Physics - Mechanics (Section 4)

Give examles of Scalar Quantities

Time, Distance, temperature, mass, speed, length, energy

What is the difference between scalar and vector quantities?

Scalars have a magnitude only, whereas vectors have a magnitude and direction.

Give examples of vectors

Weight, force, momentum, displacement, velocity, acceleration

What is important to remember about inclined plane problems?

W > W1 W>W2, therefore in right angled triangles with W1/W2 and W, W must be the hypotenuse

What are free body diagrams

Diagrams showing all forces exerted on a single object

What is Newton’s first law

If there are not resultant forces acting on an object then it will either remian at rest, or travelling at a constant speed/velocity in a straight line.

How to calculate resultant forces?

Draw forces tip to tail, the resultant force is from the first arrow to the end of the last arrow (closed vector triangles are formed in object is in equilibrium).

What is a moment?

The force x the perpendicular distance from the pivot to the line of action of the force.

What is the principle of moments?

If a system is in equilibrium then the sum of the clockwise moments = the sum of the anti-clockwise moments (CM=ACM)

What is the centre of mass?

The point in which you can consider all the weight (of the object) acts through, no matter the orientation.

How to find the mass of an object experimentally?

• Cut a piece of card into any shape'

• Hang the card from a point (with a string and plumbob attached)

• Mark a straight line along the string (repeat at least twice)

• The point of intersection is the centre of mass

What concept do engineers use to improve stability.

The centre of mass with the concept of moments:

• A low centre of mass increases stability as tilting creates a moment to tip the object back onto its base

• A low and wide base increases stability.

When does an object topple?

The object topples when the line of action of its weight (acting through the centre of mass) falls outside the the base of support

What is the principle of moments?

If a system is in equilibrium then the

What does each letter in SUVAT stand for

• s = displacement

• u = initial velocity

• v = final velocity

• a = acceleration

• t = time

What needs to be true when using SUVAT equations in questions?

For it to be uniform acceleration.

State the four SUVAT equations needed.

• v² = u² + 2as

• v = u + at

• s = ut + 0.5at²

• s = 0.5(u+v)t

What does the gradient of a displacement-time graph represent, including what is means if it is a horizontal line, straight line and curved line.

• Gradient = Velocity

• Horizontal line = Stationary

• Straight line = Constant velocity

• Curved line = acceleration

How do you find the velocity at a certain time for a curved displacement time graph?

• Draw a tangent at that point

• Find the gradient of the tangent

What do the gradient, the area under the line, a horizontal line, a straight line and a curve of a velocity time graph represent?

• Gradient = acceleration

• Area under the line = change in displacement

• Horizontal line = Constant velocity

• Straight line = Constant acceleration

• Curved line = Change of acceleration

Explain the two methods to find the area under a graph?

• Divide graph into triangles and rectangles (/into a trapezium)

• Count the number of whole squares under the line and count each partial square as ½ a square, then multiply the number of squares by the area of one square.

What does the area under the line, a horizontal line and a line under the x-axis represent in an acceleration-time graph?

• Area under the line = Change in velocity

• Horizontal line = Constant acceleration

• Line under x-axis = Negative acceleration/deceleration

Explain Newton’s first law of motion.

• If there is no resultant force acting on an object, it will either stay at rest or travel at a constant velocity/constant speed in a straight line

• Therefore, an object in motion will stay in motion in less acted upon by a resultant force

• Constant Velocity = no resultant force = in equilibrium

Explain Newton’s Second Law of Motion.

• The acceleration of an object is proportional to the resultant force acting on it

• F=ma

• Unbalanced forces/resultant force = acceleration

What is the SI base-unit expression of the Newton?

• kgms⁻²

Explain Newton’s third law of motion.

• If object A exerts a force on object B, then object B exerts an equal but opposite force on object A

• Action-reaction pairs = act on different objects, be the same type of force, be equal and opposite forces

• The forces do not cancel out

State three examples of Newton’s third law.

• The Earth pulls down on you with gravity, you put up on the Earth with gravitational force

• If a desk pushes up on a book, the book pushes down on the desk

• A rocket pushes exhaust gases backwards, the gas pushes the rocket forwards

Explain freefall.

An object is in freefall when the only force acting on, it is its weight

Explain air resistance (related to freefall)

• Air Resistance is often negligible in exam questions

Air resistance increases with speed and is proportional to the surface area and speed^2

Explain when all objects fall at the same rate

When there is no air resistance because all objects accelerate at the same rate under gravity regardless of their mass e.g. on the moon

What is gravity on earth and give two possible units?

• 9.81ms^{-2} or 9.81Nkg^{-1}

Explain Galileo’s experiment

• He rolled balls of different masses down a slope

• This was to reduce the speed making the motion easier to time and to reduce the effect of air resistance

• He found the distance was always the same regardless of mass proving uniform acceleration

What is a projectile?

• An object where the only force acting on it is its weight

• Therefore it’s an object in freefall

Explain what is key about the vertical and horizontal components of a projectile.

• The vertical and horizontal components of motion are independent of each other

• Assuming no air resistance there are no horizontal forces acting on a projectile so the horizontal velocity is constant throughout

• A dropped object and one projected horizontally from the same height will hit the ground at same time

How does air resistance affect projectile motion?

• Air Resistance adds a horizontal force, so horizontal velocity decreases overtime

• The projectile does not travel as fast

• The projectile gains vertical speed more slowly

How can you calculate horizontal and vertical motion for projectiles?

• Horizontal: S=vt as a=0 (s=vt-½at²)

• Vertical: SUVAT

Explain how to calculate projectiles fire fired at an angle.

• Use trigonometry to break the initial velocity into vertical and horizontal components

• The vertical velocity changes due to gravity, horizontal remains the same

• The trajectory is parabolic (ignoring air resistance), so time up equals time down

• The angle of launch equals the angle of impact (symmetrical)

What are the two main forces experienced as an object force through air?

• Gravity downwards

• Drag upwards

Describe the four stages of falling

1. At the start weight is greater than air resistance so the object accelerates

2. As speed increases so does air resistance, eventually air resistance equals weight so the object is travelling at a constant velocity (terminal velocity)

3. If air resistance becomes greater than weight (parachute) the result of force is directed upwards and the object accelerates

4. When forces are balanced again, the object continues at a new terminal velocity

Define terminal velocity

The maximum speed an object reaches, when the resultant force is zero (weight = air resistance)

State the equation and units of momentum

• Momentum = mass x velocity

• Units: Ns or kgms^{-1}

State the principle of conservation of momentum

• The total momentum before a collision equals the total momentum after the collision provided no external forces act

• m1u1 + m2u2 = m1v1 + m2v2

Say the difference between an elastic and inelastic collision

• In an elastic collision, both momentum and kinetic energy are conserved

• In an Inelastic collision only momentum is conserved (as some energy is transformed into heat energy)

What is a perfectly inelastic collision?

When the colliding objects coalesce (stick together), may keep moving

Explain momentum in explosions and explain two example examples

• Momentum is still conserved in explosions because the total momentum before is 0

• When a bullet is fired from a rifle, the bullet and the rifle have equal and opposite momentum’s

• An astronaut throws a hammer, the astronaut moves in the opposite direction to the hammer

What happens in an elastic head on collision of two identical masses?

The masses will exchange velocities

Why does recoil (velocity) occur?

Because momentum must be conserved

State Newton’s second law of motion in momentum form

• The resultant force of an object is equal to the rate of change of the momentum

• Force = Change in momentum / change in time

What is impulse?

• Impulse = Force x change in time

• Impulse = Change in momentum

• Units = Ns or kgms^{-1}

• The area under a force-time graph

State how to increase change in momentum

Increase time of contact

State how to reduce impact forces, and state three examples

• Increase the time of the collision

• Examples: Crumple-zones, airbags, seatbelts

Explain work done.

• Work is done when a force acts on an object over a distance = energy transferred

• W=Force x distance

Briefly explain how to calculate workdine when the force is applied at an angle

• Use W = Force x Cos(x) x distance

What is the Work-Energy Theorem?

If force varies with distance then work done = area under a force-distance graph

Define Power

• Power is the rate of energy transferred per second

• Power = change in work down / change in time

• Power = Force x Velocity

State the principal of conservation of energy?

• in a closed system the total amount of energy remains constant

• Energy cannot be created or destroyed only transferred from one form to another

Explain three types of Energy transfers (Conservation of Energy)

• Work done against gravity increases gravitational potential energy

• Work done by motion increases kinetic energy

• Energy lost due to resistive forces (friction, air resistance) is usually converted to thermal energy

Define efficiency

• A way to quantify how much of the energy/power put in is transformed into useful energy/power

• Efficiency = Useful energy out / total energy in

• Efficiency = Useful power out / total power in