Forces

Effects of forces

Effects of forces:

• Changes size of object
• Changes shape
• Changes the speed of motion
• Changes direction of motion

Load–extension practical:

1. Take a spring and suspend it from a clamp & stand.
2. Attach a ruler to the clamp.
3. Add a pointer for more accurate readings to prevent parallax error.
4. Measure initial position of pointer.
5. Add 100g masses.
6. Continue until limit of proportionality and measure the position of pointer each time.
7. Repeat to get accurate results.

Resultant forces acting along the same straight line:

An object either remains at rest or continues in a straight line at constant speed unless acted on by a resultant force.

• Friction – The force between two surfaces that may impede motion and produce heat.

Friction (drag) acts on an object moving through a liquid.

Friction (drag) acts on an object moving through a gas (e.g. air resistance).

• Spring constant – Force per unit extension

k=F/x

• Limit of proportionality – The maximum force that can be applied to a spring so that it can still return to its original length/shape when the force is removed.

When the limit of proportionality is exceeded, the spring will no longer bounce back to its original shape after the force is removed.

F=ma

Motion in a circular path due to a force perpendicular to the motion:

• Speed increases if force increases, with mass and radius constant.
• Radius decreases if force increases, with mass and speed constant.
• An increased mass requires an increased force to keep speed and radius constant.

Turning effect of forces

• Moment of a force – A measure of a turning effect of a force around a pivot.

Moment = force × perpendicular distance from the pivot

Everyday applications: A door opening around a fixed hinge, seesaws, scissors.

For equilibrium there must be: no resultant force, no resultant moment.

Opposing moments are balanced when: sum of the clockwise = sum of the anti-clockwise.

Opposing forces are balanced when: sum of upward forces = sum of downward forces.

Demonstrating equilibrium:

1. Take an object (e.g. a beam) and replace the supports with newton metres.
2. The forces acting on the beam can be found by measuring using newton meters or measuring the masses (then calculating the weights) of the beam.
3. The distance of each force from the end of the ruler can then be measured.
4. The moment of each force about the end of the ruler to be calculated.
5. The sum of clockwise moments = the sum of anticlockwise moments.

Centre of gravity

• Centre of gravity – The point an object's weight will act through.

Determine centre of gravity:

1. Hang the irregular shape with a pin and a thread of plumb line.
2. Draw a line based on the thread.
3. The point where the lines intersect is the centre of gravity.

For an object to be stable it should have a low centre of gravity and a wide base