5.6 Forces & Motion

What is distance?

How far an object moves.

Is distance a scalar or vector quantity?

Scalar

What is displacement?

The distance an object moves (measured in a straight line) and the direction of that straight line.

Is speed a scalar or vector quantity?

Scalar

What are the factors that can affect the speed at which someone walks, runs, or cycles?

• Age

• Distance Travelled

• Fitness

• Terrain

What are the typical speeds for walking, running, and cycling?

• 1.5 m/s

• 3 m/s

• 6 m/s

What are the typical speeds for a car, train, and aeroplane?

• 13 - 30 m/s

• 50 m/s

• 250 m/s

What is the typical value for the speed of sound in air?

330 m/s

What is the equation to calculate the distance travelled in a specific time for an object moving at constant speed?

s = vt

How do you calculate relative speed (when the objects are travelling in opposite directions?

speed A + speed B

How do you calculate relative speed (when the objects are travelling in the same direction)?

The difference between speed A and speed B.

What is the velocity of an object?

Its speed in a given direction.

Is velocity a scalar or vector quantity?

Vector

Motion in a circle involves ________ speed and ___________ velocity.

• constant

• changing

If an object moves along a _________ _____, the _________ travelled can be represented by a distance-time graph.

• straight line

• distance

What does the gradient on a distance-time graph represent?

Speed

If an object is ____________, its __________ at any particular time can be determined by drawing a tangent and measuring the ____________ of the distance-time graph at that time.

• accelerating

• speed

• gradient

Fill in the gaps.

• Constant Speed

• Stationary

If the speed of an object changes, it will be ___________ or ___________. This can be shown as a ___________ line on a distance-time graph.

• accelerating

• decelerating

• curved

What is the gradient and speed of section A?

Increasing

What is the gradient and speed of section B?

Constant

What is the gradient and speed of section C?

Decreasing

What is the gradient and speed of section D?

• Zero

• Stationary

What is the equation for acceleration?

a = ∆v ÷ t

What is the gradient, velocity, and acceleration of section A?

• Positive

• Increasing

• Positive

What is the gradient, velocity, and acceleration of section B?

• Zero

• Constant

• Zero

What is the gradient, velocity, and acceleration of section C?

• Negative

• Decreasing

• Negative

What is the gradient, velocity, and acceleration of section D?

• Zero

• Stationary

• Zero

What does the gradient on a velocity-time graph represent?

Acceleration

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

Distance/displacement

What is the equation that applies to uniform acceleration?

v² - u² = 2as

Near the Earth's surface any object falling freely under gravity has an acceleration of about _______ m/s².

9.8

What happens to an object falling through a fluid?

It initially accelerates due to gravity. Eventually the resultant force will be zero and the object will move at terminal velocity.

Newton’s First Law:

If the ____________ _______ acting on an object is zero and:

• the object is stationary, the object ___________ ___________.

• the object is moving, the object continues to move at the _______ _______ and in the ________ _______. So the object continues to move at the _________ _________.

• resultant force

• remains stationary

• same speed

• same direction

• same velocity

What happens when a vehicle travels at a steady speed?

The forward force from the engine balances out the backward forces of air resistance and friction, so the resultant force is zero.

The velocity of an object will only change if a ___________ ________ is acting on the object.

resultant force

What is inertia?

The tendency of objects to continue in their state of rest or of uniform motion.

Newton’s Second Law:
The acceleration of an object is ___________ to the ___________ _______ acting on the object, and ___________ _____________ to the ________ of the object.

• proportional

• resultant force

• inversely proportional

• mass

What is the equation that represents Newton’s Second Law of Motion?

F = ma

What are you investigating in RP 7 — Acceleration?

• The effect of varying the force on the acceleration of an object of constant mass.

• The effect of varying the mass of an object on the acceleration produced by a constant force.

RP 7 — Acceleration — Activity 1:

1. Use a __________ to measure intervals on the bench and draw straight lines at these intervals.

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

3. Tie a length of string to the toy car. Pass the string over the _________ and attach the _________ ________ to the other end of the string.

4. Make sure the string is ____________ and is in line with the toy car.

5. Hold the toy car at the start point.

6. Attach the full weight stack (1.0 N) to the end of the string.

7. Release the toy car at the same time that you start the stopwatch, press the stopwatch at each ____________ ____________ on the bench and for the final time at 100 cm.

8. Record the results in a table.

9. Repeat steps 5 - 8 for decreasing weights on the stack (e.g. 0.8 N, 0.6 N, 0.4 N, 0.2 N). Place the masses that are removed from the weight stack onto the top of the car each time you decrease the weight.

• ruler

• pulley

• pulley

• weight stack

• horizontal

• measured interval

In RP 7 — Acceleration (activity 1), why must you put the weights that you remove on top of the car when you decrease the weight?

To keep the mass of the object (car, weight stack, and string) the same.

RP 7 — Acceleration — Activity 2:

1. Set up the bench, pulley, weight stack, and car as in steps 1 - 5 of activity 1.

2. Select a weight for the weight stack that will just accelerate the car along the bench (use results from activity 1).

3. Put a 200g mass on the car.

4. Hold the car at the start point.

5. Attach the __________ _________ to the end of the string.

6. Release the car at the same time as you start the stopwatch, press the stopwatch at each ___________ __________ on the bench and for the final time at 100 cm.

7. Record the results in a table.

8. Repeat steps 5 - 7 for increasing more masses on the car.

• weight stack

• measured intervals

What are the sources of error in RP 7?

• Human reaction time

• Friction

• Parallax error

Explain how human reaction time is an issue in RP 7 — Acceleration and what kind of error is it?

• When using a stopwatch, there is a delay in starting and stopping it.

• Random error

Explain how friction is an issue in RP 7 — Acceleration.

The friction between the car and the bench (or in the pulley) provides a counter force, so the resultant force will be less than the weight on the string.

Explain how parallax error is an issue in RP 7 — Acceleration.

If you are not looking directly over the markers on the bench when pressing the stopwatch, the time might be recorded too early or too late.

How could the method or apparatus for RP 7 be changed to improve the accuracy and reproducibility of the results?

• Use a light gate and data logger — removes human reaction time as an error.

• Incline the bench so that the gravity acting on the car balances the friction. The car will move at a constant speed so the weight is solely responsible for acceleration.

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?

thinking distance + braking distance

What is the braking distance?

The distance travelled under the braking force.

What is the thinking distance?

The distance travelled during the driver’s reaction time.

For a given _______ _______, the greater the speed of the vehicle, the ________ the __________ __________.

• braking force

• greater

• stopping distance

What is the range for typical human reaction times?

0.2 s - 0.9 s

What can affect a driver's reaction time?

• Tiredness

• Alcohol

• Drugs

What can affect a driver’s ability to react?

Distractions

What are the 2 factors that can affect the braking distance of a vehicle?

• Adverse road and weather conditions

• Poor conditions of the vehicle

What does adverse road and weather conditions include?

Wet and icy conditions

What does the poor condition of a car include?

• Tyres

• Brakes

When a _________ is applied to the brakes of a vehicle, work done by the _________ between the brakes and the wheel reduces the _______ ______ of the vehicle and the _____________ of the brakes increases.

• force

• friction

• kinetic energy

• temperature

The greater the speed of a vehicle the ___________ the __________ _______ needed to stop the vehicle in a certain ___________.

• greater

• braking force

• distance

The greater the braking force the greater the ________________ of the vehicle.

deceleration

What can large decelerations lead to?

• Brakes overheating

• Loss of control