Download Forces Revision Notes

Forces Revision Notes
Forces Basics
1. Forces act on objects. They can change an object’s speed, its shape
and the direction in which it is travelling.
2. In order to represent forces in a diagram, we use an ARROW. The
arrow not only shows the direction in which the force is acting, its
size also indicates the size of the force.
3. The units of force are the NEWTON, named after a famous
English scientist. He also gives his name to the NEWTON
BALANCE; a device used to measure the size of forces.
In order to use a Newton balance, you must first check that it is
set to ZERO when nothing is hanging from it. You then attach your
object and see what the reading is.
4. As shown in the table below, forces can be separated into two
kinds; contact and non-contact.
Contact Force
Friction
Surface Tension
Supporting Force
Tension
Upthrust
Lift Force
Non Contact Force
Gravity
Electrical
Magnetic
Friction
1. Friction is a force that acts in the opposite direction to movement.
It creates heat and slows objects down.
2. Sometimes friction is useful. We need it for the brakes on our car
to work properly, to keep our tyres on a road and to catch a
football.
3. In some situations, we try to limit friction. We can use oil to make
a surface slippery, or streamline our car so it passes through the
air more easily.
Buoyancy
1. Buoyancy is a force that acts upwards on an object that is sitting
in a liquid or gas.
2. In the diagram below, buoyancy (A) and weight (B) are acting on a
mini-sub.
If the weight is greater than the buoyancy force, the object will
sink.
If the buoyancy force is greater than the weight, the object will
float to the surface.
3. An experiment to demonstrate buoyancy would be to put a flat
piece of tin foil into some water. It should float. If we then
scrunch up the foil into a ball, it will sink. This is due to the large
difference in surface area that is in contact with the water.
What other buoyancy experiments do you know?
4. Astronauts prepare for space walks (for example, to fix a problem
on the outside of the spacecraft) by training in a neutral buoyancy
lab.
In the lab, the buoyancy force from the water is equal to their
weight. This is achieved by adjusting the astronaut’s weight. As the
two forces are balanced, this means that the astronaut appears to
be “weightless”, similar to the conditions found in space.
Electric Force
1. Electric forces act at a distance.
2. Electric charges are either POSITIVE or NEGATIVE.
3. If two similar charges are placed together, they will REPEL each
other.
4. If two different charges are placed together, they will ATTRACT
towards each other.
5. The type of electricity that can hold balloons against walls, or give
us a nasty shock is STATIC ELECTRICITY.
Magnetic Force
1. Magnetic forces act at a distance.
2. The three elements that are attracted towards a magnet are
IRON, NICKEL and COBALT.
3. Steel is attracted towards a magnet as it contains lot of iron.
4. If a current is passed through a wire, it creates a magnetic field.
This is used to create an electromagnet.
5. An electromagnet can be built in class like shown below:
Crash Testing
1. It is important that safety is considered in the design of cars;
hence the inclusion in modern cars of crumple zones, seat belts and
air bags.
2. In the class, the following experiment can be used to show how
forces in a car accident can be reduced:
3. The force is measured for a normal car, which is then given a
“bumper” made out of a softer material (foam, card, bubble wrap).
The crash barrier experiences a smaller force when struck by this
new car.
Weight, mass and gravity
1. MASS is the amount of matter in a body, measured in kilograms.
The mass of an object will be constant throughout the universe.
2. WEIGHT is not the same as mass. Weight is a force, measured in
Newtons, caused by the pull of gravity. An object’s weight will
change depending on which planet it is on.
3. On Earth, weight = mass x 10
mass = weight ÷ 10.
Balanced or unbalanced
1. When the forces acting on an object are equal and opposite, they
are called BALANCED forces (an example is below).
2. When forces are balanced, a stationary object will stay still. If it
was originally moving, then it would continue at a constant speed.
3. If the forces acting on an object are not equal, then we call them
UNBALANCED forces. The object would speed up or slow down.
Space
1. Space contains many bodies. In order of size, from smallest to
largest, there are moons, planets and stars.
2. A black hole has such a high gravitational field that not even light
can escape from it.
3. What other facts do you know about space?
Other Stuff
1. How to draw a line graph (remember an even scale on both axis,
labels/units on each axis and accurately drawn points).
2. How to calculate the mean of a number of forces (Add up each
individual force, then divide the answer by the number of forces).