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Forces

Forces. Physical Science Chapter 12. 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. Mass and inertia.

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Forces

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  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.

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