Forces and Motion

1. Forces and Motion

2. What is a Force? • A Force is a push or pull acting on an object. • Forces can cause motion to start, stop or change the direction. • Force can be measured by a spring scale. The SI unit of force is the Newton (N).

3. Combining Force • Force is a vector so it can be represented by an • The net force is the result of all the forces acting on an object.

4. Balanced and Unbalanced Forces • Balanced forces: equal in size; opposite in directions. No change in motion. • Unbalanced forces: Not equal; do not cancel. Motion may occur.

5. Friction • Friction is the force that opposes motion. • Static friction: acts on objects not moving. • Sliding friction: • Rolling friction: • Fluid friction: interaction with a liquid or a gas.

6. Gravity and Air Resistance • Gravity pulls down towards the center of the Earth. • Anything with mass exerts a force of gravity. • A falling object reaches terminal velocity when the acceleration of gravity is cancelled by air resistance (fluid friction).

7. Projectile Motion • A projectile has both forward motion and is pulled down by gravity. This makes it follow a curved path. • Even though it is moving forward that doesn’t change how long it takes to fall! • Mythbusters!

8. How is force related to motion? • Aristotle: “force is needed to keep a constant speed” • Galileo: “motion would continue unless a force happens” • Newton: formulates 3 laws of motion based on Galileo’s concept.

9. 1st Law of Motion • An object in motion remains in motion at a constant velocity, an object at rest remains at rest – unless acted upon by an outside force. • Law of inertia – the resistance to a change in motion.

10. 2nd Law of Motion • F = ma or a = F/m • Shows the relationship between force, mass and acceleration.

11. Weight and Mass • Mass: the amount of matter in an object • Weight: the effect of gravity on mass • W = mg • Weight = mass x acceleration due to gravity. • Weight is measured in Newtons.

12. Newton’s 3rd Law • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • These “action-reaction” forces do not cancel out because they don’t act on the same thing!

13. Momentum • Momentum is the product of an object’s mass and its velocity. • p = m x v • Unit: kg • m/s

14. Law of Conservation of Momentum • In a closed system the loss of momentum by one object equals the gain in momentum by another object. • The total momentum of the system does not change.

15. Universal Forces • Electromagnetic • Gravitational • Strong Nuclear Force • Weak Nuclear Force

16. Electromagnetic Force • The only force that can both attract and repel. • Magnets: Opposite poles attract, like poles repel • Charged particles: Opposite charges attract, like charges repel

17. Nuclear Forces • Strong Nuclear Force • Attractive force between particles in the nucleus of an atom. • Protons, neutrons • Very strong, but only acts over a very short distance. • Weak Nuclear Force • Also attractive and between particles in the nucleus – but not as strong. • Even shorter range as well!

18. Gravitational Force • Attractive force between all objects • Strength of the force depends on mass and distance between objects. • Force decreases as the square of the distance 2x distance = ¼ force 3x distance = 1/9 force

19. Centripetal Force Centripetal force is the force directed towards the center of the circle. TIDES!