Module 3 Motion, Forces in action, work, energy and power, materials, Newtons laws of motion and momentum

Define speed, the equation used to calculate speed, and the respective SI units

Speed is defined as the rate of change of distance. The equation used is:

speed = distance / time

The SI units are ms-1.

Define displacement

the distance it has travelled in a given direction, so it is a vector with both magnitude and direction.

Define velocity, the equation used to calculate velocity, and the respective SI units.

The velocity of an object is defined as the rate of change of displacement, or speed in a given direction, making velocity a vector.

The equation used is velocity = change in displacement/time and the SI units are ms-1.

Define acceleration, the equation used to calculate acceleration, and the respective SI units.

Acceleration is defined as the rate of change of velocity, making it a vector.

The equation used is acceleration = change in velocity / time and the SI units are ms-2.

What does a line with a constant, non-zero gradient represent on a displacement-time graph?

An object moving with a constant velocity

What does a curved line represent on a displacement-time graph?

Acceleration (if gradient is increasing) or deceleration (if gradient is decreasing).

What does a line with a constant, non-zero gradient represent on a velocity-time graph?

An object that is accelerating (positive gradient) or decelerating (negative gradient).

What does the area under an acceleration-time graph represent?

velocity

Describe how the terminal velocity of an object can be determined using light gates.

• Set up the light gates vertically and measure the distance between them.

Connect them to a data logger and then release an object from rest above them, measuring the time it takes for the object to travel between the two gates.

• Using the time and the known distance, you can calculate the velocity of the falling object.

Describe how light gates can also be used to investigate conservation of momentum.

• Place two carts on a linear air track (to reduce friction) with repelling magnets so that they do not stick together.

• Attach card to the top of each cart so that they break the beams of the light gates when they pass.

• Keep one cart stationary and push the other towards it, measuring its velocity before the collision.

• Then measure the velocity of both carts after the collision and calculate the momentum before and after.

Define 'g'

The acceleration of free fall, 'g', is the acceleration of an object in response to the gravitational attraction between the Earth and the object. g = 9.81m s-2.

Describe the experiment in which one can determine 'g' using an electromagnet.

• An electromagnet holds a steel ball suspended a measured distance above a surface, then start the timer when the electromagnet is deactivated, and stop it when the surface is hit.

• As the ball was initially resting, u = 0.

• The distance and time are known, so we can use a SUVAT equation:

s = ut + ½ att

• Calculate 'a' which, in this case, is 'g'

A ball is projected off a castle at 6ms^-1. How does its horizontal velocity change from its launch until it hits the ground?

The horizontal velocity remains the same as there is no acceleration in the horizontal direction.

In projectile motion, what is the vertical acceleration?

The vertical acceleration is equal to gravitational field strength (g) downwards.

Describe what happens when a resultant force F acts on a body with mass m.

The body will accelerate in the direction of the resultant force. The resultant force is related to mass and acceleration by the formula: F = ma.

Give 3 examples of common forces and explain briefly what they do.

Any 3 of the following:

● Weight - the gravitational force acting on an object, through its centre of mass.

● Friction - the force that arises when two surfaces rub against each other.

● Drag - the resistive force on an object travelling through a fluid (e.g. water or air).

● Tension - the force within a stretched cable or rope.

● Upthrust - the upward buoyancy force acting on an object when it is in a fluid.

● Normal Contact Force - the force arising when an object rests against another object. It acts perpendicular to the plane of contact.

What is meant by drag?

An object moving through a fluid experiences a drag force acting on it. Drag is a frictional force that opposes motion. In air this is air resistance.

What is meant by terminal velocity?

When the forces acting on the falling object become balanced, the acceleration becomes zero and the object is moving at maximum velocity.

What is the principle of moments?

For an object in rotational equilibrium the sum of the clockwise moments about any point is equal to the sum of anticlockwise moments about that point.

What is meant by a couple?

A pair of equal and opposite coplanar forces that have equal magnitude and opposite direction, applied to a body parallel to each other but not along the same line.

If you have a uniform object, where would its centre of mass be?

At the geometric centre of the object.

Describe what is meant by density.

The mass per unit volume of a substance. The unit is kg m-3 .

Describe what is meant by pressure.

The pressure is the normal force exerted on a surface per unit area. It is measured in Pascals (Pa), where 1 Pa = 1 N m-2 .

What is the force that an object submerged in a fluid experiences?

It experiences an upwards force called upthrust. This is because the pressure at the bottom surface of the object is greater than at its top surface.

State Archimedes' principle.

The upthrust exerted on a body immersed in fluid, whether partially or fully submerged, is equal the weight of the fluid that the body displaces. An object will sink if the upthrust is smaller than the weight. For a floating object, its weight is equal to the upthrust.

Define work done.

The product of the magnitude of the force and the distance moved by the object in the direction of the force. Unit: Joules (J).

Define Kinetic Energy and give its SI base unit.

The energy associated with the motion of an object with mass. The SI base unit is kg m2 s-2

Define Gravitational Potential Energy.

energy an object possesses because of its position in a gravitational field

Define Elastic Potential Energy.

The energy stored by an object as a result of a reversible change in its shape.

State the principle of conservation of energy.

In a closed system, energy cannot be created or destroyed but only transferred from one form to another.

If we consider a closed system where an object is moving up and down, derive a formula for the velocity of an object in a gravitational field

All initial GPE is converted to KE as the object falls, and this KE is converted back to GPE as it rises.

Equating KE and GPE gives: mgh = ½ mv²

Rearranging this in terms of v² gives v² = 2gh, so v = sqr2gh

Since there is no 'm' in the calculation, the mass has no effect on the final speed - the acceleration of free fall is the same for all objects.

The rate of work done is equal to...

power

What is efficiency?

efficiency (%) = useful output/ power total input power × 100

What is meant by extension and compression of a string?

Tensile forces act away from the centre of the spring in both directions and will stretch it out causing the spring to extend.

Forces acting towards the centre of the spring in both directions is called compression.

What happens to the spring when tensile/compressive forces are exerted?

The spring undergoes 'tensile deformation' (for tensile forces) or 'compressive deformation' (for compressive forces).

What is Hooke's law?

F = kx The force applied is directly proportional to the extension up to the limit of proportionality.

What is meant by tensile stress?

Force per unit area.

What is tensile strain?

A measure of how the material stretches: the extension divided by the original length. This value is a ratio, so it has no units.

What is the difference between elastic and plastic deformation?

Elastic deformation: when the force is removed the object will return to its original shape

Plastic deformation: after the force is removed, the object will not return to its original shape (limit of proportionality has been exceeded).

How is energy stored during elastic deformation?

The work done is transferred and stored as elastic potential energy.

Describe the energy changes that occur during plastic deformation.

The material is stretched and the energy from the work done is used to break the bonds between the molecules. This causes permanent deformation.

What is breaking stress?

The amount of stress a material can take without it breaking.

What is meant when a material is described as brittle?

It does not extend much when a force is applied (tensile strain stays low). The material tends to break rather than stretch under a large force.

What is the elastic limit?

The point after the which plastic deformation occurs. It is sometimes also referred to as the 'limit of proportionality'.

What does the area underneath a force-extension graph represent?

The work done on the material

Give the equation that calculates elastic strain energy in terms of spring constant and extension.

E = 1/2k change inL ^ 2

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What is Young's modulus?

tensile stress / tensile strain

How do you find the Young's modulus from a stress-strain graph?

The gradient of the line

If an object is in equilibrium it means the object is...

Not accelerating; it is either:

● Stationary

● Moving at a constant velocity

What is Newton's second law?

Force = mass x acceleration

The resultant force of an object is proportional t the rate of change of momentum of the object and in the same direction.

What is Newton's first law?

An object at rest or moving with constant velocity will stay that way unless a resultant force acts upon it.

What is the difference between elastic and inelastic collisions?

In an elastic collision, the kinetic energy before is equal to the kinetic energy afterwards is lost. no energy

However, in an inelastic collision, the kinetic energy at the end is not equal to the kinetic energy at the start - some energy is lost to the surroundings.

True or false: linear momentum is only conserved in elastic collisions.

False, linear momentum is always conserved.

The rate of change of momentum can also be described as...

resultant force

What is impulse?

change in momentum

F changein t = change in(mv)

What is meant by the principle of conservation of energy?

Energy cannot be created or destroyed, only transferred into other forms of energy. Therefore the total energy in a closed system will always remain the same.

What is the equation used for the principle of conservation of momentum in one-dimensional collisions?

m1u1 + m2u2 = m1v1 + m2v2

What does a straight, horizontal line represent on a displacement-time graph?

A stationary object

What does a straight, horizontal line represent on a velocity-time graph?

An object moving with constant velocity

What does the area under a velocity-time graph represent?

displacement

What is a moment?

turning force

Which of Newton's Laws state 'every action force has an equal and opposite reaction force'

Newton's third law

Give an equation that can be used to calculate momentum.

momentum = mass × velocity

What does the area underneath a force-time graph stand for?

Impulse/change in momentum