, and the magnitude is the difference between the two individual forces. Force from 1 st tug Resultant force Force from 2 nd tug Force from 1 st tug Force from 2 nd tug Resultant force

8. Resultant Force When two forces act at different angles on a body, the resultant force is calculated using the parallelogram of forces... Force from tug 1 Force from tug 2 Resultant force

9. Resultant Force When the forces acting on a body produce a zero resultant, we say that the forces are balanced. Draw three lines representing three different forces. Alter the forces to see the effect on the resultant.

10. Resultant Force When the forces acting on a body produce a zero resultant, we say that the forces are balanced. Draw three lines representing three different forces. Alter the forces to see the effect on the resultant.

11. Isaac Newton Isaac Newton was a great mathematician and scientist. His discoveries made major advances to our knowledge of the physical laws that govern the Universe. He invented a new branch of mathematics called calculus, that enabled mathematicians to solve many problems. He researched into gravity, light, rotating bodies, planetary orbits, and tidal motion. He had amazing powers of concentration, sometimes studying and solving problems continuously for 19 hours a day. In 1687, Newton published his most important discoveries in a famous book called “Principia Mathematica”. Isaac Newton 1643 - 1727

12. Newton’s First Law Every object travelling at constant speed in a straight line (or at rest) will continue at this same speed, unless a resultant force acts on it. Newton’s Laws of Motion Newton stated three laws concerning force and motion.  If the resultant force acting on a stationary body is zero, the body will remain stationary. Examples:  If the resultant force acting on a moving body is zero, the body will continue to move at the same speed and in the same direction (i.e. with the same velocity).

13. When a vehicle travels at a constant speed, the frictional forces exactly balance the driving force, so the resultant force is zero. Newton’s First Law

14. When a vehicle travels at a constant speed, the frictional forces exactly balance the driving force, so the resultant force is zero. Newton’s First Law

15. An object in space will continue at the same speed forever, unless it is acted upon by the gravity of a celestial body.

16. The acceleration of a body is proportional to the resultant force and in the same direction as the force. Newton’s Second Law Newton’s second law is represented by the equation: F = m x a Forcemassacceleration  If the resultant force acting on a stationary body is not zero, the body will accelerate in the direction of the resultant force. Example:  A car accelerates when the driving force is greater than the frictional forces. A greater driving force will produce a greater acceleration.

17. Demonstrating Newton’s Second Law

18. Using F = ma F = m x a m = F / a a = F / m

19. F = ma connects the resultant force to the mass of the object and its acceleration: Force = mass x acceleration N kg m/s 2 Force is measured in Newtons (N). One Newton is the force required to accelerate a mass of 1kg by 1m/s 2. Using F = ma The formula, F = ma can be rearranged using a formula triangle...

20. If you know any two of these values, you can use the formula triangle to find the remaining value... Using F = ma F = m x a m = F a a = F m

21. A lorry with a mass of 3500kg decelerates steadily at 3m/s 2. What is the braking force applied? Using F = ma F = m x a F = 3500 x 3 F = 10500N Example 1 The braking force applied is 10500N

22. A force of 250N acts on an object with a mass of 25kg. What is the acceleration produced? Using F = ma Example 2 a = F m a = 250 25 a = 10m/s 2 The acceleration produced is 10m/s 2

23. A force of 80N acts on an object, causing an acceleration of 3.2m/s 2. What is the mass of the object? Using F = ma Example 3 m = F a m = 80 3.2 m = 25kg The mass of the object is 25kg

24. Mass & Weight Mass is a measure of the amount of matter in an object. It depends on the number and type of atoms present. Mass is a scalar quantity because it has only magnitude. Weight is the gravitational force acting on a mass. Weight is a vector quantity, because it has magnitude and direction (assumed to be acting towards the centre of the Earth). Your mass on the Earth is exactly the same as your mass on the Moon, or anywhere else. On the Moon objects weigh roughly 1/6 th their weight on the Earth. This is because the mass of the Moon is about 1/6 th the mass of the Earth and therefore its gravity is also 1/6 th that of the Earth.

25. Mass & Weight The acceleration due to gravity, g is 9.8m/s 2, although we often use 10m/s 2 in calculations. From Newton’s second law...

26. Mass & Weight The acceleration due to gravity, g is 9.8m/s 2, although we often use 10m/s 2 in calculations. From Newton’s second law... F = m x a Weight = mg Sometimes, we say “I weigh 50kg”, when scientifically we should either say “my mass is 50kg”, or “I weigh 500N”.

27. Mass & Weight In Space Mass = 90 Kilograms Weight = 0 Newtons On the Moon Mass = 90 Kilograms Weight = 150 Newtons On the Earth Mass = 90 Kilograms Weight = 900 Newtons When an astronaut has zero weight (weightless) in space it does not mean that that astronaut has ceased to exist!

28. Newton’s Third Law Examples:  If you lean on a wall, the wall pushes you back with the same amount of force in the opposite direction.  A book resting on a table applies a downward force on the table equal to the weight of the book. The table applies an equal and opposite force on the book. The book remains at rest because the forces acting on the book (the weight acting downwards and the upward force from the table) add up to zero. For every action, there is an equal and opposite reaction.

29. Newton’s Third Law Fire the cannon, then describe how a cannon works, using Newton’s third law on Motion.

30. Newton’s Third Law  When the cannon fires, the chemical energy of the gunpowder is converted into the kinetic energy of the cannonball.  The cannon applies a force to the cannonball, causing a sudden increase in velocity.  The cannonball applies an equal and opposite force to the cannon, causing a recoil effect (a sudden but brief increase in velocity of the cannon in the opposite direction). For every action, there is an equal and opposite reaction.

31. Newton’s Third Law Click on the image, then use ‘Z’ for left, ‘X’ for right, and ‘M’ to fire the jetpack. Describe how a jetpack works, using Newton’s 3 rd law.

32. Newton’s Third Law  When the jetpack fires, the chemical energy of the fuel is converted into the kinetic energy of the exhaust gases.  The gas pushes the jetpack with an equal and opposite force. The man is attached to the jetpack, so he moves with it.  The jetpack pushes the exhaust gas downwards or sideways. For every action, there is an equal and opposite reaction.

33. A mass m, resting on a floor exerts a force equal to its weight on the floor. Reaction Force The floor also exerts an opposite force on the mass. This force is called the reaction force (or just ‘reaction’). As there is no change in velocity of the mass, from Newton’s first law, the resultant force must be zero, so the weight and reaction force must be equal in magnitude and opposite in direction... R = mg

34. End of Show Copyright © 2006 SSER Ltd. and its licensors. Images are for viewing purposes only. All rights reserved. Copyright © 2006 SSER Ltd. and its licensors. Images are for viewing purposes only. All rights reserved.