1. Force

2. Objective Use the equation: net force = mass x acceleration

3. Stowmarket Physics Types of force Gravitational force between two objects with mass Magnetic force between two magnetic objects Electrical force between charged objects

4. Stowmarket Physics Force, mass and acceleration F = ma applet Newton established that: acceleration produced by a force depends on the mass of the object Also that if an object has no resultant force on it, it will not accelerate eg a car travelling at a constant velocity (balanced force) has no resultant force, and therefore no acceleration

5. Stowmarket Physics Force, mass and acceleration A resultant force always causes acceleration Zero resultant force implies constant velocity (which may be zero)

6. Stowmarket Physics Force, mass and acceleration Newton established that: acceleration is proportional to force acceleration is inversely proportional to mass, if the force is constant Newton’s Second Law: “The acceleration produced by a force when it acts on a body is proportional to the force and takes place in the direction of the force”

7. Stowmarket Physics Force, mass and acceleration F = ma where F = force in Newtons (N) m = mass in kilograms (kg) a = acceleration (ms -2 )

8. Objective F = ma practical

9. Stowmarket Physics F = ma Practical accelerating washers washers trolley mask size 1 cm light gates (resourcefulphysics.org)

10. Stowmarket Physics F = ma Practical Plot a graph of acceleration (y – axis) against force (x- axis). Use the graph to answer the following questions: 1From Newton’s 2nd Law you would expect the graph of acceleration against force to be a straight line which passes through the origin. 2Explain why you would expect this to be the case. 3Explain what the gradient of the graph represents. 4Calculate the gradient of the graph and explain whether it agrees with your answer to question (3).

11. Stowmarket Physics F = ma Practical 5Your results may show some scatter. Explain why this might be and the causes of any inaccuracy. 6Does the experimental result support Newton’s Second Law of Motion?

12. Objective Define the Newton

13. Stowmarket Physics Define the Newton One Newton is the force that will give a mass of one kilogram an acceleration of one metre per second squared 1 N = 1 kg x 1 ms -2

14. Objective Explain why F = ma cannot be used for a particle travelling at very high speeds

15. Stowmarket Physics Explain why F = ma cannot be used for a particle travelling at very high speeds According to Einstein’s Theory of Special Relativity, at speeds approaching the speed of light, the mass of a body increases This means that at very high speeds, Newton’s 2 nd Law is invalid This is unlikely to affect the majority of calculations!

16. Stowmarket Physics Example A car of mass 500kg is travelling at 20 ms -1. The driver sees a red traffic light ahead, and slows to a halt in 10 s. What force must the brakes provide to do this? a = v – u / t = 0 ms -1 – 20 ms -1 / 10 s = 2 ms -2 F = ma = 500 kg x 2 ms -2 = 1000 N

17. Questions Physics 1 Page 29 SAQ’s 1 - 4