Newton’s Laws of Motion Sections ) 1,3,4,5,6,8,12)

Objectives Define and understand Newton's First law of Motion. Define and understand Newton’s second law. Write Newton’s second law using appropriate units for mass, force, and acceleration. Demonstrate your understanding of the distinction between mass and weight. Define and understand Newton's third law.

Draw free-body diagrams for objects at rest and in motion. Apply Newton’s second law to problems involving one or more bodies in constant acceleration. understand Friction Force. Draw free-body diagrams for objects at rest and in motion with Friction Force. Apply your understanding of kinetic and static friction to the solution problems.

What is a Force? A force can most easily be described as a push or a pull. When a force is applied to an object, the velocity of that object changes. This change in velocity constitutes an acceleration. So, forces are tied to accelerations.

Give me example for the types of Forces?

Types of Forces ForceSymbol FrictionF fric or F f NormalFNFN SpringFSFS TensionFTFT WeightF grav

Newton's First law of Motion If no net force or no force acts on it (net force= 0), any object at rest will stay at rest, and any object in motion will stay in motion at constant velocity in a straight line.

Why any object at rest stay at rest?

How we find the Net Force?

The vector sum of all the forces acting on it.

Find the Net force on this object?

Law of Inertia Law of Inertia Inertia: The resistance an object has to a change in its state of motion. [Newton’s 1st Law].

Law of Inertia Law of Inertia Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

Equilibrium If the net force on an object equal zero, then it is said to be in equilibrium.

An object is in equilibrium when: at rest or moving at constant velocity.

What happen when the net force not equal zero?

Newton's second law The net force acting on an object causes it to accelerate in the same direction as the force.

Newton's second law F = m a Where: F is the net force, m is the mass, and a is the acceleration

Unit of Force Force is measured by N (Newton). 1 N = 1 Kg. m/s 2

kg. m/s 2 can be taken as the unit of : a ) force b ) power c ) work d ) none of these

If the net force acting on a moving car is zero, which of the following statements is true ? A.Its acceleration is constant. B.Its velocity is constant. C.Its mass is equal to zero. D.None of these.

When the net force that acts on a hockey puck is 10 N, the puck accelerates at a rate of 50 m/s 2.Determine the mass of the puck………. Example (1)

Equal forces F act on two bodies A and B. The mass of B is three times that of A. The magnitude of the acceleration of A is: A. three times that of B B. 1/3 that of B C. the same as B D. nine times that of B E. 1/9 that of B Example (2)

A car travels east at constant velocity. The net force on the car is: A. east B. west C. up D. down E. zero

A 4.0 kg shot-put is thrown with 30 N of force. What is its acceleration?

A constant force of 8.0 N is exerted for 4.0 s on a 16 kg object initially at rest. The change in speed of this object will be: A. 0.5m/s B. 2m/s C. 4m/s D. 8m/s E. 32m/s Example (3)

Weight The weight of an object is the gravitational force by the earth’s gravitational pull. Example: A person weighing 60 N is being pulled towards the earth with a force of 60 N. – m: mass of the body (kg) – g: gravitational acceleration = 9.8m/s 2

Example (4) Draw the force of gravity is acting on this box?

Answer: The force of gravity acts perpendicular to the horizontal ground.

What is the mass of a 47 N person?

The normal force is always exerted perpendicular to the surface that is holding up the object.

Example of The normal force

Example

Is the Normal Force Always Equal to the Weight?

To keep track of how all these forces are affecting a single object, it is a good idea to draw a free body diagram. A free body diagram is just a simple sketch of the object showing all the forces that are acting on it. Draw a quick sketch of the object. Draw an arrow showing every force acting on the object. To calculate the net force, add any vectors acting on the same axis (x and y), making sure to pay attention to the directions. Free Body Diagrams

Example 3.7 P57 Example 3.2 P52 Example 3.9 P58 Some Examples of Newton's laws

A 25kg crate is pushed across a frictionless horizontal floor with a force of 20 N, directed 20◦ above the horizontal. The acceleration of the crate is: 0.75 m/s 2 Example (5)

A 400N steel ball is suspended by a light rope from the ceiling. Find The tension in the rope ? Example (6)

When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first. All forces come in action-reaction : For every action there is an equal and opposite re-action. Newton's third law

Which of the following correctly states Newton's third law of motion? Forces occur in matched pairs that are: a) Equal in magnitude and the same direction. b) Not equal in magnitude and the same direction. c) Equal in magnitude and opposite in direction. d) Equal in magnitude and deferent in direction.

Example for Newton's third law  The rocket exerts a downward force on the exhaust gases.  The gases exert an equal but opposite upward force on the rocket.

The “reaction” force does not cancel the “action” force because: A. the action force is greater than the reaction force B. they are on different bodies C. they are in the same direction D. the reaction force is greater than the action force

If F = 4.0 N and m = 2.0 kg, what is the magnitude a of the acceleration for the block shown? The surface is frictionless. a) 5.3 m/s 2 b) 4.4 m/s 2 c) 3.5 m/s 2 d) 6.2 m/s 2

A car is moving at a constant velocity of 20 km/h,How much net force is required to raise its velocity to 50 km/h in 30 seconds? Suppose the car has a mass of 150 kg? F (net)=ma a= ∆v/ ∆t= V1= 20 km/h= 5.56 m/s V2=50 km/h = m/s a=( )/30=.278 F=150(.278)= 41.7N Example

A 70-N block and a 35-N block are connected by a string as shown. If the pulley is massless and the surface is frictionless,.when the tension force between them equal the magnitude of the acceleration of the 35-N block is: ans: 3.3

You can combine two forces into one. Suppose Jack pushed a box with a force of 30 N at 0 degree and Michael pushed it with a force of 40 N at 45 degrees. How can we find the net force acting on the box? Answer: Example

The total force on x direction would be 30 + (cos 45 * 40) = 58.3 N. [E] The total force on y direction would be 0 + (sin 45 * 40) = 28.3 N. [N] To find the combination of Jack and Michael's forces, we can just combine forces on x and y directions. Therefore, using the Pythagorean Theorem, we can calculate that N

Home Work

A 950 kg elevator is rising and its speed is increasing at 4m/s 2. The tension force of the cable on the elevator is: A N B N. C N D. None of these.