Classifying Forces Contact Forces must touch the object that they push or pull, for example, hitting a tennis ball. Another common contact force is friction. Friction is a force that opposes the relative motion of an object.
Gravity Action at a distance forces can push or pull an object without touching it. Gravity is the most common of these classification of forces. Gravity is the attraction between two objects due to their mass. The amount of attraction depends upon the amount of each objects mass and the distance between the two objects.
Mass Mass is the amount of matter in an object. The mass of a bowling ball is greater than the mass of a tennis ball because it contains more matter. For example, the mass of 1 L of water is 1 kg. Since mass is the amount of matter in an object, the objects mass does not change as a result of gravity.
Weight The weight of an object is not the same as its mass. Weight is the amount of force on an object due to gravity. Therefore, weight means the same thing as the force of gravity.
The Unit of Force The metric unit for force is the Newton (N). Since weight is the force of gravity, it is measured in newtons (N). On earth, a 1.0kg mass has a weight of 9.8 N. This value is called earth’s gravitational field strength and is symbolized by g. If you multiply any mass by 9.8 N/kg you get its weight on earth.
Calculating the Force of Gravity The force of gravity (Fg) on any mass (m) near the surface of Earth can be calculated by: Force of gravity= (mass of object) x (the strength of earths gravitational field)
Using symbols, this word equation can be expressed as: F g = mg Where mass is in kilograms (kg) and g is 9.8 N/kg
For example, to find the weight of a 50-kg student on earth: F g = mg = (50kg) (9.8N/kg) = 490N A 50 kg students weighs 490 N on earth.
Measuring Force The most common force meter is called a Newton Gauge or spring scale This consists of a hook and spring. As more force is applied to the hook, the spring stretches further. Spring scales can measure forces other than weight. If you wanted to know the force required to slide an object across your desk, a spring scale can be used to measure the friction between the object and your desk.
The effect of Friction The difference between the calculated value and the real (actual) value of mechanical advantage is friction, which is a force that opposes motion. Friction is caused by the roughness of materials. Because friction is a force in any device, additional force must be applied to overcome the force of friction. The mechanical advantage of the device will be less because of this added force that must be overcome. Friction in a system also causes heat, which can cause additional concerns.
Work and Energy The Meaning of Work Scientifically, work is done when a force acts on an object to make that object move. In order to say that work is being done, there must be movement. If there is no movement, no matter how much force is used, no work is done.
For example; a worker uses force to move a large carton up a ramp. Energy (pushing) is transferred to the carton from the worker. Thus, we say that the worker did work on the carton as long as the carton moved up the ramp as a result of the worker’s pushing action (force).
Forms of Energy When an object is moving, the energy it has is called kinetic energy However, energy does not always involve motion. An object can store its energy to do work later. Any energy that is stored is called potential energy.
Calculating Work The amount of work is calculated by multiplying the force times the distance the object moves. The formula looks like this: W = F x d Force is measured in Newtons and distance is measured in meters. The resulting work unit is called a joule, named after the English scientist James Joule.
The amount of work done therefore depends on 2 things: 1.The amount of force applied 2.The amount of distance travelled in metres An example: If you were to lift your 25 N backpack onto your shoulder 1.7 m above the floor: W = F x d W = 25 N x 1.7 m W = 42.5 J
Work and Energy Energy and work are closely related, because without energy there would be no work. Work is done when there is a transfer of energy and movement occurs. Energy provides the force needed to make an object move. The energy can be in the form of human energy (muscle power – chemical reactions in the body producing energy) or it can be in the form of another energy source, such as gasoline (for a car)
Sources of energy may differ depending on the machine, but machines help us do work by generating energy.
Work and Machines Using a machine does not mean that more work is being completed! When you use a machine you may exert less energy, but the same amount of work is being done. To prove this you can calculate work input and work output.
Work input is the work needed to use, or operate the machine Work input = Force input x d input If you exerted a force of 640 N to pull a wagon up an incline a distance of 2.5m Work input = 640 N x 2.5 Work input = 1600 J
Work output is the work done by the machine. Work output = F output x d output This same inclined plane rises to a height of 1.25 m and the wagon exerts an output force of 1280 N Work output = 1280 N x 1.25 = 1600J
Work and Friction Work input and Work output are not always the same. Friction is also the cause of this imbalance in real world situations. This difference in work input and output therefore can also be calculated by: Efficiency = Work output x 100 Work input
Machines and Mechanical Advantage A machine is any mechanical system that reduces the force required to accomplish work A machine can make work easier for you by increasing the amount of force that you exert on an object. This produces mechanical advantage which is the amount of force that is multiplied by the machine.
Input and output forces The force applied to the machine (by you) is the Input force. The force that is applied to the object (by the machine) is the output force
Machines do work in three ways Increasing the force that can be applied to an object Increasing the distance over which the force is applied Changing the direction of a force
Calculating Mechanical Advantage In order to calculate mechanical advantage, you need to know the output and input forces MA = Output Force or MA= F output Input Force F Input Where F= Force in Newtons (N)
Mechanical advantage is a ratio of forces in a mechanical device, and therefore is also referred to as the force ratio of the machine. The more a machine multiplies force, the greater its mechanical advantage of the machine.
MA = 236N / 59 N MA= 4 Therefore, the pulley system has a mechanical advantage of 4. This pulley can output 4 times the amount of force that the person pulling inputs. F in = 59 N
Three types of Mechanical Advantages Mechanical advantage greater than one Mechanical Advantage of one Mechanical Advantage less than one
Mechanical Advantage greater than one In the previous example, the pulley had a mechanical advantage greater than one (four). This is ideal in most situations for machines. The purpose of a machine is to produce more output force than the human working the machine
Mechanical Advantage of One A mechanical advantage of a machine is one only change the direction between the input and output force. These machines do not make any harder or easier but used when the force of direction needs to be changed.
Mechanical Advantage of less than one Why would you want a mechanical advantage of less than one? All machines we have discussed so far have a mechanical advantage of greater than one Machines with a mechanical advantage of less than one require more input force than is released as output force Machines with a mechanical advantage less than one are driven by moving the input force less of a distance than the distance moved by the output force.
Example In this __3__ rd class lever the input force is ___100____ N, while the output force is only ______25__ N. MA = F output F input MA = 25N / 100 N MA = 0.25
This larger gear is harder to pedal but the bicycle will move faster
When an increase in speed of motion is necessary, a mechanical advantage of less than one is sometimes necessary.
Ideal Mechanical Advantage (IMA) The mechanical advantage of a machine that has no friction. Calculated by finding the ratio between the distance over which the input force is exerted on the machine (D in ) and the distance over which the output force is exerted on the object (D out) Eventhough no real machines have a true IMA, some have very little similar to an IMA. Machines like a hammer or screwdriver have no sliding parts and therefore have minimal friction.
IMA= D in D out A hammer is used to pull a nail. If the handle of the hammer moves 30 cm and the nail moves 5 cm, what is the IMA? IMA= 30 cm 5 cm = 6 30cm 5 cm
The IMA is also the same concept as Speed Ratio To calculate speed ratio is the same formula as calculating IMA Less force but greater Distance The advantage to gaining force is offset by the disadvantage of losing distance