P5 : Forces

What is a scalar quantity?

A quantity that has magnitude only

What is a vector quantity?

A quantity that has magnitude and a direction

What is magnitude?

Size

How can vector quantities be presented?

By using an arrow

What does the length + the direction of arrow represent?

• Length of an arrow represents the magnitude

• Direction of an arrow represents the direction

Give an example of a scalar quantity (any)

• Speed

• Distance

• Mass

• Temperature

• Time

Give an example of a vector quantity (any)

• Force

• Velocity

• Displacement

• Acceleration

• Momentum

What is a force?

A push or pull that acts on an object due to the interaction with another object

What are 2 main types of forces?

• Contact forces

• Non-contact forces

Give an example of a contact force (any)

• Friction

• Air resistance

• Tension

• Normal contact force

Give an example of a non-contact force (any)

• Gravitational force

• Electrostatic force

• Magnetic force

Is force a scalar or a vector quantitity?

Vector

What is the unit for force?

Newton (N)

What is friction?

A force that opposes the motion of two surfaces that are in contact with each other

What is air resistance?

A force that opposes the motion of an object as it moves through the air

What is weight?

A force that pulls an object towards the centre of the Earth

What is upthrust?

A force that pushes up on an object in a fluid

What is tension?

A force that acts through a stretched object

What is electrostatic force?

A force that is acting between 2 charged particles

What is normal contact force?

The force when a surface pushes up on an object at a right angle to the surface

What is thrust?

A force that is generated from the engine of a motorised vehicle/movement

Label the forces (the boat is moving towards the right)

• ↑ = Upthrust

• ↓ = Weight

• → = Thrust

• ← = Friction

Label the forces (the ball is not moving)

• ↑ = Normal contact force

• ↓ = Weight

Label the forces (the ball is falling)

• A = Air resistance

• B = Weight

Label the forces (the car is moving to the right)

• A → Normal contact force

• B → Weight

• C → Thrust

• D → Friction

Label the forces (the aeroplane is moving to the right)

• ↑ = Lift

• ↓ = Weight

• → = Thrust

• ← = Air resistance

What does the size of the arrow determine?

How big a force is

Why is there a force close to the Earth?

Because of the gravitational field around the Earth

What does the weight of an object depend on?

The gravitational field strength at the point where the object is

What is the formula for weight?

What is referred to when the weight of an object is considered to act a single point?

The objects “centre of mass”

What is the relationship between the weight and mass of an object?

It is directly proportional

What is weight measured with? (2)

• A calibrated spring-balance

• A newton meter

What is the difference between mass and weight of an object?

The mass of an object is a definite value and always will be whereas the weight of an object changes due to an objects mass and the gravitational field strength of where the object is located

What is the resultant force?

The overall force acting on a point or an object

If the forces are acting along the same line how do you find the resultant force of

• Forces going in the same direction

• Forces going in opposite directions

• Same direction → add the forces

• Different direction → subtract the forces

What is a free body diagram?

A diagram which shows forces acting on an object

Calculate the resultant force

90N to the right

Calculate the resultant force

20N to the right

Calculate the resultant force

220N to the right

Calculate the resultant force

1310N to the right

Calculate the resultant force

0N

What does it mean if all the forces cancel each other out?

The object is at equilibrium

Explain how to find the resultant force using a vector diagram (7)

• Draw the 2 sides given (line A at the top and line B at the bottom) making sure the angle given fits in the space between

• Use a suitable scale (e.g. 100N = 1cm)

• Then extend the top line using the measurement of line B making sure it corresponds with the angle give to create the third line of a parallelogram

• Then join up the bottom line with the line just created

• Then join the two furthest points of the parallelogram with a line

• That is the resultant force

• Measure the line and relate back to the scale

Explain how to resolve a single force into two forces using a vector diagram (7)

• Draw a dot as a starting point

• Then draw faint horizontal and vertical lines → these will act as the axis of the diagram

• Then draw the angle, if it is to the horizontal then draw the angle using the x axis as the starting line, if it is to the vertical then do the same

• Then use a ruler to draw the force and use a suitable scale (e.g. 100N = 1cm)

• Then draw dotted lines from the tip of the line drawn to the faint axis

• Where the lines touch the axis is the tip of the two other forces

• Draw the two other forces and measure them using the scale

What is work done?

When a force causes an object to move through a distance

What causes a displacement of an object?

When a force does work on an object

What is the formula for work done?

What happens in terms of energy when work is done on an object?

Energy is transferred

What is the relationship between joules and newton metres?

1J = 1Nm

What happens when work is done against the frictional forces acting on an object?

There is a rise in temperature of the object due to energy being transferred to the thermal energy store of the object

What forces are involved in stretching, bending or compressing an object?

• Work done

• Elastic potential energy

Why does more than one force have to be applied to change the shape of an elastic object?

Because if there was only one force the object would move in the direction of the applied force instead of changing shape

What types of object can be inelastically deformed?

Elastic objects

What are the 3 ways a spring can be manoeuvred?

• Stretched

• Compressed

• Bent

What are the 2 types of deformation?

• Elastic deformation

• Inelastic deformation

What does it mean if an object is elastically deformed?

When an object can be bent, stretched or compressed, it will return to its original state

What does it mean if an object is inelastically deformed?

When an object can be bent, stretched or compressed, it will not return to its original state + the length doesn’t change after the force has been removed

What happens to an object if it is elastically deformed and has been stretched or compressed?

Energy is transferred to the elastic potential energy of the object

What is the relationship between the extension of an elastic object and the force applied?

It is directly proportional

What does limit of proportionality mean?

When the extension of an elastic object and the force applied are no longer directly proportional

What is the equation for force?

What can the e in the force = spring constant x extension (F = K x e) also be applied to?

The compression of an object → difference between the natural and compressed lengths

Explain the energy transfer when a spring is stretched or compressed

Work is done to the spring and elastic potential energy is stored in the string

What does a graph look like when an object is inelastically deformed?

non-linear

What does a graph look like when an object is elastically deformed?

Linear

What is the formula for elastic potential energy?

What does a high spring constant show about the object?

Higher spring constant = stiffer the object

Explain how to investigate the relationship between force and extension for a spring (7)

1. Set up your equipment, ensuring the spring will return to its original dimensions if stretched within its elastic limit

2. Attach the pointer to the base of the spring, ensuring that it isn’t angled (parallel to the workbench) and perpendicular to the metre ruler. Align the top of the ruler with the top of the spring

3. Measure the initial length of the spring without any weights attached

4. Add a 10g mass to the base of the spring and record the length of the spring

5. Repeat and continue to add masses, ensuring that the spring doesn’t oscillate after each weight has been added

6. Calculate the extension of the spring for each mass by subtracting the initial length of the spring from each different length of the spring

7. Convert all masses to weights using the equation: Weight (N) = mass (kg) x 9.81 (N/kg) where 9.81N/kg is the gravitational field strength on Earth

8. Plot the graph of force (y-axis) against extension (x-axis). Calculate the gradient.

What does moment of a force mean?

Th turning effect of a force

What is the formula for moment of a force?

What would cause a larger moment? (2)

• Larger force

• Longer distance

What is a pivot?

The point around which the object can rotate or turn

If an object is balanced what does that mean in terms of pivoting?

The total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

How is the maximum moment achieved?

Pushing at right angles to an object

Why are levers useful?

Because they increase the distance from the pivot at which the force is applied → make it easier to do work

Describe the structure of a gear

Circular discs with teeth around their edges

How do gears work?

Their teeth interlock so that turning one causes another to turn in the opposite direction

What are gears used for?

They are used to transmit the rotational effect of a force from one place to another

What is the benefit of using different sized gears?

They can change the moment of a force

How do different shaped gears change the moment of a force?

A force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater

Which gear would turn faster, the larger gear or the smaller gear?

The smaller gear

Why is the overall force of both gears the same?

Because gear A rotates twice for every time gear B rotates once

If a seesaw is balanced what does that mean in terms of moment?

(refer to moment = distance x force)

The clockwise moment (distance x force) must equal the anticlockwise moment (distance x force)

Why do objects topple?

The line of action of the weight of the object lies outside the base of the object which causes an unbalanced moment to act of the block

What states of matter can a fluid be? (2)

• Liquid

• Gas

What does the pressure in fluids cause?

A force normal at right angles to any surface

What is the formula for pressure at the surface of a fluid?

What is the formula for pressure at a certain depth due to the column of liquid ?

When is a liquid more dense?

When it has more particles in a given place

Why does the pressure of a liquid increase?

Because there are more particles which leads to more collisions

What happens to pressure as depth increases?

Pressure increases

Explain why, in a liquid, pressure at a point increases with the height of the column of liquid above that point and with the density of the liquid

• As the depth of the liquid increases the number of particles above that point increases

• The weight of these particles adds to the pressure felt at that point so the liquid increases with depth

What happens to an object when it is either partially or totally submerged in a fluid?

The pressure of the fluid exerts a force on it from every direction

In a fluid which force is larger, the force being exerted upwards or the force being exerted downwards + why?

The force being exerted upwards because pressure increases as depth increases

Where is the resultant force created on an object either partially or fully submerged in a fluid + what is this called?

• There is a resultant force upwards

• This is called upthrust

When would an object float? (in terms of forces)

When its upthrust is equal to its weight

When would an object float? (in terms of density) (3)

• When an object that is less dense than the fluid it is placed in, weighs less than the equivalent volume of fluid

• This means that it displaces a volume of fluid that is equal to its weight before it is completely submerged

• Then the weight is equal to the upthrust

When will an object sink? (in terms of forces)

When its weight is larger than its upthrust

When will an object sink? (in terms of density) (2)

• When an object is denser than the fluid it is placed in, it is unable to displace enough fluid to equal its weight

• This means that its weight is always larger than the upthrust, so it sinks