Motion & Forces Flashcards by Keith Anderson

Two identical cars are crash-tested. One is at 56mph, the other is 40mph.
Which car was crashed at 56mph? Explain your answer.

B was crashed at 56mph.

Crashes at higher speeds will do more damage.

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This car is stationary. What are the forces acting on the car?

Weight and normal reaction force

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What is the difference between average speed and instantaneous speed?

Average speed - measured over a distance or journey

Instantaneous speed - measured at one specific moment or point

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Are these forces balanced or unbalanced? Explain your answer.

They are BALANCED - equal size, in opposite directions.

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How can the force of friction be reduced?

Using oil / wax / ice / lubrication

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What are the forces acting on the skydiver?

Weight (down)

Air resistance (up)

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True or False: the elastic cable pulls down on the bungee jumper. This force is called tension.

FALSE.

The elastic cable pulls UP on the bungee jumper. This force is called tension.

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A car travels 420 metres in a time of 17 seconds.
What is the average speed of the car?

d = v x t
420 = v x 17
v = 420 / 17
v = 24.7 ms^-1

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A cyclist is moving at 8 metres per second. How far will they travel in 5 minutes?

d = v x t
d = 8 x (5 x 60)
d = 2400 m

!!! convert minutes to seconds !!!

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List three safety features of modern cars.

Seatbelts

Airbags

Crumple zone

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What is the name of the force that affects an object moving at speed through the air?

Air resistance

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What are the units for force?

N
(newtons)

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Describe the motion of an object when the forces are balanced.

The object will
- stay at rest
or
- move at a constant speed in a straight line.

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Are these forces balanced or unbalanced? Explain your answer.

They are UNBALANCED - there is 100 N in total, all towards the right.

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What are the units for distance?

m
(metres)

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When designing a car, how can the force of air resistance be reduced?

This is called streamlining.

lower to the ground
smooth shape
smaller surface area

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Are these forces balanced or unbalanced? Explain your answer.

They are BALANCED - equal size, in opposite directions.

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This car is moving forward at a constant speed. What are the forces acting on the car?

Weight (down), Normal reaction (up), thrust (forwards), air resistance (backwards), friction (backwards)

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A car is travelling at a constant speed of 12 metres per second.
What distance will the car travel in 30 seconds?

d = v x t
d = 12 x 30
d = 360 m

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Describe the motion of the object in this speed-time graph.

Constant positive acceleration

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Re-word this statement to make it correct:
“When you cycle really fast on your bike, you often feel the wind pushing back against you.”

“When you cycle really fast on your bike, you often feel the air resistance pushing back against you.”

What are the units for speed?

ms^-1
(metres per second)

What are the three quantities in this formula?

d - distance
v ‘bar’ - average speed
t - time

What are the units for time?

s
(seconds)

**Describe the motion of this object.**

**The forces are BALANCED so the object will stay at rest OR continue at a constant speed in a straight line.**

**For the object in this speed-time graph, what can be said about the forces acting on the object?**

**The forces are BALANCED because it is moving at a constant speed**

**What are the units for acceleration?**

**ms****^-2** **(metres per second squared)**

What **equipment** would you need to be able to determine the average speed of a cyclist?

**Stopclock (timer)** **Metre stick (measuring tape, trundle wheel)**

**What is the name of the force from a vehicle's engines that makes it accelerate?**

**Thrust**

Re-word this statement to make it correct: ***"A seatbelt will make you speed up if you are in a car accident. It does this by applying a forwards force against your body."***

"A seatbelt will make you **slow down** if you are in a car accident. It does this by applying a **backwards** force against your body."

**Describe the motion of the object in this speed-time graph.**

**Constant negative acceleration**

**How does this cyclist reduce the force of air resistance acting against them?**

**- wearing skin-tight clothing** **- crouching down and making their body as small as possible** **- wearing an aerodynamic helmet**

**What are the three quantities in this formula?**

**d - distance** **v -** (instantaneous) **speed** **t - time**

**Describe the motion of this object.** **Explain your answer.**

**The object will accelerate to the right.** **There is an unbalanced force acting on it.**

**Describe what would happen to an object if the forces on it are UNBALANCED.**

**The object would accelerate (in the direction of the unbalanced force)**

**What is the name of the force caused by gravity pulling on an object?**

**Weight**

**Describe the motion of an object when there is an unbalanced force acting on it.**

**The object will accelerate (in the direction of the unbalanced force).**

**How would you do an experiment to determine the average speed of a person walking?**

**Use a metre stick to measure the distance** **Use a stopclock to measure the time** **Use the formula d = v x t to calculate the speed**

**What is the name of the force that pushes UP on an object, when it is at rest on a solid surface?**

**Normal reaction force**

**What is the definition of acceleration?**

**Acceleration is the change in speed per second.**

**What are the forces acting on the rocket (just after take-off)?**

**Weight (down)** **Thrust (up)**

**What is the name of the force that exists when two solid surfaces are in contact?**

**Friction**

**A rocket travels 10 km in a time of 38 seconds. What is its average speed?**

**d** = **v** x **t** **10,000** = **v** x **38** **v** = **10,000** / **38** **v** = **263 ms****^-1** *!!! convert km to metres !!!*

**What are the forces acting on this person?**

**Weight (down)** **Normal reaction force (up)**

**For the object in this speed-time graph, what can be said about the forces acting on the object?**

**The forces are UNBALANCED because the object is accelerating**

**How would you do an experiment to measure the instantaneous speed of a toy car?**

**Use a lightgate (and computer) to measure the time taken to pass through the lightgate (t)** **Measure the length of the car (d)** **Use the formula d** = **v** x **t to calculate the speed**

**What is the name of the force that exists in a rope, string, cable, elastic band... ?**

**Tension**

**True or false: an object can be at rest (stationary) even if forces are acting on it.**

**TRUE** **If the forces on an object are BALANCED, it can stay at rest (or continue at a constant speed in a straight line)**

**What are the four quantities in this formula?**

**a - acceleration** **v - final speed** **u - initial speed** **t - time**

**Describe the motion of the object in this speed-time graph.**

**Constant speed (or steady speed)**