Effects Of Forces

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Notes de leçon

Resultant Forces

A force is a push or pull that acts on an object due to interaction with another object.
Forces can change an object's size, shape, or motion (speed or direction).
Resultant force is the single force that describes all forces acting on a body.
Balanced forces are equal in magnitude and opposite in direction; they cancel out (resultant = 0).
Unbalanced forces produce a non-zero resultant force, causing a change in motion.
To calculate resultant force, assign positive/negative signs to directions and sum the forces.

Forces on a book resting on a table

Newton's First Law

Newton's first law: Objects remain at rest or move with constant velocity unless acted on by a resultant force.
Constant velocity means no change in speed or direction.
If resultant force is zero, the object is either stationary or moving at constant velocity.
Example: A mug on a table stays at rest unless a force acts on it.
Example: A car moving at constant speed has balanced driving and frictional forces.

Examples of Newton's first law

Newton's Second Law (Extended)

Newton's second law: Acceleration is proportional to resultant force and inversely proportional to mass.
Equation: F = ma, where F = resultant force (N), m = mass (kg), a = acceleration (m/s²).
Acceleration occurs in the same direction as the resultant force.
For a given force, larger mass gives smaller acceleration.

Newton's second law in action

Investigating Force & Extension (Extended)

Aim: Investigate relationship between force and extension of a spring.
Independent variable: force (F); dependent variable: extension (x).
Method: Hang masses on a spring, measure extension with a ruler, repeat for different masses.
Calculate force as weight (W = mg) and extension as final length minus original length.
Plot force against extension; a straight line through origin shows direct proportionality (Hooke's law).

Equipment for investigating the extension of a spring

Hooke's Law (Extended)

Hooke's law: Extension of an elastic object is directly proportional to force applied, up to the limit of proportionality.
Equation: F = kx, where k = spring constant (N/m), x = extension (m).
Spring constant measures stiffness: high k = stiff spring, low k = stretchy spring.
Force-extension graph: linear region obeys Hooke's law; gradient = spring constant.
Beyond limit of proportionality, graph curves and Hooke's law no longer applies.

Force-extension graph for a spring

Friction

Friction opposes motion and causes heating (energy transfer).
Solid friction arises from surface imperfections; can be reduced by lubrication or smoothing.
Fluid friction (drag) occurs when objects move through gases or liquids; particles collide with object.
Air resistance is a type of drag that slows motion and heats the object and air.
Streamlining reduces fluid friction.

Friction between solid surfaces

Moments

A moment is the turning effect of a force about a pivot.
Moment = force × perpendicular distance from pivot (unit: N m).
Increasing distance from pivot reduces the force needed for the same moment.
Principle of moments (extended): For a balanced object, total clockwise moment = total anticlockwise moment.

A force applied about a pivot

Equilibrium & Centre of Gravity

Equilibrium: forces are balanced (no resultant force) and moments are balanced (no resultant moment).
Centre of gravity is the point where the weight of an object acts.
For symmetrical objects, centre of gravity is at the geometric centre.
Stability depends on centre of gravity position: stable if it lies above the base; topples if outside base.
Wide base and low centre of gravity increase stability.

Systems in equilibrium

Investigating Centre of Gravity

Aim: Find centre of gravity of an irregularly shaped plane lamina using suspension method.
Method: Suspend lamina from a point, use plumb line to mark vertical line; repeat from two other points.
Intersection of the three lines gives the centre of gravity.
Allow lamina to settle before marking; punch holes before suspending to avoid shifting centre of gravity.

The suspension method for centre of gravity

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1.What is a force?
Easy
- AA push or a pull that acts on an object due to interaction with another object
- BA measure of the mass of an object
- CThe speed of an object in a given direction
- DThe energy stored in an object
2.State two effects that a force can have on an object.
Easy
3.Newton's first law states that an object will remain at rest or move with constant velocity unless acted on by a resultant force.
Easy
True or false?
4.Complete the sentence using the correct word.
Easy
Hooke's law states that the extension of an elastic object is directly proportional to the force applied, up to the ____.
5.Match each term to its correct description.
Medium
- Moment
- Centre of gravity
- Equilibrium
- The turning effect of a force
- The point through which weight acts
- State where forces and moments are balanced
6.Arrange the steps to find the centre of gravity of an irregular lamina in the correct order.
Medium
- Hang the lamina from a clamp
- Draw a line along the plumb line
- Repeat with two other holes
- Punch three holes near the edges
- The intersection of lines is the centre of gravity
7.A spring has a spring constant of 4900 N/m. A weight compresses it from 40 cm to 33 cm. What is the weight?
Medium
- A343 N
- B3430 N
- C34.3 N
- D0.343 N
8.What is the unit of moment of a force?
Easy

Plus de questions

9.A force of 22 N is applied at a perpendicular distance of 0.15 m from a pivot. Calculate the moment.
Medium
- A5.3
- B3.3
- C2.3
- D4.3
10.Friction always increases the speed of a moving object.
Easy
True or false?
11.Complete the sentence.
Easy
The principle of moments states that for a balanced object, the total clockwise moment equals the total ____ moment.
12.Which of the following is a definition of a force?
Easy
- AA push or a pull that acts on an object due to interaction with another object
- BThe amount of matter in an object
- CThe rate of change of velocity
- DThe turning effect of a force about a pivot
13.A force can change the shape of an object.
Easy
True or false?
14.What is a resultant force?
Easy
- AA single force that describes all the forces operating on a body
- BThe force that causes an object to accelerate
- CThe force that opposes motion
- DThe force due to gravity acting on an object
15.According to Newton's first law, if the forces on an object are balanced, the object will:
Medium
- Aremain at rest or move with constant velocity
- Baccelerate in the direction of the largest force
- Ceventually come to rest
- Dchange direction
16.A car of mass 900 kg accelerates from 0 to 27 m/s in 3 seconds. What force is required to produce this acceleration?
Medium
- A8100 N
- B300 N
- C2700 N
- D24300 N
17.Match each type of force to its correct description.
Easy
- Thrust
- Friction
- Weight
- Normal contact force
- A push force that causes an object to speed up
- A force that opposes motion
- The gravitational pull on an object
- The upward force from a surface
18.Arrange the following steps in the correct order to determine the centre of gravity of an irregular lamina using the suspension method.
Medium
- Punch three holes near the edges of the lamina.
- Hang the lamina from a clamp using a thread through one hole.
- Use a plumb line to mark the vertical line on the lamina.
- Repeat with the other two holes.
- The intersection of the three lines is the centre of gravity.
19.Which of the following are conditions for an object to be in equilibrium? (Select all that apply)
Hard
- AThe resultant force is zero.
- BThe sum of clockwise moments equals the sum of anticlockwise moments.
- CThe object is at rest.
- DThe object is moving with constant velocity.
20.The centre of gravity of a uniform sphere is at its centre.
Easy
True or false?
21.A car is travelling at a constant velocity. The driving force from the engine is 3 kN. What is the frictional force acting on the car?
Medium
- A0 kN
- B3 kN in the opposite direction
- C3 kN in the same direction
- D6 kN
22.Which of the following actions can increase the stability of an object? (Select all that apply)
Medium
- ALowering the centre of gravity
- BRaising the centre of gravity
- CWidening the base
- DNarrowing the base
23.Arrange the following steps in the correct order to calculate the moment of a force about a pivot.
Medium
- Identify the force and the pivot point.
- Measure the perpendicular distance from the pivot to the line of action of the force.
- Multiply the force by the distance.
- State the moment in N m.
24.Friction can be reduced by:
Easy
- ALubricating surfaces
- BIncreasing the roughness of surfaces
- CIncreasing the weight of the object
- DUsing a larger force
25.A car is travelling at a constant velocity of 20 m/s. The driving force from the engine is 1500 N. What is the frictional force acting on the car?
Easy
- A0 N
- B1500 N
- C30000 N
- D750 N
26.A spring has a spring constant of 200 N/m. A force of 50 N is applied to the spring. What is the extension of the spring?
Easy
- A0.25 m
- B4 m
- C250 m
- D0.4 m
27.A spring obeys Hooke's law. When a force of 8 N is applied, the extension is 2 cm. What force would cause an extension of 5 cm?
Medium
- A3.2 N
- B20 N
- C0.8 N
- D10 N
28.In the experiment to investigate the relationship between force and extension of a spring, which of the following is the independent variable?
Easy
- AExtension of the spring
- BForce applied to the spring
- CSpring constant
- DOriginal length of the spring
29.Which of the following are correct procedures when investigating the extension of a spring? (Select all that apply)
Medium
- AUse a pointer to accurately read the extension from the ruler
- BTake measurements at eye level to avoid parallax error
- CAdd all masses at once and record the final length
- DWait for the spring to stop oscillating before taking readings
30.In the force-extension graph for a spring, the gradient of the linear portion is equal to the spring constant.
Medium
True or false?
31.Arrange the following steps in the correct order to determine the centre of gravity of an irregular lamina.
Medium
- Punch three holes near the edges of the lamina
- Suspend the lamina from a clamp using a thread through one hole
- Draw a line along the plumb line on the lamina
- Repeat for the other two holes
- Find the intersection point of the three lines
32.Match each term related to the force-extension experiment with its correct description.
Medium
- Limit of proportionality
- Spring constant
- Hooke's law
- The point beyond which force and extension are no longer directly proportional
- The force per unit extension of a spring
- The extension of an elastic object is directly proportional to the force applied
33.A student investigates the extension of a spring. The results show that when a force of 2.0 N is applied, the extension is 4.0 cm. What is the spring constant?
Medium
- A0.5 N/m
- B50 N/m
- C200 N/m
- D500 N/m
34.In the centre of gravity experiment, the holes should be punched after drawing the lines of action of the weight force.
Easy
True or false?
35.Which of the following is the correct definition of a resultant force?
Easy
- AA single force that describes all of the forces operating on a body
- BThe force that is always greater than any other force
- CThe force that acts in the opposite direction to motion
- DThe force that causes an object to stop moving
36.What is the unit of the spring constant k in Hooke's law?
Easy
- AN/m
- BN m
- Cm/N
- DN
37.Match each effect of a force to its correct description.
Easy
- Change in motion
- Change in shape
- Change in size
- A car speeding up
- Compressing a spring
- Stretching a rubber band
38.Arrange the steps in the correct order to calculate the resultant force when forces act in opposite directions.
Medium
- Assign positive and negative signs to directions
- Add all forces together considering signs
- Determine the magnitude and direction of the resultant
39.Which of the following are conditions for an object to be in equilibrium? Select all that apply.
Medium
- AThe resultant force is zero
- BThe sum of clockwise moments equals sum of anticlockwise moments
- CThe object is stationary
- DThe object is moving with constant velocity
40.The centre of gravity of a symmetrical object of uniform density is at its geometric centre.
Easy
True or false?
41.A spring has a spring constant of 500 N/m. A force of 200 N is applied. What is the extension?
Easy
- A0.4 m
- B2.5 m
- C0.4 cm
- D2.5 cm
42.Arrange the steps in the correct order to determine the centre of gravity of an irregular lamina using the suspension method.
Medium
- Punch three holes near the edges
- Suspend the lamina from a clamp
- Use a plumb line to mark the line of action of weight
- Repeat for each hole
- The intersection of the lines is the centre of gravity
43.A car of mass 1200 kg accelerates from rest to 20 m/s in 5 seconds. What is the resultant force acting on the car?
Medium
- A4800 N
- B300 N
- C6000 N
- D2400 N
44.A spring has a spring constant of 4900 N/m. A weight compresses it from 40 cm to 33 cm. What is the weight in newtons?
Hard
45.A force of 22 N is applied at a perpendicular distance of 0.15 m from a pivot. Calculate the moment in N m.
Medium
46.State what is meant by the term 'resultant force'.
Easy
47.Complete the sentence about Hooke's law.
Medium
The extension of an elastic object is directly proportional to the ____ applied, up to the ____.
48.A force of 12 N acts to the right on an object. A second force of 5 N acts to the left. What is the magnitude of the resultant force?
Easy
49.A resultant force of 45 N acts on an object of mass 15 kg. What is the acceleration of the object?
Easy
50.State two ways in which friction between solid surfaces can be reduced.
Easy
51.A force of 30 N is applied at a perpendicular distance of 0.5 m from a pivot. Calculate the moment.
Easy
52.Explain why a car is more stable than a bus on a slope.
Medium
53.Describe how you would determine the centre of gravity of an irregularly shaped plane lamina using the suspension method.
Hard
54.Explain why a bus is less stable than a car on a slope.
Medium
55.A force of 12 N acts to the right on an object. A second force of 5 N acts to the left. What is the magnitude of the resultant force?
Easy
- A9 N
- B7 N
- C8 N
- D6 N
56.A resultant force of 45 N acts on an object of mass 15 kg. What is the acceleration of the object?
Easy
- A4 m/s²
- B5 m/s²
- C3 m/s²
- D2 m/s²
57.A force of 30 N is applied at a perpendicular distance of 0.5 m from a pivot. Calculate the moment.
Easy
- A17 N m
- B14 N m
- C16 N m
- D15 N m

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