Force and Motion PSc.1.2 OBJECTIVE: Understand the relationship between forces and motion.

Objectives PSc Explain forces using Newton’s three laws of motion.

4 Forces of the Universe Strong: Forces holding the nucleus of an atom together Weak: Forces as a result of some atoms that undergo nuclear decay

4 Forces of the Universe Electromagnetic: A force based on electricity, magnetism and light properties Gravitational: The weakest force caused by 2 factors, MASS and DISTANCE

What is a Force? A force is an interaction between TWO objects. For example, pushes and pulls are forces.

What is a Force? Forces are vectors (they have both magnitude and direction).

Adding Forces In one dimension, note the direction using a + or – sign and then add like scalar quantities (regular numbers with no direction associated with them).

Adding Forces Examples: +3 N +4 N-4 N = 0 N += +6 N +

Newton's First Law of Motion (Inertia) An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by a net outside force.

“Consider a body on which no net force acts…” An important word here is NET. It means “total” or “sum of all” (forces).

Newton's First Law of Motion (Inertia) Inertia is the tendency of an object to resist changes in its velocity, whether in motion or motionless.

The tendency of an object to resist changes in its state of motion (inertia) is dependent upon its mass. The more mass an object has, the more inertia it has – the greater the tendency it has to resist changes in its state of motion.

Newton’s 1 st Law and You Because of inertia, objects (including you) resist changes in their motion. If a car going 80 km/h is stopped by a brick wall, your body keeps moving at 80 km/h.

Stationary and Moving Objects

Balanced and Unbalanced Forces An object is said to be in equilibrium when there is no unbalanced force acting upon it.

Balanced and Unbalanced Forces If the book is not at equilibrium, it subsequently accelerates.

For each situation, determine the net force (magnitude and direction) acting upon the object. Balanced and Unbalanced Forces A) zero NB) 5 N leftC) 0 N D) 15 N up

Newton’s Second Law of Motion The acceleration of a body is directly proportional to the net force on it and inversely proportional to the mass.

Newton’s Second Law of Motion m F a Unit of force = N (Newton) Unit of mass = kg (kilogram) Unit of acceleration = m/s 2

Force Example Problem What net force is required to accelerate a 1500 kg car at 3.00 m/s 2 ? GIVEN: F = ? m = 1500 kg a = 3.00 m/s 2 WORK : F = m a F = (1500 kg) (3.00 m/s 2 ) F = 4500 N m F a

Force Example Problem Joe pushes a 12 kg box with a force of 10. N pointing east while Jane applies a 15 N force directed west. What is the rate of acceleration of this box? GIVEN: F = 10 N N m = 12 kg a = ? WORK : a = F  m a = (- 5 N)  (12 kg) a = m/s 2 m F a

Force Example Problem If a 134 N force accelerates mass by 6.5 m/s 2, w hat is the object’s mass? GIVEN: F = 134 N m = ? a = 6.5 m/s 2 WORK : m = F  a m = (134 N)  (6.5 m/s 2 ) m = 21 kg m F a

Force Problem #1 A freight train slows down as it approaches a train yard. If a force of 3,800,000 N is required to provide a deceleration of 0.33 m/s 2, what is the train’s mass? 1.2 x 10 7 kg

Force Problem #2 A 69,000,000 kg freight train accelerates from rest to 11.0 m/s in 57.6 s. What is the size of the unbalanced force? 1.32 x 10 7 N

Force Problem #3 Suppose an empty grocery cart rolls downhill in a parking lot. The cart has a maximum speed of 1.3 m/s when it hits the side of the store and comes to rest 0.30 s later. If an unbalanced force of 65 N stops the cart, what is the mass of the grocery cart? 15 kg

Force Problem #4 A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? 7.5 m/s 2

Newton’s 2 nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

Newton’s Third Law of Motion For every action, there is an equal and opposite reaction OR for every force, there is an equal and opposite force. The action and reaction forces must act on different objects.

Problem:  How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force? NO!!!  Aren’t these “balanced forces” resulting in no acceleration? Newton’s Third Law of Motion

forces are equal and opposite but act on different objects they are not “balanced forces” the movement of the horse depends on the forces acting on the horse Explanation: Newton’s Third Law of Motion

When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. Newton’s Third Law of Motion

Flying gracefully through the air, ravens depend on Newton’s third law of motion. As the raven pushes down on the air with its wings, the air pushes its wings up and gives the raven lift.

The hammer exerts a force on the nail to the right. The nail exerts an equal but opposite force on the hammer to the left. Newton’s Third Law of Motion

The baseball forces the bat to the left. Identify the reaction. The bat forces the baseball to the right.

Newton’s Third Law of Motion The bowling ball forces the pin to the left. Identify the reaction. The pin forces the bowling ball to the right.

Newton’s Third Law of Motion The rocket exerts a downward force on the exhaust gases. The gases exert an equal but opposite upward force on the rocket.

Newton’s Third Law of Motion The enclosed air particles push the balloon wall outwards. Identify the reaction. The balloon wall forces the air particles inward.

Action-Reaction Pairs Both objects accelerate. The amount of acceleration depends on the mass of the object. F m Small mass  more acceleration Large mass  less acceleration Newton’s Third Law of Motion

Objectives PSc Explain how gravitational force affects the weight of an object and the velocity of an object in freefall.

The force of gravity on an object is called weight. At the Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass. Force of Gravity

Weight vs. mass Weight and mass are not the same. Mass is a fundamental property of matter measured in kilograms (kg). Weight is a force measured in Newtons (N). Weight depends on mass and gravity.

Weight vs. mass A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N. On the moon, the same rock only weighs 16 N.

Force of Gravity The force of gravity, by Newton's Second Law, is F g = m g where g is the acceleration due to gravity (9.8 m/s 2 ). m FgFg g

When you measure weight in pounds on a postal scale, you are measuring the force of gravity acting on an object. Force of Gravity

Gravitational Force Determine the gravitational force of 50.0 kg of physical science textbooks. GIVEN: F g = ? m = 50.0 kg g = 9.8 m/s 2 WORK: F g = m g F g = (50.0 kg) (9.8 m/s 2 ) F g = 490 N m FgFg g

Gravitational Force Determine the mass of an object that weighs 1200 N. GIVEN: F g = 1200 N m = ? g = 9.8 m/s 2 WORK: m = F g ÷ g m = (1200) ÷ (9.8 m/s 2 ) m = 122 kg m FgFg g

Gravitational Force Determine the acceleration due to gravity on Jupiter in which a 12 kg object exerts a gravitational force of 300 N. GIVEN: F g = 300 N m = 12 g = ? WORK: g = F g ÷ m g = (300) ÷ (12 kg) g = 25 m/s 2 m FgFg g

Gravitational Force Problem #1 Determine the gravitational force on a 1.25 kg object at rest on the surface of the Earth N

Gravitational Force Problem #2 A 20.0 kg block is pulled by along a surface by a horizontal force of at a constant speed. Determine the weight of the block. 196 N

Gravitational Force Problem #3 If the weight of a book is 14.0 N, what is the mass of the book? 1.43 kg

Objectives PSc Classify frictional forces into one of four types: static, sliding, rolling and fluid.

Friction Friction is a force that opposes motion, and so its vector always points away from the direction of movement.

Friction Friction is a force that resists the motion of objects or surfaces. Many kinds of friction exist.

Objects on Earth, unlike the frictionless space the moon travels through, are under the influence of friction.

57 Friction Opposes Motion Force on person by box Force on floor by box Force on box by floor It’s the sum of all the forces that determines the type of motion. Force on box by person

Friction Friction is greater... between rough surfaces when there’s a greater force between the surfaces (e.g. more weight)

Friction

Different Types of Friction As its name suggests, static friction occurs when the body is not moving (i.e. "static"). It is the force which makes it difficult to start something moving.

Different Types of Friction On the other hand, sliding (kinetic) friction occurs when the body is in motion. This is the force which causes objects to slow down and eventually stop.

Friction Static friction is always greater than kinetic friction.

Friction Rolling friction is the resistance that occurs when a round object such as a ball, tire, or wheel rolls on a surface.

Different Types of Friction Fluid friction is the friction that occurs in fluids such as water, air, and other liquids.

Different Types of Friction Air resistance is fluid friction that occurs in air.

As an object falls, the downward force of gravity causes the object to accelerate. Terminal Velocity

However, as an object falls faster, the upward force of air resistance increases. This causes the net force on a sky diver to decrease as the sky diver falls. Terminal Velocity

Finally, the upward air resistance force becomes large enough to balance the downward force of gravity.

This means the net force on the object is zero. Then the acceleration of the object is also zero, and the object falls with a constant speed called the terminal velocity. Terminal Velocity

Applied Force: Push and Pull A force which may act on an object could be any physical push or pull, an applied force. This could be caused by a person pushing a crate on the floor, a child pulling on a wagon or the wind pushing on the ship.

Friction Problem #1 The figure shows a block that is being pulled along the floor. According to the figure, what is the acceleration of the block? +2 m/s 2

Friction Problem #2 A 20.0 kg mass is pulled by along a surface by a horizontal force of at a constant speed. Friction is 20.0 N. What is the force applied to the mass? 20.0 N

Friction Problem #3 A 20.0 kg mass is pulled by along a surface by a horizontal force of 100 N. Friction is 20.0 N. What is the acceleration of the mass? 4.00 m/s 2

Friction Problem #4 A 49.0-N block is pulled by a horizontal force of 50.0 N along a rough horizontal surface at a constant acceleration of 6.00 m/s 2. What is the force of friction? 20.0 N

A 10.0 kg box rests on a horizontal floor. Static friction is 39.2 N, and kinetic friction is 29.4 N. Sample Friction Problem #5

Determine the force of friction acting on the box if a horizontal external applied force is exerted on it of magnitude (a) 0, (Answer: (a) 0) (Answer: (b) 10. N) (b) 10. N,

(Answer: (c) 20. N) (Answer: (d) 38 N) (Answer: (e) 29.4 N) (c) 20. N, (d) 38 N, and (e) 40. N