Matter and forces

• Be able to distinguish between mass and weight of an object.

Weight is the gravitational pull on an object measured in Newtons. Weight is a force. Weight is different depending on the planet (the mass of the planet changes the gravitational pull).

Mass is what many people think of as "weight" (e.g. "How much do you weigh?"). It is dependant on how many particles is in a substance. Mass is often measured in kilograms and is the same regardless of how much gravitational pull is applied to the object. 
For example: Two men, both 70kg, one is on the moon and one is on the Earth. Both men have the same mass (70kg) but different weights. This is because the gravitational field strength is different on the moon and on the Earth.

• Demonstrate understanding that mass is a property that “resists” change in motion.

Depending on how a force is applied to an object, it will take different times for the object to speed up. From the formula F=ma, we see that if we apply a constant force, then there is constant acceleration. But, if the force is not constant and is only applied for, say, a few seconds then there will just be an instant of acceleration.

If you're in a supermarket and there is a shopping cart you want to move, the difficulty of moving it depends on the mass of the shopping cart. It is much more difficult to push or change the direction of a heavier cart than it is on a lighter one. In other words, mass "resists" change in motion as it takes a lot of force to move heavy (high mass) objects.

This is also called the concept of inertia.

• Know that the Earth is the source of a gravitational field.

The Earth is the source of a gravitational field. In fact, every single object with mass has a gravitational field, but since the Earth is significantly heavier than most other things, we count the Earth as being the source of our "gravity" (i.e. gravitational field). The more mass an object holds, the stronger the gravitational energy.

• Describe, and use the concept of, weight as the effect of a gravitational field on a mass.

Gravity = Force / Mass. 

You can rearrange this equation to find other variables depending on what you're given in the question. 

• Describe an experiment to determine the density of a liquid and of a regularly shaped solid and make the necessary calculations using the equation: Density = Mass / Volume.

Density = Mass / Volume.

For both the liquid and the regularly shaped mass you have to find the mass and the volume of the object and use this equation. Be careful so that the units are correct!

• Describe the determination of of the density of an irregularly shaped solid by the method of displacement, and make the necessary calculations.

The same formula as the one above applies. Begin by finding the mass of the object. Now, you put the object in a measuring cylinder filled with water. When you put the object in the water the water volume level will rise because of displacement. The change in volume is the volume of the object (e.g. if the level of water went from 20cm3 to 50cm3, then the object has a volume of 30cm3). Use these values in Density = Mass / Volume.

• Know that a force is measured in Newtons (N).

Force is usually measured in the SI unit of Newtons.

• Describe how forces may change the size, shape and motion of a body.

If you apply a force to a body(/an object), both the size, shape and the motion of this body can change (depending on how much force you add).

• Plot extension/load graphs and describe the associated experimental procedure.
• Interpret extension/load graph.

The more load (force) you add on a spring, the longer the extension is going to be. The relationship between the extension of the spring and the load (force) is proportional (a straight line graph). Towards the end, the spring can't extend any further and the line on the graph begins to curve. This is called the limit of proportionality. 

One can test this through adding different masses to a spring and calculate the extension. 

• State and use Hooke’s Law and recall and use the expression Force = Extension (x) x Constant. (F=kx)

Force applied (N), Spring constant (N/m), Extension (m)

• Recognise the significance of the term “limit of proportionality” for an extension/load graph.

The limit of proportionally, where if you extend the spring any further, force will no longer be proportional to extension. On a graph you can see this when the line starts to curve (indicating that the limit of proportionality has been reached).

• Recall and use the relation between between force, mass and acceleration (including the direction) Force = Mass x Acceleration.

• Find the resultant of two or more forces acting along the same line.

When we want to find the "resultant force" of something, we want to find the overall force. We do this through adding or subtracting forces (when in a straight line).

Here is an example:

The larger force = 350N

The smaller force= 50N

Resultant force = larger force – smaller force = 350N-50N = 300N to the right.

• Explain how a system is in equilibrium when there is no resultant force.

A system is in equilibrium mainly when both opposing forces have equal magnitudes and the resultant force is 0. The vectors cancel each other out.

• Relate (without calculation) pressure to force and area.

Pressure is the force exerted divided by the area the pressure is exerted on (use the formula to define it).

• Recall and use the equation P = F/A.

 

Pressure (N/cm2), Force (N), Area (cm2)

N/m2 = Pa

• Know that energy and work are measured in joules (J), and power in watts (W).

Energy = Joules

Work = Joules

Power = Watts