Newton's Laws if Motion

What does newton’s first law of motion state?

Objects will remain at rest, or move with a constant velocity unless acted on by a resultant force

What does this mean?

• A stationary object will remain stationary

• An object moving with a constant velocity will continue to move at that constant velocity

• Unless the object is acted upon by a resultant force

How does a tab resting on a desk link to this law?

The two forces acting on it, weight and contact force are equal

What is Newton’s second law?

The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object's mass

or

An object with a resultant force will undergo acceleration such that f = ma

What does this mean?

• An object will accelerate (change its velocity) in response to a resultant force

• The bigger this resultant force, the larger the acceleration

• For a given force, the greater the object's mass, the smaller the acceleration experienced

What equation expresses this law?

• F = ma

• F = resultant force on the object in Newtons (N)

• m = mass of the object in kilograms (kg)

• a = acceleration of the object in metres per second squared (m/s²)

What can newton’s second law be used to estimate?

What symbol can show that an answer is approximate?

• The sizes of forces and accelerations in realistic scenarios

• ~

What type of quantity is resultant force?

When could its value be negative?

• Vector

• happens when the resultant force acts in the opposite direction to the object’s motion

What is the equipment list for Required Practical 7: Investigating Force & Acceleration?

Metre ruler – To measure intervals of distance
A toy car (or trolley) – The object for which acceleration is measured
Pencil or chalk – To mark intervals of distance
Bench pulley and string – To connect masses to the toy car or trolley
Weights and weight hanger – To provide a force on the toy car or trolley
Stopwatch – To time the toy car or trolley between distance intervals
Blu tac – To attach extra weight to the toy car or trolley if needed

What is the aim of the investigating the effect of force on acceleration experiment (1) from required practical 7?

The aim of this experiment is to investigate the effect of varying force on the acceleration of an object of constant mass

What are the variables of this experiment?

• Independent variable = force, F

• Dependent variable = acceleration, a

• Control variables:

  • Mass, m

Describe the Newton’s second law experiment method in 9 steps

1. Use the metre ruler to measure out intervals on the bench, e.g. every 0.2 m for a total distance of 1 m. Draw straight lines with pencil or chalk 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 you or a partner starts the stopwatch. Press the stopwatch (in lap mode) at each measured interval (line) 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 would the analysis of results be?

• Use the table of results to determine the average speed of the trolley between intervals

  • Use the distance between each interval (0.2 m) and the average time it takes for the toy car or trolley to travel that distance to calculate the average speed per interval

• Compare the average speed between the first and last intervals for different weights

• Use the equation below to calculate the acceleration between the first and the last intervals:

• Do this for each different weight, comparing how the acceleration varies

What is the aim of: Experiment 2: Investigating the Effect of Mass on Acceleration

The aim of this experiment is to investigate the effect of varying mass on the acceleration of an object produced by a constant force

What are the variables?

• Independent variable = mass, m

• Dependent variable = acceleration, a

• Control variables:

  • Force, F

What is the 10 step method?

1. Use the metre ruler to measure out intervals on the bench, e.g. every 0.2 m for a total distance of 1 m. Draw straight lines with pencil or chalk across the table at these intervals

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

3. Put a 200 g mass on the car

4. 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

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

6. Select a weight to put on the weight hanger that will gently accelerate the car along the bench. This provides the constant force on the car or trolley and will not change

7. Hold the car at the start point

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

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

10. Repeat steps 6-9 for increasing mass on the car, e.g. 400 g, 600 g, 800 g and 1000 g

What is the analysis of the results?

• As in Experiment 1, use the table of results to determine the average speed of the trolley between intervals

  • Use the distance between each interval (0.2 m) and the average time it takes for the toy car or trolley to travel that distance to calculate the average speed per interval

• Compare the average speed between the first and last intervals for different weights

• Use the equation below to calculate the acceleration between the first and the last intervals:

• Do this for each different mass on top of the toy car or trolley, comparing how the acceleration varies

Evaluate the experiments

Systematic Errors:

• Experiment 1: ensure any weights removed from the weight hanger are transferred to the toy car or trolley

  • This is to ensure the total mass of the system remains constant

Random errors:

• A main cause of error in this experiment is the measurements of time

  • Ensure to take repeat readings when timing intervals and calculate an average to keep this error to a minimum

• Start the toy car by releasing it, allowing it to accelerate under the force of the weights attached by the string

  • Ensure not to give it a 'push'

  • Use small masses so that the car/trolley moves at a relatively slow pace to make the time measurements more accurate

What does newton’s third law describe?

the effects of the forces involved when two different objects interact with one another

What does newton’s third law state?

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

or

for every force acting on an object there is an equal and opposite force acting in the other direction

What does it mean?

• When two objects interact, the forces arise in pairs

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

• Force pairs are of the same type of force

  • For example, if object A exerts a gravitational force on object B, then object B exerts an equal and opposite gravitational force on object A

Describe newton’s third law in a person walking

• The foot and the ground are the two objects interacting

• The foot exerts a push force on the ground

• The ground exerts a push force on the foot

• The forces are equal in magnitude and opposite in direction

What are 3 rules to help identify a third law pair?

1. The two forces in a third law pair act on different objects

2. The two forces in a third law pair always are equal in size but act in opposite directions

3. The two forces are always the same type: weight, reaction force, etc.

   • If you are struggling to name the force, just describe it as a push or pull force

What is inertia?

The tendency of an object to continue in its state of rest, or in uniform motion unless acted upon by an external force

What is inertial mass?

The property of an object which describes how difficult it is to change its velocity

What is inertial mass defined as?

The ratio between the force applied to it and the acceleration it experiences

What is the equation for inertial mass?

What does the equation show?

• inertial mass = force/acceleration

• m = f/a

• shows that for a given force, inertial mass is inversely proportional to acceleration

  • Larger inertial masses will experience small accelerations

  • Smaller inertial masses will experience large accelerations