# Forces

OP4 understand the concept of force,

what is a force

OP5 investigate examples of friction and the effect of lubrication

Friction

OP6 investigate the relationship between the extension of a spring and the applied force

Hookes Law

OP7 understand that weight is the force of gravity and that weight varies with location; recall that mass in kilograms multiplied by 10 is approximately equal to weight in newtons on the surface of the earth

the force of gravity (weight) and its relationship to mass

Gravity is a force.

It is a force of attraction between any two objects with mass.

The Earth’s gravity pulls things down toward the centre of the Earth

The force of gravity acting on an object is called the weight of the object.

In science, weight is measured in newtons.

Weight = mass x gravity

On Earth g= 10m/s2

On Earth a mass of 1 kilogram has a weight of 10 newtons,

If you want to find the weight of a body, multiply its mass by 10.

Gravity has a noticeable effect close to the Earth.

Gravity works out in space. but to a lesser effect, the further away you are the less it seems to works on you.

When you drop an object it starts slowly, but gets faster. It accelerates.

This acceleration is due to gravity (g).

Why does rain fall faster than snow?

When something is falling Gravity pulls it down

But another force, AIR RESISTANCE, or DRAG pushes it up.

When something starts to fall, gravity makes the accelerate, they are not very fast, so drag is small.

As they fall faster, the drag gets bigger, so the acceleration gets less.

After a while , the drag force becomes the same size as the force of gravity.

The object stops accelerating.

It is now moving at its terminal velocity

What happens when a parachute opens?

The surface area gets bigger.

The drag force gets bigger

The parachutist slows down, but continues to fall.

The falling object has slowed down.

It has a lower terminal velocity

So, Why does rain fall faster than snow?

It used to be said that you’re taller in bed than standing up.

Is this true? How could you find out?

To verify that all objects fall at the same speed

Apparatus

• 3 tennis balls
• Syringe with needle

Method

1. Drop 2 balls from the same height, observe
2. Compare and make judgement on gravity
3. Inject 1 ball with water, increasing the mass of the ball
4. Repeat steps 1 & 2 with water filled ball.

Result

Conclusion

taken from "Science on Stage3"

OP8 find the centre of gravity of a thin lamina;

investigate the role of centre of gravity in design for stability and equilibrium

Title

find the centre of gravity of a thin lamina

Apparatus

Retort Stand, Cork, pin, thread, thin sheet of cardboard or plastic, masses

Method

1. Cut out an irregular shape out of cardboard,
2. Attach a small mass to a thin piece of thread
3. Suspend the lamina and the thread from a pin inserted into a cork held in the retort stand, pin the lamina in a corner
4. Observe the line, and sketch along the line, MARK THE LINE OF THE THREAD
5. Repeat steps 3 & 4 for two other corners.

Safety Concerns

Be careful using the pins,

make sure the arm of the retort stand is over the base.

Results / Observations

Draw at least 3 lines,

Conclusions

The found center of gravity is the point through which the mass of the body appears to act through, the lamina can be supported on a pencil, accurate work on the 'lead end' and less accurate on the rubber end.

make sure that you suspend the lamina from a corner.

investigate the role of centre of gravity in design for stability and equilibrium

centre of gravity and its importance in design

Stability, an objects ability to resist toppling over.

The center of gravity is the point (in 3 dimensions) that all the weight of a body seems to act through. If a line from a bodys CoG to the Center of Earth lines up outside the supports of the body the object tends to topple.

In order to make a system / machine more stable

1 Keep the center of gravity as low as possible

2 Keep the center of gravity through the middle of the body

3 Build the body with a wide base

A Body will fall over if the line of the center of gravity is outside the supporting legs / wheels of the body

Trees

How many nails can you get to balance on 1 nail?