Motion and Forces

Motion describes how objects travel in space and time The main variables in the description of motion are: Speed (velocity) Travel time Acceleration

Speed or Velocity (v) How fast an object is travelling Measured in km/h or most often in science as m/s

Travel time (t) The amount of time the object was moving Measured in seconds (s)

Acceleration (a) The change in speed over a given time Measured in m/s2

How do you calculate the speed of an object? Formula d=vt d = distance travelled (m) v = velocity (m/s) t = time (s) The units of velocity will be m/s

What is the speed of a car that travels 10 km in 12 minutes? Table of Values Distance (d) = 10km = 10 000m Travel time (t) = 12 min = 720 sec

d = vt 10,000 m= (v)(720 s) 10 000 m = v 720 s v = 13.89 m/s = 50 km/h

Where to now? Now that we know d=vt, how does that help us understand force? Well, in order for an object to move, force has to be applied. When force is applied it moves an object in a specific direction.

What forces can do to an object Can cause an object to start moving or increase the speed of an already moving object. This is called Acceleration. Can cause an object to stop moving or slow down the speed of a moving object. This s called Deceleration. Can cause a moving object to change direction.

Examples of Forces A force is just a push or pull with the end result having an object move. Examples: an object’s weight tension in a rope as you pull an object attraction between an electron and proton friction Bodies don’t have to be in contact to exert forces on each other, e.g., gravity.

Unit of Force Force is measured in Newton’s (N) 1 Newton (N) is the amount of force required to accelerate a 1 kg object at a rate of 1 m/s2 1 N = 1 kg m/s2

Newton’s Laws of Motion Inertia: “An object in motion tends to stay in motion. An object at rest tends to stay at rest.” Net force is equal to mass times acceleration  Fnet = ma Action – Reaction: “For every action there is an equal but opposite reaction.”

First Law - Inertia A moving body will continue moving in the same direction with the same speed until some net force acts on it. A body at rest will remain at rest unless a net force acts on it. Summing it up: It takes a net force to change a body’s velocity.

Inertia Example 1 An astronaut in outer space will continue drifting in the same direction at the same speed indefinitely, until acted upon by an outside force.

Inertia Example 2 If you’re driving at 110 km/h and have an accident, your car may come to a stop in an instant, while your body is still moving at 110 km/h. Without a seatbelt, your inertia could carry you through the windshield.

Second Law – Fnet = ma The acceleration and the mass of an object moving is directly proportion to the net force acting on it. F=ma where F = Force in Newton’s (N) m = mass in Kilograms (kg) a = acceleration in meters/second (m/s2)

What is Acceleration Acceleration = Change of velocity Time Taken Example – An F1 car starts at 0 m/s and gets to 27 m/s in 3 seconds, what is the F1’s acceleration? a = 27m/s – 0m/s = 9m/s2 3 s

Force example How much force is required to accelerate a 2 kg toy car at 3 m/s2? Table of Values Mass = 2 kg Acceleration = 3 m/s2

Equation F=ma F=(2 kg)(3 m/s2) F=6 kg m/s2 F= 6 N

Net Force or Resultant Force Objects are usually subjected to several forces at once. The RESULTANT force is equal to the combination of all forces acting on an object at the same time.

Net Force or Resultant Force Force # 1 + Force # 2 = Net Force 400N + 300 N = 700 N

Net Force or Resultant Force Force # 1 =300 N Force # 2 = 400 N Force # 1 + Force # 2 = Resultant Force 300N + (-400 N) = -100 N 100 N

Net Force or Resultant Force Force # 1 =300 N Force # 2 = 300 N Force # 1 + Force # 2 = Resultant Force 300N + (-300 N) = 0 N 0 N

What is Net Force? F1 When more than one force acts on a body, the net force (resultant force) is the combination of all the forces, i.e., the “net effect.” F2 F3 Fnet

Net Force & the 2nd Law 32 N 15 N 2 kg When forces act in the same line, we can just add or subtract their magnitudes to find the net force. What is the acceleration of this object? 32 N 15 N Fnet = 32-15 = 27 N 2 kg Fnet = ma 27 N = (2 kg)a a= 27 N 2 kg a = 13.5 m/s2

Third Law – Action/Reaction If you hit a tennis ball with a racquet, the force on the ball due to the racquet is the same as the force on the racquet due to the ball. If you drop an apple, the Earth pulls on the apple just as hard as the apple pulls on the Earth. If you fire a rifle, the bullet pushes the rifle backwards just as hard as the rifle pushes the bullet forwards.