Forces

Newton’s Second Law Newton’s second law describes how force, mass, and acceleration are all connected! An object accelerates when a force acts on its mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

Force = mass x acceleration Newton’s Second Law F = M x A Force = mass x acceleration

Newton’s Second Law Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, can you compute how much force Mike is applying to the car?

Newton’s Second Law A 0.06kg tennis ball is served with a racket. The racket applies a force of 330N to the ball and it leaves the racket with a speed of 100 km/hr. What was the tennis ball’s acceleration?

You push a skateboard with your hand You push a skateboard with your hand. According to Newton’s first law the skateboard should continue at a constant speed in the direction you pushed it forever unless acted on by another force. In reality, what happens to the skateboard? Why?!

Friction Friction – the force that opposes motion between two surfaces that are touching each other Surface material Amount of force pressing the surfaces together

Types of Friction Static friction – friction between two surfaces that are not moving past each other Sliding friction – the force that opposes the motion of two surfaces sliding past each other.

Types of Friction Rolling friction – less than static and sliding, due to friction between rolling object and surface it is rolling on

Air Resistance Air resistance – the opposing force on something that is falling due to gravity, “friction of the air”

Air Resistance What is there is no air?!

Air Resistance The force of air resistance increases with speed When speed increases to the point that air resistance cancels the force of gravity you reach terminal velocity Terminal Velocity – forces on a falling object are balanced and the object no longer accelerates. The highest velocity a falling object can reach

Force = mass x acceleration How much force is needed to accelerate a 66 kg skier at 2 m/sec2? A 50 kg skater pushed by a friend accelerates 5 m/sec2. How much force did the friend apply? A force of 20 N acts upon a 5 kg block.  Calculate the acceleration of the object.    An object of mass 30 kg is falling in air and experiences a force due to air resistance of 50 newtons. Determine the net force acting on the object and calculate the acceleration of the object.

Gravity Anything with mass is attracted by the force of gravity Law of Gravitation – any two masses exert an attractive force on each other

Gravity

Force due to Earth’s gravity… Gravity is a basic force Near Earth’s surface the acceleration due to gravity is 9.8m/s2 F = m a Force due to Earth’s gravity… F = m (9.8m/s2)

Gravity If you are sitting at your desk you are not accelerating Does this mean that gravity has disappeared? Of course not! Gravity constantly pulls on you, this is measured as weight Weight – gravitational force on an object Weight = mass x 9.8m/s2

Weight vs. Mass The further apart two objects with mass are, the less gravitational pull there is between them The further away from the surface of the Earth, the less you will weigh!

Weight A man stands on a scale and measures his weight. He then stands on the scale in an elevator. Will his weight be affect when the elevator travels upward? Will his weight be affected when the elevator travels down? If so, how will it be affected and when?

Projectile Motion If you were to throw a ball will it continue in a straight line? Objects down travel is straight lines, they tend to curve down Gravity is pulling the object down to the Earth!

Projectile Motion As you release a baseball you stop applying a force, its horizontal motion is constant When you let go, gravity takes over and begins to pull the ball down

Projectile Motion If you throw a ball as hard as possible in a perfectly horizontal line, and drop a ball of the same mass from the same height, which would hit the ground first?

Centripetal Force Remember… Acceleration is the rate of change of velocity due to a change in speed, direction or both Centripetal Acceleration – acceleration toward the center of a curved or circular path

Centripetal Force What is required for an object to accelerate in any direction? A FORCE!!! F = M x A! Centripetal Force – an unbalanced force acting on an object causing acceleration toward the center of a curved path

Every action has an equal and opposite reaction Newton’s 3rd Law Newton’s 3rd Law of Motion – when one object exerts a force on a second object, the second object exerts a force on the first that is equal in size and opposite in direction Every action has an equal and opposite reaction

Newton’s 3rd law even works in SPACE! Rocket Science! Newton’s 3rd law even works in SPACE! Rocket engines work by forcing gases out of the back of the rocket Those gases exert an equal and opposite force on the rocket itself

Momentum = mass x velocity Moving objects have a property called momentum Momentum – the product of mass and velocity; how much force is needed to change the motion of an object Momentum = mass x velocity p= m x v

Momentum kg m/s Our units for momentum… Momentum has a direction because velocity has a direction!

Newton’s Second Law – the law of inertia! Momentum Which of Newton’s laws is momentum related to? Newton’s Second Law – the law of inertia!

Momentum Knowing that momentum is related to Newton’s 2nd law… Has your hand ever stung when you caught a ball?! A force is exerted on your hand as its momentum is changed

Momentum Does anybody remember the formula for acceleration!? How about the formula for force?!

Momentum Now if we combine the two formulas… F = 𝑚𝑣𝑓 −𝑚𝑣𝑖 𝑡

Momentum A 1000kg car has stalled in the middle of the road. It must be pushed in order to get it out of the middle of traffic. Two men stop to help and push the car for 15s. The car reaches a speed of 4.5 m/s. With what force did they push the car?

A force of 250 N is applied to an object that accelerates at a rate of 5 m/sec2. What is the mass of the object? An object of mass 300 kg is observed to accelerate at the rate of 4 m/s2.  Calculate the force required to produce this acceleration. What is the momentum of a bird with a mass of 1kg and a velocity of 5m/s? What is the mass of a bowling ball with a momentum of 25 kgm/s and a velocity of 5 m/s?

A 50 kg clown is launched from a cannon at 20 m/s A 50 kg clown is launched from a cannon at 20 m/s. What is the recoil speed of the cannon if the cannon has a mass of 250 kg?

Conservation of Momentum Momentum of an object does not change unless its mass, velocity, or both change Momentum can be transferred from one object to another

Newton’s Laws! 1st - An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. “Law of inertia!”

Newton’s Laws 2nd - Acceleration is produced when a force acts on a mass. The greater the mass the greater the amount of force needed. F = m x a

“when you push an object, it pushes back” Newton’s Laws 3rd - For every action there is an equal and opposite re- action “when you push an object, it pushes back”