Forces

1. Forces Physical Science Chapter 12

2. Inertia • The tendency of an object to resist any change in its motion. • If it is moving, it tends to keep moving at the same velocity unless a force acts on it. • If it is not moving, it tends to remain at rest unless a force acts on it.

3. Mass and inertia • The more mass an object has, the greater its inertia is. • Objects with greater inertia take more force to change their velocity.

4. Newton’s first law of motion • An object in motion maintains its velocity unless it experiences a net force. • An object at rest stays at rest unless it experiences a net force. • Also called the law of inertia.

5. Seat belts • When a car stops or turns suddenly, the passengers’ inertia keeps them moving in the original direction of travel. • Slide sideways when turning • Slide forwards when stopping • Seatbelts (and friction) stop passengers’ motion.

6. Car seats • Children are too small for regular seat belts. • Having two straps helps distribute the force more evenly. • Rear-facing car seats distributes the force over the infant’s entire body, making the force on each individual part smaller.

7. Airbags • Supplement seatbelts • Sensors detect sudden changes in speed and start a chemical reaction in the air bag. • Nitrogen gas (from reaction) inflates air bag.

8. Discuss • Explain how the law of inertia relates to seat belt safety. • How is inertia related to mass?

9. Newton’s second law • A net force acting on an object causes the object to accelerate in the direction of the force. • The acceleration is affected by two things • size of force • mass of object

10. Size of Force • A larger force will give an object greater acceleration.

11. Mass of object • A more massive object will take a larger force to give it the same acceleration.

12. Equation for Newton’s 2nd law

13. Units on force • Last chapter we learned that force has units of Newtons. • From Newton’s second law equation,

14. Example • How much force is needed to accelerate a 70-kg rider and the 200-kg motorcycle the rider is on at 4 m/s2?

15. You try • How much force is needed to accelerate a 60-kg person and the 500-kg car the person is in at 6 m/s2?

16. Rearranging the equation • We can rearrange the equation in the two following ways:

17. You try • A 63-kg skater pushes off from a wall with a force of 300 N. What is the skater’s acceleration?

18. Gravity • Every object in the universe exerts a force on every other object • That force is gravity • Often, the force is too small to notice • Example – the force of gravity between two people

19. Gravitational force • The amount of gravitational force between two objects depends on two things • Their masses and the distance between them. • The mass of a person is small, so the gravitational force between two people is weak. • The mass of the Earth is large, so the gravitational force between the Earth and a person is strong.

20. Gravitational force • Gets weaker as the distance increases

21. Weight • The measure of the force of gravity on an object. • usually used for the gravitational force between the Earth and an object near its surface

22. Weight • Related to mass • Objects with greater mass have greater weight.

23. Weight and mass • Weight is measured in Newtons (N). • 1 kg of mass at the Earth’s surface has a weight of 9.8 N

24. Weight on Earth vs. Weight on Moon • Since the Earth has more mass than the moon, it exerts a greater gravitational force than the moon. • So, things weigh less on the moon than they do on Earth. • Do they have less mass on the moon?

25. Scales • Use the principal of balanced forces to measure how much something weighs. • Scales use springs to balance the force of an object’s weight.

26. Weightlessness • To be truly weightless, an object would have to be free from gravity. • To feel weightless, something has to be in free fall along with its surroundings.

27. Weightlessness • How does a scale measure weight? • If it can’t push back, it would read zero. • You would feel weightless.

28. Acceleration due to gravity • Near Earth’s surface, gravity causes all falling objects to accelerate at a rate of 9.8 m/s2, regardless of their mass. • Acceleration due to gravity is represented by the letter g.

29. Reality check • Do all objects really fall with the same acceleration?

30. Air resistance • The force air exerts on a moving object • Acts in the opposite direction to which an object is moving • For falling objects, air resistance pushes up while gravity pulls down

31. Air resistance • Depends on the speed, size, and shape of the object • The larger the object, the more air resistance affects it • The faster an object is moving, the more air resistance affects it

32. Terminal velocity • As on object falls, air resistance gradually increases until it equals the pull of gravity. • At this point, the object stops accelerating and moves with a constant velocity – called its terminal velocity.

33. Projectiles • Anything that’s shot or thrown through the air. • Always follow a curved path. • Motion can be split into independent vertical and horizontal parts.

34. The horizontal part • Once the object is released, there is no force acting on it horizontally. • It maintains a constant horizontal velocity. (Newton’s 1st)

35. The vertical part • There is a force acting – gravity • The object is pulled downward with a constant acceleration of 9.8 m/s2. (Newton’s 2nd)

36. Discuss • Explain why your weight would be less on the moon than on Earth even though your mass would not change. • Use examples to expain how changes in mass and changes in distance affect gravitational force.