P8 Forces in Balance

Question 1

What is Newton’s first law?

Answer 1

According to Newton’s first law of motion, an object remains in the same state of motion unless a
resultant force
acts on it. If the resultant force on an object is zero, this means:

a stationary object stays stationary
a moving object continues to move at the same
velocity (at the same speed and in the same direction)

Newton’s first law can be used to explain the movement of objects travelling with uniform motion (constant velocity). For example, when a car travels at a constant velocity, the driving force from the engine is balanced by the resistive forces such as air resistance and frictional forces in the car’s moving parts. The resultant force on the car is zero.

Other examples include:

a runner at their top speed experiences the same air resistance as their
thrust
an object falling at
terminal velocity
experiences the same air resistance as its weight

Question 2

What is newton’s second law?

Answer 2

Newton’s second law of motion can be described by this equation:

resultant force = mass × acceleration

This is when:

force (F) is measured in newtons (N)
mass (m) is measured in kilograms (kg)
acceleration (α) is measured in metres per second squared (m/s2)
The equation shows that the acceleration of an object is:

proportional to the resultant force on the object
inversely proportional to the mass of the object
In other words, the acceleration of an object increases if the resultant force on it increases, and decreases if the mass of the object increases.

Question 3

What is inertial mass?

Answer 3

A measure of how difficult it is to change the velocity of an object. It is defined as the ratio of force over acceleration.

Question 4

What is non uniform motion?

Answer 4

Newton’s first law can also be used to explain the movement of objects travelling with non-uniform motion. This includes situations when the speed changes, the direction changes, or both change. For example, when a car accelerates, the driving force from the engine is greater than the resistive forces. The resultant force is not zero.

Other examples include:

at the start of their run, a runner experiences less air resistance than their thrust, so they accelerate
an object that begins to fall experiences less air resistance than its weight, so it accelerates

Question 5

Newton seconds law required practical:

Answer 5

Cut an interrupt card to a known length (such as 10 cm) and attach it to an air track glider.
Set up the equipment as shown in the diagram. Make sure that the air track is level, and that the card will pass through both gates before the masses strike the floor.
Set the data logging software to calculate acceleration.
Use scales to measure the total mass of the glider, string and weight stack. Record this value.
Attach the full weight stack (6 x 10g masses) to the end of the string.
Make sure the glider is in position and switch on the air blower. The glider should accelerate.
Remove one weight and attach it to the glider using blu-tack. This will keep the total mass constant. (The weight stack is being accelerated too.)
Repeat steps 6-7 removing one weight from the stack each time. Remember to attach each weight to the glider as it is removed from the weight stack.

Analysis
Plot a scatter graph with force on the vertical axis, and acceleration on the horizontal axis. Draw a suitable line of best fit.
Describe what the results show about the effect of increasing the force which is accelerating the object.
Extension: calculate the gradient of your graph and compare this to the to total mass of the glider and weight stack measured in step 4 above.
Evaluation
Acceleration is directly proportional to the force applied to the object. This means that a graph of force against acceleration should produce a straight line that passes through the origin.

Question 6

What is newton’s third law?

Answer 6

Newton’s third law
According to Newton’s third law of motion, whenever two objects interact, they exert equal and opposite forces on each other.

This is often worded as ‘every action has an equal and opposite reaction’. However, it is important to remember that the two forces:

act on two different objects
are of the same type (eg both contact forces)
Examples of force pairs
Newton’s third law can be applied to examples of equilibrium situations.

A cat sits on the ground
There are
contact
gravitational forces between Earth and the cat:

the cat pulls the Earth up
the Earth pulls the cat down
These forces are equal in size and opposite in direction.