Forces, Mass, and Acceleration: Understanding Newton's Laws

1. Chapter 3 Forces

2. Forces Forces, Mass, and Acceleration • the greater the force applied on an object, the greater the acceleration • the greater the mass of an object, the less acceleration

3. Forces • Newton’s Second Law of Motion – the net force acting on an object causes the object to accelerate in the direction of the net force • Formula for force: • F = force m = mass a = acceleration F = ma

4. Forces • Newton’s Second Law of Motion – the net force acting on an object causes the object to accelerate in the direction of the net force • units for force: Newtons F = ma

5. Forces Sample Problems • example: A person that weighs 70 kg jumps out of a plane at 9.8 m/s2. What is the force that gravity is exerting on the person?

6. Forces Sample Problems • If gravity is exerting a force of 98 Newtons on an object in air, and the acceleration due to gravity is 9.8 m/s2, what is the object’s mass?

7. Forces Sample Problems • If an object weighs 10 kg and has a force of 200 Newtons acting on it, what is the acceleration?

8. Forces • friction – the force that opposes motion between two surfaces that are touching each other • Newton’s first law would say that if you push a skateboard, it would continue to move in a straight line at constant speed. We know however, that the skateboard slows down due to friction

9. Forces Types of Friction • static friction – the friction between two surfaces that are not moving past each other • example: when a box is so heavy you try and push it but it doesn’t move NOT Moving

10. Forces Types of Friction • sliding friction – the force that opposes the motion of two surfaces sliding past each other • example: when you push a box it slides across the floor but the sliding friction makes it hard to push (but it DOES move)

11. Forces Types of Friction • rolling friction – the friction between a rolling object (ex. tire) and the surface that it rolls on • example: a car’s tire rolling over the pavement or a train rolling on the rails • usually much less friction than static or sliding friction

12. Forces • air resistance - a force that acts opposite to the motion of an object • example: running against the wind or a parachute falling to the ground • air resistance depends on the speed, size, and shape of the object • if no air resistance exists, two objects will fall at the same rate (even a feather and apple)

13. Forces

14. Gravity • law of gravitation – states that any two masses exert an attractive force on each other • the force of gravity increases when the mass of either object increases • the force of gravity increases when the two objects move closer together

15. Gravity

16. Gravity Gravitational Acceleration • the gravitational attraction of Earth causes falling objects to have an acceleration of 9.8 m/s2 • remember that F = ma so the force of gravity on an object near Earth’s surface is: F = m x 9.8 m/s2

17. Gravity • weight (W) – the gravitational force exerted on an object • measured in newtons (N) • weight and mass are NOT the same because mass does not change based on location but your weight does Weight = mass x 9.8 m/s2

18. Gravity • example: How much does a person with a mass of 70 kg weigh on Earth? • on the moon, the acceleration due to gravity is only 1.6 m/s2 so you will weigh less on the moon

19. Gravity projectile motion • the motion of anything tossed, thrown, shot, etc. will travel in a curved path • the object (projectile) will follow a curved path because of Earth’s gravitational pull

20. Forces • centripetal force – an unbalanced force that acts in the direction toward the center of motion • centripetal acceleration then, is the acceleration of an object toward the center of a curved or circular path

21. Forces • centripetal force – an unbalanced force that acts in the direction toward the center of motion • centripetal acceleration then, is the acceleration of an object toward the center of a curved or circular path • acceleration occurs during a curve because the direction is changing therefore making velocity change • example: centripetal force (friction between the tires and road surface) causes a car on a curve to stay inward while the car’s inertia forces it outward

22. Motion • Newton’s third law of motion – describes action-reaction pairs by stating that every action force has an equal and opposite reaction force. • “for every action, there is an equal and opposite reaction” • example: if an object hits water, water splashes back up

23. Motion • A rocket works by igniting fuel which causes the gases to exert a downward force, resulting in air being pushed downward exerting a force upward, pushing the rocket up

24. Motion • momentum (p) - property that a moving object has that is related to how much force is needed to change its motion • the unit for momentum is kilogram meters per second momentum = mass x velocity p = m x v

25. Motion • momentum (p) - property that a moving object has that is related to how much force is needed to change its motion • two trucks might have the same velocity but the bigger truck has more momentum • example: Compare the momentum of a 50-kg dolphin swimming at 10.4 m/s and a 6,300-kg elephant walking 0.11 m/s. momentum = mass x velocity

26. Forces Law of Conservation of Momentum • If no other force acts on bodies in motion, the momentum before collision is equal to momentum after collision • momentum is not lost or created – it is conserved • total momentum is zero.