AQA GCSE Single Science Physics: Forces

How do scalar and vector qualities differ?

• Scalar quantities have magnitude only

• Vector quantities have magnitude and direction.

What is a force?

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

What differentiates a contact and non-contact force? Name examples

• Contact forces occur when objects are physically touching- friction, air resistance, tension and normal contact force.

• Non-contact forces occur when objects are physically separated- gravity, electrostatic force, magnetic force

What is an object’s center of mass?

The point at which the whole weight of the object may be considered to act

What two things does an object’s weight depend on?

• Gravitational field strength

• Mass

What does it mean for work to be done on an object?

A force has caused an object to move through a distance

How would you convert between newton-metres and joules?

• 1 joule = 1 newton-metre

• This is because one joule of work is done when a force of one newton causes a displacement of one metre.

Work done against the frictional forces acting on an object causes a rise in which energy store?

• Thermal energy

• An example of this is generating warmth through rubbing hands together

What is the relationship between extension and force?

• The extension of an elastic object, such as a spring, is directly proportional to the force applied

• This stops when the limit of proportionality is exceeded.

What is the equation for elastic potential energy and force applied?

• elastic potential energy = 0.5 × spring constant × (extension)²

• force = spring constant × extension

What is a moment and how can it be calculated?

• A moment is the turning effect of a force.

• moment = force × perpendicular distance from line of action

What two things can be used to transmit the rotational effects of forces? How do they work?

• A simple lever and a simple gear system can both be used to transmit the rotational effects of forces.

• They increase the moment by increasing the perpendicular distance from the line of action and so allow a larger force to be supplied without requiring a larger effort.

How is pressure at the surface calculated? In which direction does the force it produces act?

• Pressure (Pa) = force (N) / area (m²)

• It produces a force at right angles to a surface

How can pressure be calculated and what is the relationship between pressure and depth?

• pressure = height of column × density of the liquid × gravitational field strength

• Pressure increases as depth increases due to the increase in weight of the water above

How does the weight of an object relate to whether it will sink or float?

• If the upthrust is less than the weight of the object, the object will sink.

• If the upthrust is larger than the weight of the object, the object will float.

What are some key factors of the atmosphere?

• It is thin compared to the size of the Earth

• It becomes less dense as the altitude increases

Why does atmospheric pressure decrease as altitude increases?

• the number of air molecules decreases

• the weight of the air above decreases

What is the typical speed for: sound in air, walking, running, cycling, car, train, plane

• Sound in air: 330 m/s

• Walking: 1.5 m/s

• Running: 3 m/s

• Cycling: 6 m/s

• Car: 25 m/s

• Train: 50 m/s

• Plane: 250 m/s

What are the two acceleration formulas?

• (final velocity)² - (initial velocity)² = 2(a x d)

• acceleration = change in velocity/time taken

What can you discover from velocity-time graphs?

• Acceleration is the gradient of the line on velocity-time graphs

• The area under a velocity-time graph is the displacement of an object

Explain how a parachutist reaches two terminal velocities

1. Immediately on leaving the aircraft, the skydiver accelerates downwards due to the force of gravity. There is no air resistance acting in the upwards direction, and there is a resultant force acting downwards. The skydiver accelerates towards the ground.

2. As the skydiver gains speed, their weight stays the same but the air resistance increases. There is still a resultant force acting downwards, but this gradually decreases.

3. Eventually, the skydiver's weight is balanced by the air resistance. There is no resultant force and the skydiver reaches terminal velocity.

4. When the parachute opens, the air resistance increases. The skydiver slows down until a new, lower terminal velocity is reached.

What is Newton’s First Law and what is inertia?

If the resultant force acting on an object is zero and:

• the object is stationary, the object remains stationary

• the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.

  • Inertia is the tendency of objects to continue in their state of rest/of uniform motion

What is Newton’s Second Law?

• The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

• Resultant force = mass × acceleration

What is inertial mass and how is it calculated?

• Inertial mass is a measure of how difficult it is to change the velocity of an object.

• It can be measured through the ratio of force over acceleration

Required practical: Investigate the effect of varying the force on the acceleration of an object of constant mass

1. Use the metre ruler to measure out intervals on the bench. Draw straight lines across the table at these intervals

2. Attach the bench pulley to the end of the bench

3. Tie some string to the toy car or trolley. Pass the string over the pulley and attach the mass hanger to the other end of the string

4. Make sure the string is horizontal (i.e. parallel to the bench) and is in line with the toy car or trolley

5. Hold the toy car or trolley at the start point

6. Attach the full set of weights (total = 1.0 N) to the end of the string

7. Release the toy car or trolley at the same time as a partner starts the stopwatch. Press the stopwatch (in lap mode) at each measured interval on the bench and for the final time at 1.0 m

8. Record the results in the table and repeat step 7 to calculate an average time for each interval

9. Repeat steps 5-8 for decreasing weights on the weight hanger, e.g. 0.8 N, 0.6 N, 0.4 N, and 0.2 N. Make sure you place the masses that you remove from the weight stack onto the top of the car, using the Blu-tac, each time you decrease the weight

What is Newton’s Third Law?

Whenever two objects interact, the forces they exert on each other are equal and opposite.

What is the stopping distance of a vehicle and its components? How does it change with speed?

• The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

• For a given braking force the greater the speed of the vehicle, the greater the stopping distance.

What’s the typical thinking, braking and total stopping distance for a vehicle travelling at 20mph, 40mph and 70mph?

At 20mph:

• Thinking distance: 6m

• Braking distance: 6m

• Total stopping distance: 12m

At 40mph:

• Thinking distance: 12m

• Braking distance: 24m

• Total stopping distance: 36m

At 70mph:

• Thinking distance: 21m

• Braking distance: 75m

• Total stopping distance: 96m

What can reaction time/thinking distance be impacted by?

Typical values range from 0.2 s to 0.9 s.

• Tiredness

• Drugs

• Distractions

• Alcohol

What can stopping distance be impacted by?

• Adverse weather conditions (wet or icy)

• Poor brakes

• Poor tyres

Explain the forces involved in rapid deceleration of a vehicle

• When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.

• The greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance.

• The greater the braking force the greater the deceleration of the vehicle. Large decelerations may lead to brakes overheating and/or loss of control.

How is momentum calculated and what is conservation of momentum?

• momentum = mass × velocity

• Conservation of momentum means that in a closed system, the total momentum before an event is equal to the total momentum after the event.

Give examples of car safety features and how they help safety of passengers

These increase the time taken for collision, decreasing the rate of momentum, and therefore the force experienced.

• Seatbelts

• Airbags

• Crumple zones