National 5 Physics- Dynamics

What is the speed of an object?

The speed of an object tells us how far it will travel in a certain amount of time.

What are the two different types of speed?

• Average speed - Instantaneous speed

What is average speed?

The speed of an object measured over a large distance or time.

What is instantaneous speed?

The speed at any given instant of time.

What are the instructions for the measuring average speed in the lab practical?

• Measure the distance between the two light gates - Let the trolley roll down the slope- don't push it - The TSA will record the time taken for the trolley to move between the light gates - Enter the distance and times values into a table - Calculate the average speed of the trolley as it runs along the track - Conduct multiple tests - Repeat this procedure, using a different distance between the light fates

What measurements do you need to conduct for the measuring the average speed in the lab experiment?

• The distance between the two light gates - The time taken by the trolley to move between the light gates

What headings should you have in the table for the measuring average speed in the lab practical?

• Test - The distance travelled (in metres) - Time time taken (in seconds) - The average speed (in metres per second)

Describe how you measuring the average speed of the trolley.

• We measured the distance between the two light gates using a meter stick - We used a clamp stand, to hold the light gates - We measured the time taken for the car to travel between the light gates (using a mast to shade the light) (the car was traveling down a ramp) <- using a fast timer - We used v= d/t to calculate the average speed

What are the instructions for the measuring instantaneous speed practical?

• Place the light gate near the middle of the ramp. - Connect up the light gate to the TSA computer and set the TSA computer to measure short time intervals. - Measure the length of the mask in metres. - Release the trolley so that it runs down the ramp and the mask cuts the light beam. - Record the time interval for the mask to cut the light. - Calculate the instantaneous speed. - Place the light gate at a different position and try again.

What headings should you have in the table for the measuring instantaneous speed practical?

• Test - The length of mask (in metres) - The time taken for mask to cut light beam (in seconds) - The instantaneous speed of trolley (in metres per second)

What are the measurements that you need to conduct for the instantaneous speed experiment?

• Measure the length of the mask in metres. - Record the time interval for the mask to cut the light.

Describe how you measure the instantaneous speed of the trolley.

• We used 1 light gate, and a clamp stand to stabilize it - We calculated the length of the cars mask (as our distance) - We measured the time taken for a car (coming down a ramp) to travel through the gate using a fast timer - We calculated the speed using v= d/t

What groups can everything we measure in physics go into?

• Vectors - Scalars

What is a scalar quantity?

One which is only defined by its magnitude (size).

What is a vector quantity.

One which has both a magnitude (size) and a direction.

What is a magnitude?

Size.

Name 5 scalar quantities.

• Mass - Time - Energy - Voltage - Temperature

Name 3 vector quantities.

• Force (all forces) - Acceleration - Weight

What can some quantities have?

There are some quantities that have a scalar and a vector version.

Provide an example of a pair with a scalar and vector quantity.

One of these pairs is distance (scalar) and displacement (vector).

What is distance?

The total distance travelled (scalar). How far something has travelled.

What is displacement?

• How far something is from where it started - The shortest distance between the start and end point, with a direction.

How can you add scalars?

Just add the quantities.

How can you add vectors?

You should look at the direction and the values, for example if you have a value left and one right you would subtract the left value from the right to get the right value.

What formulas should you use to calculate a vector quantity (displacement).

• The Pythagoras theorem for the magnitude - Trigonometry (SOHCAHTOA) for the direction (angle), specifically tan

What does resultant mean?

The result of adding (the two) vectors.

How do we usually measure directions?

Clockwise from north.

What are bearings?

Bearings are a 3 digit "angle" from north (they are implied from north) (without the degrees symbol)

Provide an image of a bearings diagram.

How do you add vectors?

Tip to tail.

How can you find vectors apart from through the use of formulas?

By using a scale diagram.

Describe speed.

• Speed is a scalar. - Speed tells us how much distance an object has covered in a certain time.

Describe velocity.

• Velocity is a vector. - Velocity tells us the displacement of an object in a certain time.

What do we need to do because velocity is a vector?

• We need to give a direction - We add velocities using Pythagoras and trigonometry

What do we do with bearings?

Round to whole numbers as bearings are 3 digits (start with a 0 if they are not 3 digits).

Are angles directions?

No.

What can speed-time graphs allow us to see?

By using speed time graphs, we can how an object is moving and carry out calculations about its journey.

What can speed time graphs let us see?

By using speed-time graphs, we can see how an object is moving and carry out calculations about the journey.

Provide an example of a speed-time graph for an object moving with constant speed.

Provide an example of a speed-time graph for an object moving with constant acceleration.

Provide an example of a speed-time graph for an object moving with constant deceleration.

Describe using graphs to describe motion.

• In real life, the speed of objects changes. - They get faster, slow down, stop. - You need to be able to use a graph to describe what's happening.

What is on the axis of a speed/velocity-time graph?

• (speed/)Velocity (measured in metres per second) is on the vertical axis - Time (measured in second) is on the horizontal axis

How is constant speed portrayed in a speed-time graph?

Using a straight line.

How is constant acceleration portrayed in a speed-time graph?

Using a line that is sloping upwards.

How is constant deceleration portrayed in a speed-time graph?

Using a line that is sloping downwards.

How is velocity different from speed?

Velocities have a direction.

What can we show on a velocity-time graph?

• In a velocity-time graph, we can show not just the size of a velocity but also its direction- as long as we are just working with "opposite" directions like up/down or left/right. - Anything we draw on the + velocity axis we take as one direction. - Anything we draw on the - velocity axis count as the other direction

Describe how you'd write a velocity from a velocity-time graph.

• The sign (+/-) represents the direction - The number tells you the magnitude

What does the area under a speed time graph give you?

Its distance travelled.

What does the area under a velocity time graph give you?

Its displacement. The areas add up like a vector.

How do we add vectors?

Tip to tail, move them so they are this way.

What is the (bold) line on a velocity-time graph?

Just the direction.

Describe how to find how far something has travelled if its velocity is changing.

• If an objects velocity is changing, we cannot use d= v x t to find how far it has travelled. - This formula only works when an object is travelling at a constant velocity.

What do you do to work out the displacement from a velocity-time graph?

• Calculate the area under the graph. - Note: displacement= area under the graph for v-t, distance= area under the graph for speed-time)

How do we calculate the area of velocity time graphs?

We break up the velocity-time graph into rectangles and triangles, then use simple maths to calculate the area.

Describe calculating the area under a velocity-time graph.

• We can not use d=vt because its instantaneous and not average speed- the speed is constantly changing - We break this down into shapes

What happens if the graph goes below the axis?

If the graph goes below the axis- because the object is travelling in the opposite direction- then we count it as a negative displacement.

Define acceleration.

Acceleration is the change in velocity each second.

What does a positive acceleration mean?

Positive number= velocity increasing.

What does a negative acceleration mean?

Negative number= Velocity decreasing.

What is deceleration?

Negative acceleration.

Describe deceleration using the acceleration formula.

• If an object is decelerating, its final velocity will be less than the initial velocity - So when you work out v-u you get a negative value, and your answer for a will be negative. - That's fine- your answer shows how much the velocity decreases by every second. - If a question tells you that an object is decelerating- then use a negative value for a.

Is acceleration a vector quality? Describe this.

Acceleration is a vector the sign (the +/-) is what shows this- unless you have a magnitude (+ direction).

What does the negative in acceleration mean?

That its slowing down.

What can speed/velocity time graphs be used for?

Calculating acceleration.

What can a velocity-time graph tell us?

• The initial velocity of an object - The final velocity of an object - The time taken for the change in velocity - That's everything we need to know to calculate the acceleration

What is the gradient of a velocity-time graph?

Gradient= rate of change

What is the area under a velocity-time graph?

• Distance travelled (displacement)

Describe how we mathematically find the acceleration?

• Mathematically, finding the acceleration from a velocity-time graph is the same as finding the gradient of the line. - acceleration= gradient of a line on a velocity-time graph (This is related to Advanced Higher) - Whatever method you use for finding gradients in maths- you can now use in physics

Describe the practical- measuring acceleration.

• When the trolley passes light gate A- we can use the width of the mask and the time to pass the light gate to calculate the initial velocity of the trolley at A. - When the trolley passes light gate B- we can use the width of the mask and the time to pass the light gate to calculate the final velocity of the trolley at B. - We can also measure the time it takes for the trolley to go from A to B.

What does the method of measuring acceleration use?

It uses a double mask and a single light gate.

What does Newton's first law tell us?

What happens to the velocity of an object when the forces on it are balanced.

What does balanced forces mean?

Equal in size, but opposite in direction.

What does Newton's 1st law say about the forces on an object being balanced?

• Objects at rest- remain at rest - Objects in motion- continue at a constant speed in a straight line

Why do things slow down if we don't keep pushing them? Describe this.

• Because of friction. - To keep something moving at a constant velocity, we have to keep pulling (or pushing) it. - Our pulling force balances the friction force. - In space, there are no friction forces. - If you give something a push, it keeps on moving at a constant velocity- for-ever.

What do we look at in the experiment for the Newton's 2nd law practical?

The relationship between force and acceleration.

What do we do for the Newton's 2nd law practical?

• In this experiment, we apply an unbalanced force to a trolley and measure its acceleration. - The track is very smooth, so frictional forces can be ignored.

What would the Newton's 2nd law experiment tell you?

Your experiment should have shown that the acceleration of the trolley was directly proportional (to) the force- double the force, double the acceleration.

What would happen to the acceleration of a trolley, if you used the same unbalanced force but doubled the mass of the trolley?

The acceleration would decrease as a= F/m, and F would be divided by a larger value.

Describe Newton's 2nd law.

• Newton's 2nd law looks very simple. - It is very simple. - But it lies at the heart of physics. - Understand it properly- and you have gone a long way to understanding physics.

Describe more complex Newton 2 calculations.

• Often, there is more than one force acting on an object. - Newton 2 applies to the unbalanced force. - So you need to work out the unbalanced force first, before using Fun= ma.

What do you substitute into F=ma?

You have to substitute the unbalanced force.

How do you work with forces at right angles (Newtons second law)?

• Same as displacement and velocity vectors. - This is a bit harder. - Remember that forces are vectors, so you need to find the resultant force using trigonometry. - You also need to add the vectors "tip-to-tail"- this means that you arrange the vectors so that the tip of one vector joins the tail of the other vector.

What are the steps of a Newton's 2nd law question with forces at right angles (eg. if 2 forces are acting on a ship)?

• You need to re-draw the arrows (or you get 0 marks) - Arrange the vectors tip-to-tail - Add the resultant, and use the Pythagoras to find the size (use Pythagoras to find the size of the resultant force). - Use F= ma to find the acceleration

Describe the relationship between mass and weight.

• In everyday language the words mass and weight get interchanged. - In physics, they have very definite- and different- meanings. - You must learn the difference!

What is mass?

Mass is the amount of matter in a an object. - Weight is the downwards force of an object due to gravity.

Describe the weight.

• The weight of an object is caused by the pull if gravity. - Gravity pulls more or less in different places. - We measure this using gravitational field strength, or g. - g tells us the force gravity applies to mass of 1kg. - On Earth, the value of g is about 9.8Nkg^-1.

Describe Newtons laws.

• Newton's 1st Law and 2nd Law are all to do with the forces on a single object. - Newton's 3rd Law tells us what happens when two objects are in contact.

Describe what Newton's 3rd Law tells us.

• When OBJECT A exerts a force on OBJECT B, then OBJECT B exerts an equal but opposite force on OBJECT A. - (Every force has an equal but opposite reaction) - OR For every action, there is an equal but opposite reaction.

Describe what Newton 3 can be called?

Newton 3 deals with two forces, these are often called Newton pairs.

Provide an example of a Newton pairs description.

• The paddle applies a force to the water (F). - So the water applies an equal and opposite force to the paddle (F'). - It's the F' force that makes the canoe accelerate.

Describe a canon (Newton's 3rd law).

• The canon exerts a force on the canon ball (friction). - The canon ball pushes back at the canon (reaction). (- The canon (A) only moves a little bit- large mass - The canon ball- B moves alot- small mass - Conservation of momentum- higher)

Describe balloons (which can be an example of rockets).

• If you've ever blown up a balloon and let it go- then you have used the same physics that rockets use. - Balloon pushes on air - Air pushes on balloon - When you blow up a balloon; the stretched rubber applies a force to the air- so the air applies a force to the balloon. - This makes the balloon accelerate.

Describe the water rockets practical.

• We filled the water rocket with water and pressurised it - The pressure allowed it to exert a force, the ground exerted a force on the rocket (Newton's 3rd law) - As the rocket moves up, its unbalanced force decreases (it uses up its fuel- water- loosing mass) (Newton's 2nd law)- so it reaches the ground. - A larger mass causes a smaller acceleration a= F/m

What is terminal velocity?

• The constant speed achieved by an object when the weight and air resistance are balanced (for an object in free fall) - (Can happen to a car but this is for N5)

Describe a boy falling out of a plane.

• At the moment he falls weight is acting on him - This is an unbalanced force - The boy's velocity increases (he is accelerating), due to F= ma (Newton's 2nd law) - As the boy's velocity increases, air resistance is a force that starts to act on the boy - As the boy falls faster the size of the force increases. - Air resistance increases with speed (and also surface area). - Eventually, the two forces acting on the boy become balanced - His velocity is now constant (Newton's first law) - We call his velocity- terminal velocity

What does a steep line on a velocity-time graph mean?

The acceleration is large.

What does a gradually changing gradient on a velocity-time graph mean?

The acceleration is small.

Describe free-fall.

• If there is no air resistance, then the only force acting on the falling object is its weight. - Because this is an unbalanced force, the object will keep accelerating (at 9.8 metres per second^-2 on Earth) until it hits the ground. - Motion like this is called free-fall. - (Although it's difficult to get rid of air resistance, we sometimes assume there is no air resistance. This makes it easier to work out what falling objects do).