Chapter 8 Section 8.1 FORCES

Objectives By the end of the lesson you should be able to: Explain the differences and similarities between the 6 types of forces Explain the difference between mass and weight

FORCES What is a force? A push or a pull that acts on an object A force can cause an object to move, can stop an object, can change the motion of an object or can change the shape of an object

Types of Forces Many different types of forces Classified into two categories: Contact forces Action-at-a-distance forces

Contact Forces Contact Forces can only have an effect on objects they touch, like when you contact a box and apply a force to move it Contact forces can be applied to bend, tear, stretch, compress or twist an object Types: Tension Force Friction Force Elastic Force

Tension Force A tension force is a force experienced by a wire or a rope when it is pulled at either end Example?? Clothes line, tight rope, tug-of-war

Friction Force A friction force works to slow down or stop motion because of surfaces rubbing against each other Example?? One example is skate blades cutting into and rubbing against the ice, bringing the skater to a stop

Elastic Force An elastic force is exerted when a spring-like object (like a trampoline) restores itself to normal shape after it has been compressed or stretched

Action-at-a-distance Forces These can apply forces to an object without touching it Examples:-gravitational force -electrostatic force -magnetic force

Gravitational Force This is the force of attraction between objects because they have mass The force of gravitation between 2 objects depends on the masses of the objects and the distance between them The closer they are and more massive they are, the stronger the force of gravity they exert on each other

Electrostatic Force AKA – static electricity Causes pushing and pulling forces Remember – all mater is made of small particles….well some can be charged (+/-) Static electricity is the excess of positive or negative charges Like charges REPEL Opposite charges ATTRACT

Magnetic Force Acts on certain metals and compounds like iron, nickel and cobalt Natural magnets (ex: Earth!) and electromagnets have a field around them that can exert pushing and pulling forces

Measuring Mass Mass = the amount of matter in a particle or object We use the SI system (Le système international d’unités) of measuring mass – based on a comparison started in 1889 Mass measured with the units - kg Best to measure mass with a balance scale where a known mass is used to determine the unknown mass on the other side of the scale

Weight What’s the difference between mass and weight? Does your mass change when you go to the moon? Does your weight? Weight = the amount of force on an object due to gravity So…on the moon would you weigh more or less? You would weigh ~ 1/6 th the amount you weigh on Earth On Jupiter you would weigh 2.35 times more than on Earth

Bathroom Scale So how does a bathroom scale work?? They actually measure force not mass Inside the scale is a spring that is stretched when you stand on it More mass = more gravity pulls down on them = more spring stretches Therefore – scale actually is a weight scale because it measures the force exerted by an object downward due to gravity

The Newton Newton = a famous person AND the measuring unit of force A Newton is a small amount of force Equal ~ to force you’d exert to hold up an apple Weight is a force so is expressed in newtons (not kg)

Measuring Force We use force meters Force meters have a spring or elastic device that stretches or compresses in response to being pulled or pushed Most common type = spring scale (AKA Newton Gauge)

Measuring Force Mass and Weight are directly proportional If you hang a 1kg mass from a spring scale, it will read 9.8N Multiply 9.8m/s 2 by mass to convert mass to weight Ex: If your mass was 50kg you would weigh how much??

Forces and Motion Balanced Forces = equal in strength and opposite each other in direction These sometimes keep an object still Can also be on an object in motion – example? (cyclist) Unbalanced Forces = cause a change in speed or direction of object

How to keep track of forces? Use arrows in the direction of the force To compare – draw larger arrows for larger forces Draw a suitcase being lifted Then draw all the forces that act when you lift it

Try… Reading check questions pg 278 (#1), pg 281 (#1-5) and pg 285 (#1-4)