Dynamics flashcards

Effects of Force

When a force is exerted on an object, it can:

• Start or stop its movement.

• Slow down or speed it up.

• Change the direction of its motion.

Resultant Force

• When multiple forces act on a body, their effects combine as if a single force is acting.

• Resultant force is the single force that represents the combined effect of all forces acting on the body.

Newton’s First Law

• If the resultant force acting on a body is zero, the forces are balanced.

  • Stationary object: Remains stationary.

    • Example: A stationary object with 10 N force acting from both sides, resulting in a resultant force of 0 N.

  • Object moving at constant velocity: Continues moving at a constant speed in a straight line.

    • Example: An object moving at constant velocity with 20 N force acting from both sides, resulting in a resultant force of 0 N.

• Important Information:

  • When there is no resultant force (forces are balanced), the object will either be at rest or travel at constant velocity.

  • Conversely:

    • If an object is at rest, there is no resultant force.

    • If an object travels at constant velocity, there is no resultant force.

    • If there is no acceleration, there is no resultant force.

Applications of Newton’s First Law

• Explains why people should wear seat belts.

  • If the driver suddenly applies the brakes, they will continue to move forward.

  • A seat belt provides the necessary opposing force to stop the driver from crashing into the windshield.

Example 1

A boy is pushing a box at a constant speed on a straight path. Which statement is correct?

• Correct Answer: A. The resultant force is zero.

Example 2

A meteorite is traveling at a constant velocity of 40,000 km/s in Earth’s atmosphere. The forces acting on the meteorite are its weight (Earth’s gravitational pull) and air resistance against its motion. Which statement is true?

• Correct Answer: C. The weight of the meteorite is equal to the air resistance.

Newton’s Second Law

• If the resultant force acting on a body is not zero, the forces are unbalanced.

  • Stationary object: No longer stationary.

    • Example: A stationary object with 10 N force acting on one side and 5 N on the opposite side, resulting in a resultant force of 5 N.

  • Object moving at constant velocity: Velocity changes.

    • Example: An object moving at constant velocity with 20 N force acting on one side and 10 N on the opposite side, resulting in a resultant force of 10 N.

• When velocity changes, the object undergoes acceleration or deceleration.

• When there is a resultant force acting on an object, the object will accelerate in the direction of the resultant force.

• The product of the mass and acceleration of the object is equal to the resultant force.

  F_{net} = ma

  Where:

  • F_{net} = resultant force (N)

  • m = mass of object (kg)

  • a = acceleration of object (m/s^2)

• Change in velocity can happen when:

  • There is a change in the magnitude of the speed.

  • There is a change in the direction of the motion.

  • Both a change in magnitude of the speed and the change in direction.

Worked Example 3

A boy pushes a box of mass 20 kg with a force of 50 N. What is the acceleration of the box? (Assume no friction.)

• Solution:

  • Given: mass m = 20 kg, force F = 50 N

  • From Newton’s Second Law: F_{net} = ma

Worked Example 4

A car of mass 1000 kg accelerates from rest to 20 m/s in 5 s. Calculate the forward thrust of the car (assume no friction).

• Solution:

  • Given: mass m = 1000 kg, initial speed u = 0 m/s, final speed v = 20 m/s, time t = 5 s

  • From Newton’s Second Law: forward thrust F_{net} = ma, where a = acceleration produced.

Worked Example 5

The force on an aircraft of mass 725 kg due to the engine is 15,000 N. The air resistance on the plane is 9925 N. Find the acceleration of the plane.

Quick Check 1

• Fill in the blanks related to force concepts.

• Questions about balanced and unbalanced forces.

Newton’s Third Law of Motion

• For every action, there is an equal and opposite reaction.

• Four characteristics of forces:

  • Forces always occur in pairs (action and reaction).

  • Action and reaction are equal in magnitude.

  • Action and reaction act in opposite directions.

  • Action and reaction act on different bodies.

  • Action and reaction pairs are of the same nature of forces.

Newton’s Third Law Examples

• A box rests on a table.

  • Weight W of the box and normal reaction F are NOT an action–reaction pair.

  • Gravitational force of Earth on box (F{EB}) and the gravitational force of box on Earth (F{BE}) are an action–reaction pair.

Example 6

Examples of action and reaction forces

1. Force pushing a bullet out of a rifle.

2. Contact force that A exerts on B

3. Frictional force the ground exerts on A

4. Tension force exerted on A by B

Example 7

State whether the following are action-reaction pairs or not.

• Weight of book (W) and reaction from the floor (R).

• Tension (T) of string pulling on block and friction of block with floor (F).

• Wind blows away from fan and force on the fan.

• Tension (T) of string pulling on object and weight on the object.

Free-Body Diagram

• Simple diagrams with arrows that represent forces acting on individual objects.

• To draw a free-body diagram, identify all the forces acting on each object.

• Examples:

  • Box suspended by strings (tension, weight).

  • Rocket in flight (thrust of engine, weight, air resistance).

Example 8

A car is traveling at a constant velocity on a horizontal road. Draw a free-body diagram of all the forces acting on it.

• Forces: engine thrust, friction, weight, normal force.

• Note: Length of arrow of engine thrust and friction are equal as they have same magnitude so the force is balance and the car travels at a constant velocity

Example 9

Draw the forces acting on a block that is stationary on the slope.

• Forces: friction, weight, normal force.