chap. 9 force and energy

• Force is an interaction that takes place between objects
• Force can make a stationary object move, stop a moving object, speed up a moving object, slow down a moving object, change the direction of a moving object, change the shape and size of an object, create a turning effect to make an object turn or rotate
• A force cannot change the mass of an object
• The unit for force is the Newtons (N)
• Force can be measured using a force meter or newton meter, and a spring balance attract
• Strong forces of nature can be found in earthquakes, typhoons, tsunamis, and volcanoes
• Speed is a measure of how fast an object moves
• Speed is defined as the distance travelled per unit time, with units of metres per second (m/s) or kilometres per hour (km/h)
• A larger force will make an object speed up more or slow down more
• When an object's mass increases, the force needed to change its speed also increases
• Sir Isaac Newton formulated his theories for motion based on these ideas
• Types of forces:

  • Non-contact forces: gravity, magnetism
  • Contact forces: friction
• Friction is a force that opposes the movement of an object and acts in the opposite direction to the object's movement
• Friction becomes greater when surfaces are rougher or when the object has a heavier weight
• Friction can be a nuisance because it causes surfaces to heat up, wear down objects, and slow down movement
• Friction is useful because it allows animals to walk, allows us to grip and hold things, and allows us to light a matchstick
• Ways to reduce friction include making surfaces smoother, using wheels and ball bearings, applying lubrication, and streamlining
• Gravity is a force associated with objects with large masses like planets and stars
• Weight of an object is the amount of gravitational force acting on the object
• The amount of gravity can be different at different locations, for example, the gravity on the Moon's surface is 1/6 as strong as on Earth's surface
• Difference between mass and weight:

  • Mass is a measure of the amount of matter in an object
  • Weight is a measure of the force of gravity on an object
• Magnetic force is exerted by magnets and can attract or repel other magnets
• Contact forces involved in rolling a ball on the ground: friction, gravity
• Contact force involved in a stone falling when dropped from a cliff: gravity
• Non-contact force involved in a magnet sticking to the refrigerator door: magnetic force
• Contact force involved in the surface of an airplane becoming hot after flying in the air at high speed: friction
• Contact and non-contact forces involved in a magician rubbing a balloon and sticking it to the wall: friction, magnetic
• Force applied to an object cannot change the mass of the object
• Forces that can be exerted on an object without requiring physical contact: air resistance, magnetic force
• A force can bend an object, make an object rotate, stop a moving object
• Turning effect of a force occurs when a force is applied off-center, making the object turn about a pivot
• Pressure is defined as the force acting per unit area
• Base SI unit for pressure is the pascal (Pa) or newton per square meter (N/m2)
• Atmospheric pressure is exerted due to the weight of the air acting on you
• Pressure increases when the area of contact is smaller and decreases when the area of contact is larger
• Work is done on an object when a force is applied to make it move in the direction of the force
• Work done depends on the size of the force and the distance moved in the direction of the force
• Energy is transferred to an object when work is done on it
• Energy is the ability to do work
• All living things require energy for processes to keep them alive