Forces doing work and their effects

What is work done?

The amount of energy transferred

What is power?

The rate of work being done (How fast energy is transferred)

What is the formula for work done?

Work done (J) = force (N) x Distance moved (m) or E=F x d

What is the equation for power?

Power (W)= Work done (J)/Time taken (s) or P= E/t

How can the energy of a system be changed?

• Work done through forces

• Electrical equipment such as a cell in a circuit transferring energy into other forms like heat energy

• Heating a material which increases the kinetic energy of the material

What are non-contact forces?

Forces that can be exerted on an object without them being in contact with it

What are contact forces?

Forces that are exerted due them them being in contact with an object

What are the three main non-contact forces?

• Gravity

• Magnetism

• Electrostatic charges

What is normal contact force?

It is a force that acts upwards in opposition to the weight of an object

What is the force of friction?

• A force that acts in opposition to a pushing force trying to change the motion of an object

• Friction always acts to slow an object down

What are the components of a force?

• Any force can be resolved into it’s horizontal component F_x and it’s vertical component F_y

• The horizontal and vertical components do not affect one another, they are independant

How to resolve forces by scale drawing

1. Decide on an appropriate scale such as 1 cm=10N

2. Using a ruler and protractor, draw the line to represent the force at the correct angle

3. Draw the horizontal and vertical components

4. Measure the lengths and convert to force using the same scale

Note that if you are given the vertical components instead you have to find the resultant force by making a box then drawing a line to the opposite corner and measure that

What do free-body force diagrams show?

They show the forces acting on a single object

What do free-body force diagrams need to contain?

• The body the forces are acting on- this can be simplified to a box or a dot

• The forces acting on the body

• If there are two forces acting on different objects it is NOT a free-body force diagram

How can we see if forces are balanced on a free-body force diagram?

• If the forces are balanced the line will be of equal length

• If the forces are not balanced then the forces will not be of equal length

If the arrows on a free-body force diagram are not balanced, what does that tell us?

• The object is moving in the direction of the biggest force

• The object is accelerating

If the arrows on a free-body force diagram are balanced, what does that tell us?

• The object is stationary (not moving)

  OR

• The object is moving at a constant speed

Why is force a vector quantity?

Because it has a size and direction

What do resultant forces determine?

Whether an object is stationary/moving at a constant speed or accelerating

What is the resultant force?

The single force that would have the same effect as all of the other forces acting on the object

If two forces are pushing a box forward with a force of 10N east each what will the resultant force be?

20N east

If one force is pushing a box forward with a force of 10N and one is pushing back with a force of 10N what will the resultant force be?

0N

If one force is pushing a box east with a force of 10N and one is pushing west with a force of 8N what will the resultant force be?

2N east

If the resultant force on an object is 0, what does that tell us?

The object is either stationary (not moving), or is moving at a constant speed

What direction will a resultant force be in if the forces act perpendicular (at right angles) to each other?

The resultant force will be a diagonal line

How can you calculate the resultant force of two angles acting perpendicular (at right angles) to each other?

• If you have a scale (1cm=10N for example) you can measure the length of the diagonal line you draw to find the resultant force, so if the line was 10 cm the resultant force would be 100N

• You can use Pythagoras’ theorem to calculate the hypotenuse (The slanted side)

What is a moment?

Forces which act at a distance from a pivot that can cause a turning effect or rotation

What is the formula for a moment?

Moment (Nm)= force (N) x distance from the pivot (m)

How should we measure distance between the pivot and the force?

The distance must be measured at right angles to the direction of the force

What is a pivot?

The point around which an object rotates

What is the principle of moments?

When a body is balanced, the sum of the clockwise moments is equal to the sum of the anticlockwise moments

What do levers and gears do?

Transmit the rotational effect of their forces

What does the nature of the rotation of a lever depend on?

• The position of the input force- the force provided by the user of the lever

• The output force- the force that results from the input force

• The fulcrum- the turning point about which both forces act

What are gears?

Gears are toothed wheels put together to transmit a rotational force and motion

If one gear is moving in a clockwise direction, which way will the other gear turn?

Anti-clockwise

What is a high gear?

• When a large input gear turns a smaller output gear

• This leads to a high speed and a low turning effect

What is a low gear?

• When a smaller input gear turns a larger output gear

• This leads to a low speed and a high turning effect

What is the speed and turning effect of a large gear compared to a small gear?

• A large gear will move slower than a small gear but will have a greater turning effect

• A low gear leads to a low speed and high turning effect

What do levers and gears both do?

Cause rotation and can alter the size of the output force